Investigation of long-term exposure effects under stress on supersonic transport structural alloys

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INVESTIGATION OF LONG-TERM EXPOSURE E F F E C T S

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UNDER STRESS ON SUPERSONIC TRANSPORT STRUCTURAL ALLOYS

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George M a r tin

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P r e p a r e d u n d e r C o n t r a c t No. NASw 921 by

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NORTH AMERICAN AVIATION, INC I Los Angeles Division, Los Angeles, California for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION ABSTRACT The report covers the first two phases of a 30,000-hour exposure test. Six materials - two titanium alloys, two P H steels, and two superalloys - were tested under stress in five environ- mental conditions comprising combinations of salt and braze coatings and constant and cyclic exposure at 650" F. Braze-coated titanium alloys, salt-coated Titanium 8Al-1V-1Mo alloy and cyclically exposed AM 350 steel were the only alloys subject to stress-corrosion failure within the 20,000-hour exposure covered. In t h i s period relative phase changes occurred in the titanium alloys without significant mechanical property changes and aging reactions occurred in both steels and the superalloys with appropriate mechanical property changes. A hypothesis for corrosion reactions on titanium alloys in salt environments h a s been developed.

n ACKNOWLEDGEMENTS The work done in this report w a s carried out in the Research and Development Labora- tories, North American Aviation, Inc, Los Angeles Division, which are under the general direc- tion of N. Klimmek. The electron microscopic work was carried out by G. C. Thomas, the microscopic work by R. Brose and the X-ray diffraction studies by E. LaRocca and G.C. Thom- as. The mechanical testing part of the work was carried out under the direction of G . R . Martin and F. Rutkosky . Stress calculations and the equations and graphs plotted in the appendix were computed by L. M. Lackman and R. M. Ault.

i TABLE O F CONTENTS Contents Page TITLE PAGE ABSTRACT i ACKNOWLEDGEMENTS i TABLE O F CONTENTS iii LIST OF ILLUSTRATIONS V LIST OF TABLES i x SUMMARY x i INTRODUCTION SURVEY OF PREVIOUS WORK ON STRESS-CORROSION EXPERIMENTAL PROCEDURES EXPOSURE EFFECTS OX METALLURGICAL STRUCTURE CHANGES IN MECHANICAL PROPERTIES AND STRESS CORROSION EFFECTS CORROSION 11 CONCLUSIONS AND RECOMMENDATIONS REFERENCES APPENBiX PRECEDING PAGE BLANK NOT FILMED.

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LIST OF ILLUSTRATIONS Figure No. Title Page

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1 Dimensions of Exposure Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 42 Exposure Specimens in Test Frame. . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 43 Exposure Test Frames in Furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System of Marking and Cutting Up of Exposure Specimens and Other Test

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Specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5 Notched and UnnotchedMiniature Tensile Specimens . . . . . . . . . . . . . . . . . 45

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6 Miniature Test Specimens Test Rig - Components . . . . . . . . . . . . . . . . . .

7 Miniature Test Specimens Test Rig - Assembly With Extensometer . . . . . . 47

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8 Microstructure of Titanium 6A1-4V Alloy P r i o r to Exposure t o Test Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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9 Microstructure of Salt-coated Titanium 6Al-4V Alloy After Exposure in

Circulating Air at 650°F (Specimens AB1 and AB2) . . . . . . . . . . . . . . . . 49

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Microstructure of Titanium 6A1-4V Alloy After 20,000 Hours Exposure. . . .

11 Microstructure of Titanium 8A1- 1Mo-1V Alloy Prior to Exposure t o Test

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Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12 Microstructure of Titanium 8A1- 1Mo- 1V Alloy After Exposure in Circulating Air at 650°F (Specimen BA2) . . . . . . . . . . . . . . . . . . . . . . .

i 3 Microstructure of Titanium 8Ai- i V - iIvio Aiioy MLer 20,000 Iiuurs Exposure (BA3) . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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14 54 Microstructure of PH15-7Mo Steel P r i o r to Exposure to Test Environment .

15 Microstructure of Salt-coated PH15-7Mo Steel After Exposure in

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Circulating Air at 650°F (Specimens CBl and CB2). . . . . . . . . . . . . . . . .

16 Microstructure of PH15-7Mo Steel After 20,000 Hours Exposure (CA3) . . . . 56

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Microstructure of AM 350 Steel Prior to Exposure to Test Environment . . . 57

I 18 Microstructure of Salt-coated AM 350 Steel After Exposure in Circulating

A i r at 650°F (Specimens DB2 and DB6) . . . . . . . . . . . . . . . . . . . . . . . . 5%

Microstructure of AM 350 Steel After 20,000 Hours Exposure (DB3) . . . . . .

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Microstructure of Inconel 718 Alloy P r i o r to Exposure to Test Environment.

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Microstructure of Salt-coated Inconel 718 Alloy After Exposure in

Circulating A i r at 650°F (Specimens EB1 and EB2) . . . . . . . . . . . . . . . . 61

I PRECEDING PAGE BLANK NOT FILMED.

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Figure No . Title Page

22 Microstructure of Inconel 718 After 20. 000 Hours Exposure (EA3K) . . . . . . . 62

23 Microstructure of Rene 41 Alloy Prior to Exposure to Test Environment . . . . 63

24 Microstructure of Salt-coated Rene 41 Alloy After Exposure in Circulating 25 65 Microstructure of Rene 41 After 20. 000 Hours Exposure (FA3) . . . . . . . . . .

26 Titanium 6Al-4V Alloy Unstressed Exposure Tensile Test Results . . . . . . . . 66

27 Titanium 6Al-4V Alloy Stressed Exposure Tensile Test Results . . . . . . . . . .

28 Titanium 8A1- 1Mo- 1V Alloy Unstressed Exposure Tensile Test Results . . . . .

29 69 Titanium 8A1-1Mo-1V Alloy Stressed Exposure Tensile Test Results . . . . . .

30 PH15- 7Mo Steel Unstressed Exposure Tensile Test Results . . . . . . . . . . . . . 70

PH15- 7Mo Steel Stressed Exposure Tensile Test Results . . . . . . . . . . . . . . 71

32 AM 350 Steel Unstressed Exposure Tensile Test Results . . . . . . . . . . . . . . .

AM 350 Steel Stressed Exposure Tensile Test Results . . . . . . . . . . . . . . . . 73

Inconel 718 Alloy Unstressed Exposure Tensile Test Results . . . . . . . . . . . . 74

35 Inconel 718 Alloy Stressed Exposure Tensile Test Results . . . . . . . . . . . . . .

36 Rene 41 Alloy Unstressed Exposure Tensile Test Results . . . . . . . . . . . . . . 76

37 Rene 41 Alloy Stressed Exposure Tensile Test Results . . . . . . . . . . . . . . . .

39 Fractographs of Stress Corrosion Cracks in Titanium 6A1-4V Alloy . . . . . . . 79

Fractographs of Stress Corrosion Cracks in Titanium 8A1-1V-1Mo Alloy (Braze Coated) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41 Fractographs of Surface of Stress Corrosion Cracks in AM 350 Steel After

Alternate Exposure at 650°F and 100.F . Failure Time 1152 Hours (DE8) . . 81

42 Microstructure of Braze and Salt- coated Titanium 6A1-4V Alloy After

Exposure in Circulating A i r at 650°F (Specimens AD3 and AD5) . . . . . . . . . 82

43 Microstructure of Braze and Salt- coated Titanium 6A1-4V Alloy Notch

Specimen AD5 Showing Surface Crack . Specimen Failed After 15. 000 Hours

Exposure in Circulating A i r at 650 .F ...........................

44 Microstructure of Braze-coated Titanium 8Al- 1Mo- 1V Alloy After Exposure in Circulating A i r at 650°F (Specimen BC2) ......................

45 Microstructure of Braze and Salt-coated Titanium 8A1- 1Mo-1V Alloy After Exposure in Circulating A i r at 650°F (Specimen BD3) . . . . . . . . . . . . . . . .

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Figure No. Title Page

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46 Microstructure of Salt-coated Titanium 8Al- 1Mo- 1V Alloy After Exposure

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47 Microstructure of Salt- coated Titanium 8Al- 1Mo- 1V Alloy After Alternating 14-day Exposure in Circulating A i r at 650°F and in Humidity Cabinet at

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48 Microstructure of Salt-coated AM 350 Steel Notch Specimen DB6 at Fracture in Notch Showing O l d and New Crack Areas. Specimen Failed After 15,000 Hours Exposure in Circulating Air at 650°F ............

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49 Microstructure of Salt-coated AM 350 Steel After Exposure in Circulating A i r at 650°F (Specimen DB5) ...............................

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50 Free Energy Change in Titanium Corrosion Reaction . . . . . . . . . . . . . . . .

51 Electron Microprobe Analysis of Titanium 8A1- 1Mo- 1V Alloy

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52 Electron Microprobe Analysis of Titanium 6A1-4V . . . . . . . . . . . . . . . . . .

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Comparison of Fine Structure of Titanium 8A1-1Mo- 1V Alloy Fractured Under Salt Coating After 2640 Hours Exposure (Specimen BB2) ........

54 Deflection and Moment Arm Versus Load - Stiffness P a r a m e t e r s . . . . . . . .

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55 Moment Coefficients Versus Load - Stiffness Parameters for Intermediate Seam Locations ........................................

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LIST O F TABLES Table No. Title Page ...................................

I Candidate Materials 20 .................

I11 Summary of Surface and Exposure Treatments 22 ...................

N Maximum Stress Levels in Test Specimens. 23 V Summary of Titanium 6A1-4V Alloy Specimen Failures to 20,000 Hours Exposure and Unnotched and Notched Tensile Properties of These VI Summary of Titanium 8A1- 1Mo- 1V Alloy Specimen Failures t o 20,000 Hours Exposure and Unnotched and Notched Tensile Properties of VI1 Summary of PH15-7Mo Steel Specimen Failures to 20,000 Hours Exposure and Unnotched and Notched Tensile Properties of These VIII Summary of AM 350 Steel Specimen Failures to 20,000 Hours Exposure and Unnotched and Notched Tensile Properties of These Specimens. ..

M Summary of Inconel 718 Alloy Specimen Failures to 20,000 Hours Exposure and Unnotched and Notched Tensile Properties of These X Summary @f Rend41 Alloy Specimen Failures to 20,000 Hours Exposure

a d 'v'iinotzhed a d N~ttched Teiisile r"i*opei+ies u f These Specimens. .. 29

XI ........................ Summary of X-ray Diffraction Tests

XI1 Summary of Ra Hardness Measurements Taken on Titanium 6A1-4V Alloy Through 20,000 Hours Exposure ......................

XI11 Summary of Ra Hardness Measurements Taken on Titanium 8A1- 1V- 1Mo Alloy Through 20,000 Hours Exposure ......................

XIV Summary of Ra Hardness Measurements Taken on PH15-7Mo Stainless Steel Through 20,000 Hours Exposure ......................

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Summary of Ra Hardness Measurements Taken on AM 350 Steel Through 20,000 Hours Exposure. .........................

XVI Summary of Ra Hardness Measurements Taken on Inconel 718 Through 20,000 Hours Exposure ..........................

/ XVII Summary of Ra Hardness Measurements Taken on Rene 41 Through i x PRECEDING PAGE BLANK NOT FILMED.

Table No. Title Page .

Analysis of Salt Coating From Ti- 8A1- 1V- 1Mo Alloy, Exposed to 650 "F X M

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X M Calculations of Effective Beam Lengths .......................

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SUMMARY

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This report describes the results of the first two phases of an investigation of the metal- lurgical and mechanical property changes occurring in six candidate materials for supersonic I transport vehicles on exposure t o 650°F in the stressed and unstressed condition under various surface environmental coatings. The first two phases consisted o r the examination of specimens exposed for 10,000, 15,000, and 20,000 hours and specimens which have failed by stress- corrosion within the period.

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The six candidate alloys tested were two titanium alloys, two precipitation hardening steels, and two superalloys. The specimens, in t h e shape of cantilever strips loaded to stress levels varying from 23 to 90 percent of the yield stress, were exposed to a temperature of 650°F. Sur-

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face treatments consisted of salt coating, braze coating and salt-coated braze coating, and cyclic exposure at 650°F and a humidity cabinet.

After a period up to 20,000 hours, most of the braze-coated and braze-coated plus salt- 2oated titanium alloys have fractured by s t r e s s corrosion. In addition, two out of six salt-coated titanium 8A1-1V-1Mo alloy specimens and two out of six of salt coated AM 350 alloy specimens have failed. Four out of four AM 350 alloy specimens exposed to a cyclic environment of furnace

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temperature and humidity cabinet failed, all after approximately 3000 hours.

that all alloys except the steels are affected by the braze The mechanical tests indicated

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alloys. The titanium alloys are strongly affected. Steels and superalloys undergo aging reac- tions during the exposure period, which affected both the strength levels and the ductility. S t r e s s during exposure did not appear to affect any of the changes in mechanical properties observed.

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No gross changes in metallurgical structure of any of the alloys could be observed. X-ray diffraction studies, however, do indicate probable changes in the relative amounts of alpha and beta phase after exposure of the titanium alloys.

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Stress-corrosion fractures are of a typical intercrystalline nature in both the titanium alloys and the A M 350 alloys. Microscopic studies have so far indicated no structural change directly related to the corrosion mechanism. However, examination of fractured titanium alloy specimens

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by means of an electron microprobe indicate evidence of segregation of heavy alloying constitu- ents near the crack. Evidence of segregation is also evident from electron microsco-pe replicas.

I The corrosion products on titanium specimens exposed under coatings of natural and artifi- cial sea salt have been examined by X-ray diffraction and there appears to be tentative evidence for the existence of NaOH formed during the exposure process. Thermodynamic calculations show the feasibilityof reactions resulting in t h i s product.

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INTRODUCTION

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The institutions responsible for material selection for high-speed aircraft designed for long time service, such as the National Aeronautics and Space Administration, the Federal Aviation

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Agency, and the Aerospace Industry Association, have, for considerable time, been keenly aware of the problems involved in the selection of materials for such aircraft. Materials re- quired for such aircraft are either radically new families which have to be developed and evalu- ated o r they consist of known materials exposed to a new type of environment. Materials likely

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to be of prime importance are titanium alloys, high-strength steels, and superalloys. Most of these materials must be heat-treated to develop suitable properties. However, such properties can only be obtained in what a r e basically meta-stable metallurgical structures. It is a matter of

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concern whether long time exposure to elevated temperatures with o r without stress would lead to changes of such meta-stable structures and therefore to changes in the mechanical properties.

In addition to this standard type of information, such as the strength of the various temperature

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levels, creep data, and fatigue information, a very high degree of assurance is required that the materials chosen will not be subject to a sudden type of failure such as stress-corrosion. The environments likely to lead to stress-corrosion are braze coatings, which may have been used for joining, and particularly the possibility of sea salt incrustation covering the external sur-

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faces of the aircraft. It is estimated that t h e type of aircraft considered here may be exposed on t h e external surfaces t o temperatures up to 650°F during the service life in excess of 30,000 hours. Complex interactions between t h e coatings, the temperature, and any structural changes

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appearing in the material after long time elevated temperature exposure can therefore be expected.

A general program t o determine the likelihood of a stress-corrosion failure in appropriate

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candidate materials w a s started by North American Aviation, Inc, in 1962. The present program consists of a comprehensive evaluation of specimens that failed by stress-corrosion o r remained exposed to periods of 10,000, 15,000, and 20,000 hours. This program was aimed at the deter-

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mination of any possible degradation in mechanical properties, at a search for evidence of pos- sible changes in the metallurgical structures, and at an evaluation of a possible reaction involved in stress-corrosion. The specimens exposed under the program organized by North American Aviation, Inc, were to form the basis of information. Specific tests carried out include notched microscopic examination, examination of the metallurgical structure and unnotched tensile tests, I hT7 nlantwfin ---l:--L:-- u J vll-IllLLI uDLupc I cpltLdLlu11, X-ray difii*acituii studies oi pvssibie phase changes, and aiso examination of surface products with a view to a better understanding of corrosion and stress

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corrosion mechanisms. The entire program is planned t o investigate the effects on materials of I a total exposure time of 30,000 hours.

I This report covers the first two phases of the investigation. The final third phase will deal with materials after 30,000 exposure. The design of the experiment, that is the number and type is such that at the completion of 30,000-hour investigation, all test results of specimens chosen, w i l l be available in duplicate.

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SURVEY O F PREVIOUS WORK ON

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STRESS- CORROSION

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The susceptibility of titanium alloys to stress-corrosion cracking when in intimate contact with sodium chloride at elevated temperatures has been known for many years. The limits of the corrosion reaction were not known nor was the corrosion mechanism established, although

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many theories have been proposed.

Evidence (Reference 1) had established that titanium alloys are subject t o stress corrosion cracking when in intimate contact with sodium chloride at temperatures above 500°F. No service

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failures were reported (to mid-1957) which were attributed to this type of corrosion. Laboratory tests had shown that various coatings (oxide films, anodic films, aluminum and nickel metallic coatings) would mitigate this type of corrosion. The limits of the corrosion reaction were not

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known nor was the corrosion mechanism established, although various theories have been pro- posed. Further studies were recommended t o identify the corrosion product, establish differ- ences between types of titanium alloys, and the effects of salt concentration and thickness of salt

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coatings. A theory of the corrosion mechanism was advanced which suggests that titanium in the presence of oxygen and a reducible chloride forms TiC12. Sodium chloride w a s established as a crack nucleating agent. It was shown that moving air across the specimen surfaces during corrosion exposure increases the resistance of the material to stress-corrosion. Glass bead

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peening and a sodium hydroxide anodizing treatment were demonstrated to afford protection against stress-corrosion of Ti-6A1-4V alloy.

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Apart f r o m salt, three other environments were found to stress-corrode titanium: (Refer- ence 2) these were molten cadmium, red fuming nitric acid (RFNA), and hydrochloric acid formed by the decomposition of a chlorinated diphenyl compound in air at 600°F. The molten cadmium corrosion occurred on a Ti-4Mo-4Al alloy in contact with a cadmium-plated bolt at

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600°F t o 750°F. Stress-corrosion cracking of Ti-5Al-2.4 Sn alloy w a s found t o take place in t h e presence of halides (trichloroethylene) during heat treatment at temperatures of 1150" F and 1500°F for 16 hours (Reference 3). Severe cracking, other than stress corrosion cracking,

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w a s a l s o found to occur with s t r e s s present if a surface oxide coating was present.

This effect accentuated any difficulty encountered by halide contamination.

I Ti-6A1-4V and Ti-8A1-1Mo-1V were incapable of withstanding an exposure of 25,000 psi at

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650°F for 1000 hours, but Ti-8Al-1Mo-1V did not fail at 450°F. A possible corrosion mechan- ism involves gaseous chlorine attack and also galvanic corrosion. Exposure of notched titanium specimens in a 650°F sea salt environment prior to stressing at 25 ksi was found to prolong spe-

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cimen life (Reference 4). Stress-corrosion cracks were found in Ti-8Al-1Mo-1V specimens in which sea salt had been packed into a notch consisting of a 1/16-inch diameter hole in the center of a sheet specimen and the specimens then exposed at 650°F and stresses of 25 and 63 ksi for 50 I and 200 hours, respectively, and at 800°F and 25 ksi for 100 hours (Reference 5). Materials Re- search Laboratory reported evidence that Ti-6A1-4V alloy stress corrosion in hot salt is electro- lytic in nature, with the titanium becoming anodic to chloride ions in a postulated thin film of eutectic o r low-melting salts. In this hypothesis free chlorine does not enter directly into the

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corrosion mechanism and is not essential to it. The entire problem of elevated temperature stress-corrosion of titanium alloys w a s summarized recently by Boyd and Fink (Reference 6).

Their criterion for stress-corrosion attack is cracking o r fracture, and no allowances a r e made

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for possible property degradation. The authors conclude that the stress-corrosion problem appears to b e unimportant at temperatures below 500"F, but that at temperaturesabove that level data are too conflicting for any conclusions regards mechanisms o r design parameters to be drawn up. In particular, extended work on the corrosion mechanism problem is recommended.

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A s far as other SST alloys are concerned, data are less conflicting than those for titanium.

The superalloys (Inconel W and cobalt-base V-36 alloy) were unaffected by heavy coatings of dry sea salt when exposed at 40,000 psi for 1000 hours at 650°F and 850°F (Inconel W only). AM 350 was also unaffected at 659°F (Reference 4).

Specimens of AM 350 CR and PH15-7Mo (RH 1050) did not exhibit any cracking after expo- sures at 650°F and 800°F and stresses of 40 and 70 ksi for up to 1000 hours, and at 650°F and 100 ksi s t r e s s for 1000 hours (Reference 5).

EXPERIMENTAL PROCEDURE Six materials were chosen for investigation. Two titanium alloys, Ti-6Al-4V and Ti-8A1- 1Mo-1V appeared to b e the most likely titanium alloy candidate materials. PH15-7Mo steel and were selected to represent the whole family of age-hardening meta-stable aus- AM 350 steel tenite stainless steels. Superalloys selected were Rene' 41 and Inconel 718. Analysis of the various alloys as well as their heat treat condition and mechanical properties as determined by standard tensile specimens are shown in table I. It should be noted that Ti-8A1-1V-1Mo used f o r experiment had been subjected to a single solution treatment because at the time of commence- ment of the investigation, the advantages of the duplex annealing process of that alloy had not yet been discovered, Materials selected were in the form of sheet ranging in thickness from 0.020 t o 0.050 inch. From these sheets, strips approximately 12 inches long and 1-1/2 inches wide were cut in 12-inch lengths in the longitudinal rolling direction. The details of the heat treatment used are shown in table 11. All brazing and heat treatment of titanium alloys was carried out in retorts filled with argon in order to minimize contamination, Subsequent to heat treatment, the speci- mens were machined t o dimensions shown in figure 1. Two types of specimens were prepared.

One type was exposed in the plain sheet form, while the other type was notched at a right angle to the principal axis at two points; one slot was close to the support area, while the other slot was near the loading points. All specimens were clamped between mica-strips f o r insulation, at one end in a stainless steel frame and loaded on the other end to produce a cantilever-type specimen. This cantilever-type specimen was chosen in preference to the usual constant- strain U-type stress-corrosion specimen. It was considered possible that on the long exposure per- iods some relaxation in a constant-strain-type specimen could take place and thus change the stress level. Another advantage of the cantilever-type specimen is that a large number of spe- cimens can be accommodated in a limited space. A test involving direct tensile loading would in order to produce the required stress levels.

have required an extensive and expensive setup, A cantilever specimen allows direct comparison of the effects of stress-corrosion and corrosion only, as one end of the specimen is under a condition of maximum stress and the other end is virtually unstressed. The assembly of the specimens in a test frame is indicated in figure 2, and figure 3 shows the loaded frame placed in a furnace. A total of six frames, each accommo- dating 24 specimens, was available. The heating device chosen as a n air-circulation-type fur- nace equipped with dual control. The air circulation furnace is run constantly, except f o r two periods of breakdowns. During those breakdown periods, the specimens remained untouched and loaded, but at room temperature.

Each of the s i x materials tested was exposed with a variety of five different surface condi- tions or surface treatments.

These conditions are summarized in table 111. Prior t o all s u r - face treatments strict attention w a s paid to cleanliness of specimen surfaces. All surface grease and stains had been carefully removed by a degreasing and pickling treatment and no handling of , specimens after this treatment with b a r e hands w a s permitted. The surface treatments were applied as follows: Exposure in as-received condition, except for surface cleaning and pickling treatments 1.

Specimens prepared as above, and subsequently coated by brushing with a suspension 2.

in water of synthetic sea salt comprising six parts sodium chloride and one part magnesium chloride. This suspension was brushed on, and after drying resulted in an even coating of approximately 1/32 inch.

3 . Specimens prepared as (1) above and subsequently coated with a coating 0.001- to 0.003- inch thick of a braze material considered suitable and likely at the time of the com- mencement of test. Braze coatings were selected from t h e following brazing alloys: Titanium Alloys: Dynabraze B (94.8 percent silver, 5 percent aluminum, 0.2 percent manganese) Steel and Superalloys: Premabraze 130 (72 percent gold, 6 percent chromium, 22 percent nickel) Specimens prepared as (3) above but coated subsequently with a salt coating as under 4.

(2).

5 . Specimens prepared as under (4) above but exposed cyclically by maintaining them in the furnace at exposure temperature for a fortnight, then removing the f r a m e into a humidity cabinet for exposure in water saturated air at 100°F for a fortnight, followed by return to the furnace for a fortnight's exposure and so on.

Suspension weights were machined from stainless steel and connected to the specimens by means of stainless steel wire and small insulating bead. In this manner, accidental electric contact between weights frame and specimens w a s minimized. During exposure in the fur- nace the air circulation caused a small oscillatory movement of all specimens. This was not considered significant , as this small movement extending to perhaps 0.050 inch on either of the equilibrium conditions would not produce significant changes in the stress levels.

The f r a m e taken out f o r cyclic treatments at fortnightly intervals was hznrlled as ge~ltly as pss%,!e, but a certain amount of joggling could not be avoided. Furthermore, it was found that after the 2 years exposures, the specimens became coated with a certain amount of dust, particularly brick dust f r o m furnace flues. These vibrations, jolts, and dust coatings therefore do constitute an unknown environmental factor. However, it is not considered that this factor was very sig- nificant. In addition t o the regular thermocouples forming part of the furnace equipment and operating controlling mechanisms, temperature checks were carried out at three separate instances. For the purpose of the temperature checks, six thermocouples were distributed at various points inside the furnace and the temperatures measured by means of a potentiometer.

In all cases, the thermal temperature variation in the furnace w a s found to be plus 0 minus 20 degrees as that indicated by the regular furnace thermometer. A slight temperature drop was indicated near the furnace door; this leak could not be sealed completely. Furnace atmosphere can thus b e assumed t o be fairly static, although a certain admixture of fresh air did take place.

The size of the weight loading the specimens was selected such that two s e r i e s of stress levels was obtained. In the plain specimens the maximum stress level w a s 25 to 30 percent of t h e yield point approximately; in the notched specimens a stress level approaching the yield stress w a s achieved, theoretically at least, at the bottom of the notch. Stress levels a r e sum- marized in table N for each type of material, and for each type of exposure, both for the notched stress levels in individual specimens had to b e calculated and the unnotched specimens. Actually using t h e theory of beams with large deflections. The general method of calculation and equations

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involved is given inthe Appendix. A computer program was developed to allow the calculation for

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each specimen for each lever a r m length and for each specimen thickness. Appendix A also gives a method of calculation of stress levels at position intermediate between the point of load support and the point of specimen support.

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At fracture or at any indication of the specimen obtaining a permanent bend both the speci- men and portion held within the clamp were removed from the furnace and stored for further These specimens together with one specimen for each material and set of surface examination.

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conditions removed after 10,000, 15,000, and 20,000 hours were then cut up for further examination Details of method of sectioning of specimens is shown in figure 4. The pieces of both ends are reserved for microscopic, electron microscopic and X-ray investigations. The section adjacent t o the edge of the support beam and adjacent t o the hold carrying the weight w a s used for notched

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Each specimen provided a total of two notched and two and unnotched tensile specimen testing.

unnotched tensile test specimens. As the original test s t r i p s were all cut in the longitudinal rolling direction, the tensile specimens therefore represent the transverse properties.

I Considerable trouble w a s experienced in the design of a suitable tensile specimen. This w a s due to the fact that the total tensile specimen lengths were limited by the 1-1/2 inch width

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of the exposure strip, while the extensometer with the shortest available gauge length required a 1/2-inch length. One purpose of cutting tensile specimens in the transverse direction as indicated was to assure that the stress distribution across the tensile specimen would be vir- tually constant.

Had the tensile specimens been cut the same direction as the s t r i p lengths, then

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the s t r e s s during exposure would have varied appreciably across the tensile specimen gauge length. A number of specimen design configurations and specimen holding grips were tried and discarded after it was found that specimens either tended to slip or break in the grip. It w a s

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essential t o u s e a pinned-type tensile specimen, because the rough surface, after the exposure, made friction grips quite unreliable. The final specimen configurations are shown in figure 5, and the types of grips are shown in figure 6. A complete set up showing specimen, grips, and extensometer is shown in figure 7 .

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EXPOSURE EFFECTS ON METALLURGICAL STRUCTURE

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Metallurgical Examinat ions

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Cross sections of all specimens were examined just prior to exposure and after 10,000, 15,000, and 20,000 hours exposure or prior failure. During examination particular attention w a s paid to the top surface of the specimens. In the case of specimens which had failed due t o

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stress-corrosion failure, sections were also taken in the plane of the specimen across the cracked zone. After completion of microscopic examination, electron microscopic examination was carried out by means of two-stage replicas. Replicas were prepared from the etched micro- specimen surface in the usual manner. A collodion replica was made of the surface which was 1 then shadowed with carbon. After dissolving away the collodion, the carbon copy w a s placed on the specimen holder of a Hitachi HU-11 electronmicroscope and examined. All microscopic examinations were carried out at a magnification of 500, and electron-microscopic examinations

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were reproduced at magnifications of 2,500 and 15,000 times.

Titanium 6A1-4V Alloy The structure of the alloy prior to exposure is shown in figure 8.

The microstructure shows the typical alpha beta phase distribution. There is very little evidence of surface con- tamination, Some of the grain boundaries near the surface in the photomicrograph are somewhat heavier than in the body of the material. The structure is resolved further in the electronmicro- graphs shown below the photomicrograph. The metallurgical structure after 10,000 and 15,000 hours is shown in figure 9. There is definite evidence on this photomicrograph of the grain size In the 15,000-hour case some coagulation of the light etching phase into and the grain shapes.

larger grains has resulted. This observation is born out by the electron micrographs which quite distinctly show coagulation of the light etching phase after the 15,000-hour exposure. The continuation of this process is indicated in figure 10.

Titanium 8Al- 1Mo- 1A Allov Figure 11 indicates the structure prior to exposure. Comparing this structure to the structure of an uncoated specimen exposed to 15,000 hours, shown in figure 12, it appears that t h e r e is no significant change in the structure, which consists primarily of the alpha phase.

Exposure for 20,000 hours also indicates no structure changes (figure 13).

PH15-7Mo Steel Neither photomicrographs nor electron micrographs indicate any apparent changes in the metallurgical structure. Figure 14 shows the structure prior t o exposure. Figure 15 shows the structures after 10,000 and 15,000 hours of exposure and figure 16 after 20,000 hours exposure. The distribution of austenite and Martensite phases appears t o b e unchanged. There is no indication of any widening of grain boundaries nor is there any indication of the appearance o r disappearance of any precipitates. There is, however, a slight indication of possible change in the Martensite structure on exposure. A high magnification electron micrograph shows a fairly coarse Martensite structure prior to exposure which after exposure is progressively refined.

AM 350 Steel AM 350 Steel exhibited definite change in t h e fine structure. Figure 17 shows unexposed structure which can compare t o figure 18 showing the 15,000 hours structure and figure 19 showing the 20,000-hour exposure structure. A most significant change i s a loss of fine There is no evidence of austenite stringers in any way dis- structure in the Martensite grains.

appearing o r interfering with stress corrosion cracking. Electronmicrograph indicates some absorption of intergrannular precipitates after exposure.

No major changes in the structure of Inconel 718 alloy after exposure a r e indicated, as can a slight indication of the be seen by comparison of figures 20, 21 and 22. There is, however, I absorption of some of the intergrannular precipitates both from the photomicrographs and electronmicrographs and also an indication of a coarsening of the precipitation hardening phase.

There is some indication of precipitate coagulating in the grain boundary areas, particulary as indicated by the electronmicrographs.

Rene' 41 Alloy Microstructurally Rene' 41 shows no gross structural changes after exposure up t o 20,000 hours. The surface structure effect in this alloy, where the grain boundaries tend t o disappear Electronmicrograph near a f r e e surface, is typical of this alloy and found in all specimens.

show an absorption of a precipitate phase after exposure both in 2,500 and 15,000 magnification electronmicrographs. There also appears to be an absorption of grain boundary precipitates on prolonged exposure (figures 23, 24, and 25).

X-rav Diffraction Analvsis Samples of all specimens were examined by X-ray diffraction after 10,000, 15,000, and 20,000 hours exposure. Diffraction patterns were ta.ken from- a flat sample snrface &PI- f removing corrosion layers by grinding and all traces of disturbed metal by electrolytic polishing.

Wherever possible, similar orientations of samples were adopted for analysis in order t o elimi- nate the effects of preferred orientation. The results of the analysis of the unexposed samples and samples after 20,000 hours exposure are summarized in table XI. Changes in relative amounts of peaks may be due to changes in preferred orientation. Generally, from the obser- vations at the various exposure stages, the following comments can b e made: Titanium 6-Al-4V alloy: The amount of beta phase tends t o diminish f r o m a small initial amount of the order of 1 percent during the first 10,000 hours of exposure and then appears to remain constant at about 0 . 3 percent.

Titanium 8-Al-1Mo-1V alloy: X-ray diffraction tests at the completion of the 10,000- and 15,000-hour stages indicated a 50-percent reduction in the amount of beta phase after the first 10,000 hours, with subsequent stabilization. Comparative tests at the 20,000 hours stage show no difference in beta phase content between the exposed and the unexposed sample.

PH15-7Mo steel: Diffraction analysis shows a definite reduction in the amount of retained austenite from an original 19 percent to about 3 percent after 20,000 hours. The reduction appears to be approximately linear.

A M 350 steel: The alloy contains a small amount of approximately 5 to 6 percent of retained austenite, which appears t o be stable throughout the exposure.

Superalloys: Changes in relative peak amplitude detected are probably due t o reorientation phenomena. No phase changes are apparent.

CHANGES IN MECHANICAL PROPERTIES Stress-corrosion phenomena eventually lead to premature failure. They can, and do, how- ever lead to a premature property degradation, which is not apparent by most nondestructive test methods. A major portion of the effort of this program is therefore directed to the determination of such property degradation prior to failure. Mechanical properties as exemplified by the ultimate strength, the yield strength, and the elongation for the case of unnotched tensile speci- mens and by the ultimate tensile strength in the case of notched tensile specimens were deter- mined on all specimens removed from the test after 10,000 hours and after 15,000 hours and on all specimens which fractured prior to the 15,000-hour period, in a manner previously de- scribed. Each exposure specimen thus yielded two tensile specimens; one from the stressed and one from the unstressed portion of exposure specimen. Data a r e therefore given for the un- notched and notched tensile properties for the stressed and unstressed condition. For the pur- pose of analysis, each material will be considered separately.

When considering the tensile data, the fact whether specimens tested are cut, in the direction transverse to the direction of rolling and the curve of the specimen must be born in mind. Apart from the anisotropy due to rolling direction, the transverse rolling test direction employed eliminates

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the effect on strength of any minute edge cracks due to corrosion. Due to the bend curvature of I the exposure specimen such cracks are most likely to be in the directiontransverse to the length Of the exposure specimens, i.e., parallel t o the direction of tensile load in the tensile test speci- mens. A clear distinction can therefore be made between t h e effects of uncontrolled and unmeas- urable corrosion surface deterioration and true material property changes. I t is felt that in

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materials subject t o excessive corrosion, s t r e s s corrosion failures would occur within the test period and thus point up the need for a protective coating.

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In addition to testing specimens cut in the transverse rolling direction, i.e., across the test cantilever, tensile specimens were also cut in the longitudinal rolling direction from the test cantilever. These were also of the miniature type, and the main purpose of this procedure was the determination of possible strength degradation through microcracks not visually observ-

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able. The stress level under which these specimens were exposed was approximately 50 percent of the maximum stress of the un-notched specimens, i.e., the stress levels, varying from about 8 to 19 percent of the yield strength. The results of these tests are included in tables V through X with the other tensile data. When comparing these tensile data with those of the ten- sile specimens cut in the transverse direction, allowances must be made both for the anisotropy of the material and the effects of exposure on mechanical properties.

A comparison has been made between the results obtained from the miniature specimens I employed and standard tensile test specimens. As the data in tables show, there is very little difference in the test results, and the test results obtained on the minature specimen can there-

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fore be considered to be quite representative.

In the c a s e of braze-coated specimens, no special allowance has been made for the lower strength of the thin layers of braze alloy. Braze alloy thickness is of the order of 0.001 inch to

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0.002 inch maximum and all braze-coated specimens have a thickness of 0.040 to 0.050 inch.

B r a z e alloy strength is of the order of 30,000 psi. The total e r r o r introduced by not considering t h e fact that braze coated specimens do in fact represent a composite beam in tension is there-

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f o r e of the order of approximately 2 percent only.

I Titanium 6Al-4V Allov

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Test results a r e given in table V and shown diagramatically in figures 26 and 27. There dues not appear i u be any significant difference h the pruperiies ui tile rrderiii expused in tile s t r e s s e d and the unstressed condition. Strength levels of materials exposed without braze

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coatings do not vary significantly except for the case of 10,000 hours exposure of the stressed specimens, which appear to have a lower strength in t h e as treated surface condition. There is an indication of an increase in the ductility after exposure as shown by the elongation. Braze-

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coated specimens, exposed both with and without a salt coating, have a significantly lower strength and ductility, although there is considerable scatter of data.

The longitudinal specimen tested from unexposed and uncoated material shows a lower

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However, strength levels of all strength level than those cut in the transverse rolling direction.

exposed specimens are very similar in both the longitudinaland the transverse rolling direction for given exposures and surface coatings. This indicates that for the s t r e s s level chosen no stress

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corrosion cracking has occurred.

The notched/unnotched tensile strength ratio appears to be little affected by exposure time and remains above unity for the type of notched specimen used.

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Titanium 8A1-1V-1Mo Alloy

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This alloy exhibits a stress behavior similar to the other titanium alloy above, except that t h e salt coated specimens, too, exhibit a loss in strength on exposure. Ductility losses on -4 t<+-nb.- E A 1 47.1 n l l n r r T h a omhrittlin ex-paiire appezr to be lesa p r o n ~ i y l ~ & C h - - C h n n n ~ i i u i ~ c UA i L a i i L u u i u n A - I u i i v j . I AIL LUAU* A G G I A A A ~ L u a u

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I I effect of braze coating is considerable and appears to increase with time. Both notched and unnotched tensile strength are similar, indicating no significant change in notch toughness. Test data are given in table VI and are summarized in figures 28 and 29.

The mechanical properties of specimens cut in the longitudinal rolling direction are signifi- cantly lower for the braze-coated specimens. A s the material is not notch sensitive, i.e., a corrosion point is not likely to act as a strength reducing crack starter, it can be concluded that underneath the braze coatings a number of cracks exist, which reduce the net strength and can- not be seen. A s this behavior appears to be approximately the same degree in both transverse and longitudinal specimens, rolling direction does not appear to be a factor in the stress corro- sion susceptibility of silver-braze-coated alloy.

PH15-7Mo Steel Relevant data are shown in table VI1 and illustrated in figures 30 and 31. The mechanical property determinations of the alloy indicate quite clearly that the aging process is continuing during the first 20,000 hours exposure. Both stressed and unstressed specimens exhibit similar after behavior. The unnotched tensile properties show that there appears to be an aging peak approximately 10,000 hours exposure. However, the fact that both the cyclically and the con- tinuously exposed specimens show similar properties indicates that the aging peak occurs some- where prior to the 10,000 hour exposure level, because the cyclically exposed specimens actu- ally only spent 5,000 hours at elevated temperature. The aging peak is accompanied by a loss in ductility and a loss in notched tensile strength, as compared with the ultimate strength of the unnotched specimens.

This deterioration in the notched/unnotched tensile ratio is decreased after prolonged exposure t o 15,000 hours, but does not reach the ratio of the unexposed material again. The effect of the aging process on mechanical properties can be considered to be quite significant from a design point of view. There is also a considerable spread of test results, especially in the case of notched tensile data after 10,000 hours, for the different surface treatments.

Specimens cut in the longitudinal rolling direction show a consistently lower strength except for the unexposed specimen. It can be concluded that rolling direction does affect the stress corrosion susceptibility of this alloy to salt and to braze coatings. The slight lowering of the comparative strength of the exposed but uncoated specimens cut in the longitudinal direction indicates that, in addition, grain orientation factor seems to occur as the result of exposure.

The limited data allow no clear distinction between the effects of these two factors.

Ah4 350 Steel The results of the test data are shown in table VI11 and in figures 32 and 33.

Like the PH15-7Mo steel, this material too undergoes an overaging process during the 20,000 hours exposure period. The spread of the test results, however, is wide and it is more difficult to draw definite conclusions at this stage from the available data. It appears that the aging peak occurs somewhere before 10,000 hours in the braze coated specimens, but is in excess of 20,000 hours in the other specimens. Notched tensile data, too, indicate a braze coating effect. The possibility of a component of the braze alloy diffusing into the steel and changing the precipitation reaction can therefore not be excluded. This material did exhibit stress corrosion failures and the test data on the failed specimens are somewhat lower than on the specimens which have not failed, particularly with regard to ductility. Notched/unnotched tensile ratios are around unity in all cases, except f o r the case,of braze coated specimens after 10,000 hours exposure, where they are significantly below unity.

Tensile test data on specimens cut in the longitudinal rolling direction show a.consistently slightly lower strength and slightly higher ductility than data f r o m specimens with similar expo- sure cut in the transverse direction. There is no evidence of any increased stress corrosion susceptibility due to rolling direction.

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Inconel 718

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The test data given in table M and drawn in figures 34 and 35 show that this material undergoes an aging process, without, however, reaching a strength peak. The aging process appears to be stress insensitive. There is some spread of data, particularly at the 10,000

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hour level, with the braze coated specimen persistently showing the lowest results. Thenotched/ unnotched tensile ratio remains around unity f o r all conditions and exposure periods.

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The spread of data does not allow any very f i r m conclusions to be drawn as to rolling direc- tion effects, on either the mechanical properties, stress corrosion, o r exposure response.

However, it appears that specimens cut in the longitudinal rolling direction give lower test strength results and higher ductility than specimens cut in the transverse rolling direction, and

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that there seems t o b e a strength-reducing effect of the braze coating, as mentioned above.

Rene' 41

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Test data are shown in table X and figures 36 and 37. Several observations appear t o be significant. The material undergoes an aging reaction with a peak strength somewhere before I t h e 10,000 hours period. This aging reaction is not connected with a reduction in ductility o r notched strength, both of which appear t o increase on the average. There is no s t r e s s sensi- tivity in any property. The aging reaction appears to be affected considerably by the braze coating. The notched/unnotched tensile ratio is considerably below unity for all c a s e s of surface

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treatment and exposures, being a minimum at the aging peak and approaching unity after pro- longed exposure.

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Specimens cut in the longitudinal rolling direction give, after exposure, consistently lower test strength results and increased ductility as compared with those cut in the transverse rolling direction. However, there is no evidence that rolling direction affects the stress corrosion behavior of the alloy under the exposure test conditions.

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In addition t o mechanical property tests hardness measurements were made on all speci- mens. These a r e summarized in tables XI1 to XVII. It appears from these tables that hardness

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measurements are inadequate to differentiate between the changes in mechanical properties , which occur, as indicated by the tensile tests.

I I CORROSION AND STRESS CORROSION EFFECTS

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General Surface Corrosion Effects Specimens exposed in the furnace atmosphere were examined several times each week for f r a c t u r e s and bending. Specimen frames (except the one carrying specimens undergoing cyclic

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exposure) were removed from the furnace for a thorough examination at five intervals only, after 2800 hours, 4,700 hours, 8,900 hours, 10,000 hours, 15,000 hours and 20,000 hours exposure.

I I I A photograph of typical surface appearances is shown i n Figure 38. The photograph indi- cates the main observations made on the various surfaces: I (1) Titanium alloys without braze coatings form white a r e a s on a predominantly black- ground. These a r e a s a r e either in spots o r stringers.

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(2) Braze coatings on titanium alloys tends to flake off completely after even a few thou- sand hours exposure. The surface beneath the flakes is coarse crystalline i n appearance and very rough.

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Precipitation hardening steels a r e attacked if salt coatings a r e present. Particularly (3) heavy was the attack on AM 350 exposed under cyclic conditions.

(4) Uncoated specimens suffer discoloration only.

Superalloys exhibited the greatest resistance to corrosion under all conditions.

(5) Stress Corrosion Fractures Of the six materials exposed, only three materials have so far shown any evidence of s t r e s s corrosion.

These are the two titanium alloys and AM 350. Table XVIII summarizes the total number of failures found. In titanium 6A1-4V alloy only the braze coated specimens were prone to failure. No failures have been found in salt coated o r cyclically exposed specimens of this alloy. Titanium 8A1-1V-1Mo alloy is likewise prone to s t r e s s corrosion failure after braze coating, with or without salt; and failures of this alloy have also been found in two salt coated specimens. Failure of AM 350 steel after cyclic exposure occurred following significantly closely-related failure periods. Of five specimens exposed, four failed after 3,000 hours f 10%. None of the other materials have so far failed in s t r e s s conditions.

Fractographs were prepared from what appeared to be typical areas on fracture surfaces on specimens which had failed by s t r e s s corrosion.

The surfaces were cleaned by repeated replication until the appearance of the surface remained constant on successive replicas.

Fractures on Titanium 6A1-4V (Figure 39) show an apparently unattacked phase existing in angular particles or areas of cleavage surrounded by a pitted matrix. Similar pitting is revealed also in the fractgraphs of titanium 8A1-1V-1Mo (Figure 40) alloy and AM 350 steel (Figure 41). It cannot be determined from the fractographs whether this pitting occurred during crack growth o r after formation of the crack, but prior to failure. There is no evidence of any structural singularity influencing crack initiation, crack propagation or crack direction.

Stress Corrosion Mechanism In addition to a study of direct s t r e s s corrosion failure data and the changes in mechanical and metallurgical properties on exposure, the present study also aims at an attempt to obtain more information on the s t r e s s corrosion mechanisms involved. To further this aim, both vertical and horizontal sections through cracked specimens were prepared and examined by means of both the conventional microscope and the electron microscope, using replica techniques for the latter. These studies were supported by X-ray diffraction studies of the corrosion pro- ducts, particularly on titanium alloys, and by electron microprobe studies of the a r e a s immedi- ately adjacent to the cracks.

The results of the metallographic investigations a r e shown in Figures 42 through 49.

Very strong evidence of surface corrosion is exhibited in the braze coated specimens.

Figure 42 shows the cross section through two braze coated specimens of titanium 6A1-4V alloy which fa'iled after 7,124 hours and 15,000 hours exposure respectively. A photomicro- graph shows evidence of an interdiffusion zone between the braze coating and complete dis- integration of the grains below that braze coating diffusion interface. Of particular interest is the electron microscope replica of the specimen which failed after 7,124 hours. This rep- lica shows the crack preceding through the alpha-beta grain boundary in most cases, but there are at least two incidences where the crack traverses a grain. The shape of the alpha and beta grains appear to b e somewhat altered, possibly due t o the results of a diffusion reaction.

It should be noted that in this electron microscope replica the border on the top left hand cor- ner is not the specimen surface, but the shadow of the specimen holding grid in the electron microscope. Figure 43 is a microstructure of the specimen which failed after 15,000 hours exposure, sectioned in a plane parallel to the specimen's surface. The fracture is a very typ- ical stress corrosion branch-type of fracture proceeding intergranularly, through the mate- rial. The bottom right hand corner of the specimen indicates cracked zones which are not con- nected t o the surface.

Titanium 8Al-1V-1Mo shows a similar effect to the Titanium 6A1-4V alloy. Figures 44 t o 46 show the tendency of cracks t o move through the heavy intergranular precipitation zone, al- though some examples indicate that short cuts of the cracks through grains are possible. Fig- u r e 42 shows the structure of a salt coated specimen of titanium 8Al-1V-1Mo alloy, which had not failed after 10,000 hours. It is interesting to note that t h e structure of this specimen does not indicate any heavy and continuous grain boundary precipitates.

The third material prone t o stress corrosion failure w a s AM 350 steel. Figure 48 shows the heavy grain boundaries formed and incipient cracks at a section through the surface.

Figure 49, the structure of a failed specimen, however, does not indicate any material struc- tural changes which may be responsible for such failures.

Very interesting data were indicated by the X-ray diffraction examination of the corrosion It had generally been assumed that the culprit in the s t r e s s cor- products on titanium alloys.

rosion attack of salt on such alloys is the formation of chlorine o r titanium chlorides. How- ever, the presence of such chemical products has never been proven. Our X-ray diffraction results show the complete absence of any spectral lines due to the presence of titanium cmo- rides or sodium titanate. which has also been suggested as a by-product of chloride reactions.

One specimen of Ti-8A1-1V-1Mo alloy, exposed for 15,000 hours with a coating of synthetic sea salt, did indicate positive evidence of the existence of NaCl and MgO, and also shows an ad- ditional five lines which have been tentatively identified with NaOH. However, the diffraction pattern w a s diffuse, which was probably due to particle size distribution. The intensity and spacing of the lines obtained on this sample a r e summarized in Table XM. The MgO lines are probably derived from dust contamination originating f r o m the furnace bricks. Another sample of the corrosion products of a specimen exposed to less than 5000 hours w a s boiled in distilled water in an effort t o eliminate the diffuse pattern. In this material a positive identification of anatase (a form of Ti02) was made, and again three weak lines corresponding t o NaOH w e r e found. Now, anatase is t h e titanium oxide which is preferentially precipitated from alkaline solutions in preference t o the more common titanium oxide, rutile.

The thermodynamics of possible chemical reactions of the ingredients present which could result in t h e formation of NaOH was then studied. One example is the reaction: 2 NaCl + T i + 1/2 02 + H20 = Tic12 + 2NaOH The free energy change of that reaction is shown in Figure 50. It must be stressed that the hy- pothesis that NaOH is actually formed relies on a very few preliminary data, and requires fur- t h e r study f o r the positive identification of the surface reaction. The possible presence of NaOH could result in the formation of a number of compounds which a r e liauid at temperatures of 650°F and slightly below. The proof of such a reaction could lead to the establishment of a minimum temperature of s t r e s s corrosion susceptibility, corresponding to the lowest melting point of the reaction products.

Electron MicroDrobe Analvs is In order to gain further insight into the stress corrosion mechanism of titanium, electron microprobe analysis was carried out on sections cut at right angles to the crack in titanium al- loy s t r i p s which had failed by stress corrosion. Figure 51 shows the results of the examination of Ti-8Al-1V-1Mo alloy, both in the unexposed condition and also after exposure under a salt coating to failure which occurred after 4000 hours at 650 O F . In the sample current image, dark areas indicate concentration of elements with high zt,t=miz rimbei;s. h tie backscatter images such areas show up light. It can be seen that exposure resulted in a distinct coarsening of the size of the areas containing elements with high atomic numbers, in this case molybdenum. The central areas in the photos relating to the exposed specimens indicate the crack. There is a dis- tinct concentration of elements with high atomic numbers (again molybdenum) in the area adjoin- ing the crack. Analysis of the composition of the light and dark areas gave the following results: Backscatter Image Dark Areas Light Areas (Percent) (Percent) Molybdenum 0.65 3.25 Vanadium 0.86 1.04 Aluminum 8.52 7.71 These ana rses are estimated to be accurate to about percent of the amount of the element re- port. The segregation of molybdenum in local areas was confirmed by a random traverse taken over a distance of about 100 microns, analyzing the Mo K . 4 line with a lithium fluoride crystal.

In titanium 6A1-4V alloy, somewhat similar element segregation phenomena could be ob- served relating to concentration changes in vanadium and aluminum. Backscatter electron im- age photographs of the alloy are shown in Figure 52. The sample taken for this investigation (No. AD3K) was a specimen which had been both braze coated and salt coated and had failed by s t r e s s corrosion. The electron microprobe measurements were carried out on a transverse section. Corrected analysis results were as follows for the various structural areas: Vanadium Aluminum (Percent) (Percent) Bright Areas in Matrix 5.73 6.41 Dark Areas in Matrix 3.12 8.07 Small Crack Area 3.80 10.36 Bright Area in Vicinity of Small Crack 4.79 8.33 Bright Area in Network Region 2.55 8.57 Dark Area in Network Region 3.39 5.40 Tests were also carried out to detect the possible presence of silver (from the brazing al- loy) and sodium (from the salt coating). Of the latter, the lower limit of detectability is around 2 percent and none but a possible indication near the edge could be found. Silver was detectedqual- itatively near the edge and in various locations within the cracks. There was therefore, within the limits of detectability, no evidence of thediffusionof either of these elements into the titanium alloy.

The possibilityof segregation phenomena within the titanium alloy being responsible f o r stress corrosion susceptibility is further enhanced by the changes in structure which appear to occur in the vicinity of a crack. Figure 53 shows replicas at various magnifications of the fine structure of a hor- izontal section througha cracked specimenof titanium 8Al-1V-1Mo alloy. There is adistinct change in appearance between the structure in the immediate vicinity of the crack and the structure at portions further removed. The latter structure is typicalof the structure of alloy generally (see Figure 13).

CONCLUSIONS AND RECOMMENDATIONS The report presents interim results obtained during the first 20,000 hours of a 30,000 hours exposure test. Most conclusions must therefore b e considered tentative only and subject to correction and amplification after completion of the t e s t .

1. Of the candidate materials test titanium 6A1-4V, titanium 8Al-lV-lM0, PH15-7Mo steel, AM 350 steel, Inconel 71Q, Rend41) only the titanium alloys and AM 350 are

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subject to stress corrosion failure in the presence of salt on exposure at 650°F and

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s t r e s s levels above approximately 25% of the yield strength.

Silver braze coating on titanium alloys cause rapid surface deterioration and there is a 2.

complete loss of adhesion between the braze coating and the parent material. Gold

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base brazing alloy does not appear to affect the corrosion behavior of ferrous and su- peralloys.

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3. All alloys appear t o undergo slight changes in the metallurgical structure, which are reflected in the mechanical property changes and possibly also in the stress corrosion behavior.

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Mechanical properties are changed in precipitation hardening alloys on exposure. The 4.

change is most pronounced in precipitation hardening steels, least pronounced in the superalloys.

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5. The results on examination of surface films and microprobe tests allow the establish- ment of a very tentative hypothesis of factors affecting the stress corrosion mechanism

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of titanium alloys. It appears that t h i s mechanism is related t o the formation of NaOH from salt coatings and segregation phenomena in the alloys. Hypothetically, these compositional changes produce local potential differences, which, under the possible presence of a liquid phase containing NaOH, a r e capable of propagating s t r e s s corro-

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sion cracking.

Future work under this program will consist of determinations of properties and structures

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substantially along the lines outlined here , of specimens exposed for 30 , 000 hours and on the 15 , 000 and 20 , 000 hours which will be available at the duplicate specimens exposed to 10 , 000 I same time. The 30,000 hours exposure period will be reached during the late Autumn of 1965, barring accidents. In addition, more complete investigations will be made by microprobe and

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It is also planned to use electron dif- other methods of the segregation phenomena found so far.

fraction to c a r r y out a more thorough analysis of the corrosion products; the shorter wave This work will length of electrons as compared to X-rays may yield additional information.

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involve the examination of standards for comparison.

it 1s recommended that this work be amplified by the following studies:

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1. Investigate further the hypothetical s t r e s s corrosion mechanism for titanium alloys sug- gested here. Such an investigation would lead not only to a better understanding of the mechanism, but also t o the establishment of guidelines for the development of alloys of

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improved stress corrosion res ist ance.

2. Investigate aging effects of various heat treatments on P H steels and superalloys to re-

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duce the effects of elevated temperature exposure on mechanical properties.

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I I REFERENCES 1. Titanium Metallurgical Laboratory: Progress Report on the Salt Corrosion of Titanium Alloys at Elevated Temperature and Stress. TML Report No. 88, Battelle Memorial Institute, November 20, 1957.

2. Titanium Metallurgical Laboratory: Memorandum on Notes on the Stress Corrosion of Titanium. Battelle Memorial Institute, July 27, 1956.

H. Brown: Stress Corrosion Cracking of Ti-5A1-2.5 Sn. DMIC Memorandum 60, 3.

August 4, 1960.

4. R. L. Kirchner and E. J. Ripling: Elevated Temperature Stress Corrosion of High Materials Research Strength Sheet Materials in the Presence of Stress Concentrators.

Laboratory, Inc., Quarterly Progress Reports Nos. 1, 2, 4 and 9 for NASA Contract NASr-50, November 1961, March 1 t o May 31, 1962 and June 1 to August 31, 1963.

F. A . Crossley: Research on the Basic Nature of Stress Corrosion for Various struc- 5 .

USAF Technical Documentary Re- tural Alloys at Room and Elevated Temperatures.

port No. ASD-TR-61-713, Part 11, February 1963.

6. W. V. Boyd and F. W. Fink I ' The Phenomenon of hot salt stress corrosion cracking of Titanium Alloys". NASA CR-117 Oct. 1964.

7. K. E. Bisshopp and D. C. Drucker: Large Deflections of Cantilever Beams. Quarter- l y of applied mathematics, Vol. 3, No. 3, 1945.

8. R. E. Peterson: Stress Concentration Design Factors, John Wiley & Son, 1959.

APPENDIX STRESSES AND DEFLECTIONS IN CANTILEVER BEAMS Introduction Due t o the small bending stiffness of many of the cantilever beams relative t o the loading, it was necessary to resort to a large deflection theory for this analysis. The material prop- erties at 650°F listed on Table XX were used for this analysis.

Large Deflection Theorv Figure 54 shows the relation between moment a r m and maximum deflection versus a load stiffness index (reference 7 ) . The maximum decrease in lever a r m for specimens was found t o be about 20%, i.e. from Figure 54 L* - A was found to be about .800.

L* Effective Length of Beams I Figure 54 shows a typical loaded beam, where P is the applied load, L is the length of the beam from load point to angle support, and Lo is the distance inside the angle to the fixed point of the beam. Since temperature the total length of the cantilever beam can be expressed as L* = ( L o + L) (1 + a A T ) o r Lo = Equation (1) + a h T ) - L L* where: L* = total length of beam (in.)

a = coefficient of thermal expansion at 650" (in/in°F) AT = (650°F - 70°F) F o r five test beams the 6 deflection readings at the load point were recorded.

F o r each beam, the following procedure w a s then used t o obtain the value of Lo.

A value of L* was assumed and the corresponding value of P r *2/B was calculated.

where: B = E1 bending stiffness (# in2)

I = bt3 moment of inertia (in

beam thickness (in) t = b = beam width (in) Poisson's ratio Y = Using this value of PE*2/B and Figure 45, the corresponding value of 6/L* was found.

This value w a s compared to ( 6 measured)/L*. If the two values were different a new value of L * w a s assumed and the process w a s repeated until 6/L* from Figure 54 equaled ( 6 meas- ured)/L*. Then, by the use of Equation (l), Lo w a s calculated. TableXXI gives the values of LO for the beams on which the deflections were measured. Based on Table XXI, it was decided to make Lo equal to . 3 for all test beams. Equation (1) then becomes: L* = ( . 3 + L) (1 + 5 8 0 a ) Equation (2) Unnotched Beams Using these corrections the maximum deflections and stresses for the unnotched beams here calculated. These calculations are based upon the total thickness of beams and do not take into account the material properties effects of those beams coated with braze alloy. I f the actual s t r e s s in the braze alloy and parent material are wanted, the following procedure can be used. From the tables, the nominal maximum s t r e s s of the specimen can be determined by the following equations: 1 + e]; c: ,r\\ L -I u max = omax braze alloy nominal E1 t t E2 u max = umax Equation (4) x - x - parent material braze alloy t+d El where: = Young's modulus of braze alloys El = Young's modulus of parent material E2 t = total thickness of parent material d = total thickness of braze alloy Notched SDecimens ~ A similar set of calculations w a s carried out to determine the maximum deflection at the load point and the maximum stress at the interior notch for the notched beams. The correspond- ing s t r e s s concentration factor Kt is also given (Reference 8). These calculations are also based on the nominal thickness of the specimens and exact values of s t r e s s e s can again be obtained with the use of Equations (3) and (4).

Determination of Stresses in Un-Notched Beams at Intermediate Locations The stresses at any point intermediate between the support and the point of load application in an un-notched beam can be determined from the following procedure:

I

Obtain the maximum bending stress and load-stiffness index (PL*/B) from the appro- a.

priate table.

From Figure 5 5 , read the factor ( L - A - X ) / ( L - A ) for the appropriate beam

b.

position and load stiffness index. Intermediate values of S/L must be interpolated from the curves given.

c. The desired stress is the beam's maximum stress multiplied by the factor obtained from Figure 55.

I b( Q,

*

I Table I1 SUMMARY OF HEAT TREATMENTS Braze at 1725°F Titanium 6A1-4V A i r cool Unbrazed specimen: Age at 1000°F for 4 hours Braze at 1725°F Titanium 8A1-1V- 1Mo A i r cool Unbr az ed s pe cimen : No heat treatment Braze at 1900°F (brazed specimen only) PH15- ~ M o Cool t o room temperature Heat to 1730°F Cool to -100 "F, hold 4 hours Age at 1075°F for 1 hour A i r cool to room temperature Braze at 1900°F (brazed specimen only) AM 350 Cool to room temperature Heat to 1710°F Cool to -100 "F Age at 850°F for 3 hoiirs A i r cool to room temperature Braze at 1950°F (brazed specimen only) R e n k 4 l A i r cool to room temperature Age at 1400°F for 6 hours A i r cool to room temperature Braze at 1900°F (brazed specimens only) Inconel 718 A i r cool to room temperature Stress relieve at 1600°F for 4 hours Air cool to room temperature Age at 1325°F for 16 hours A i r cool to room temperature Table 111 SUMMARY O F SURFACE AND EXPOSURE TREATMENTS Exposure Coating Tern pe rat ur e Treatment A 650°F None B 650°F Synthetic sea salt 650 "F Braze coating C Titanium alloys: Dynabraze B

(94.8% Ag , 5% A l , 0.2% Mn)

Other alloys: Premabraze 128 (72% Au, 6% C r , 22% Ni) 650 "F Braze coating as above plus D synthetic sea salt Synthetic sea salt E Alternating every 14 days 650°F and humidity cabinet at 100°F

I

-

Table IV

I

MAXIMUM STRESS LJ3VELS IN TEST SPECIMENS

I

Notched Ti- 6A1- Ti-8Al- PH15- Inconel / Specimens 1V-1MO 4v 7MO AM 350 718 Rene 41

!

Maximum 44.6/ 56.2/ 140.6/ 129.3/ 92.4/ 126.8/ specimen 58.0 66.6 168.4 160.3 105.5 147.2

I

stress (ks i)

I

% Yield 34.0/ 41.5/ 72/8 6 77.3/ 65.5/ 85.6/ strength 38.4 46.6 96 74.8 100 (stand a r d

I

treatment)

I

% Yield 35.8/ 42.2/ strength 38 43.7 (brazed

I treatment)

I Unnot ched Specimens I Maximum 23.0/ 22.5/ 53.7/ 50.0/ 34.0/ 48.8/ s pe c im en 36.8 31.4 61.3 59.1 40.8 57.4 t c t v n ” m O C A G3J (ks i) t % Yield 19/22 22/ 27.5/ 30/ 24.1/ 33/ strength 26.5 31.4 35.4 28.9 38.9 (standard treatment) % Yield 23/ 1 7/ s t r en@h 26.5 24.1 (brazed treatment) NOTE Variations between specimen stress levels are due to differences in the free length of the cantilever a r m .

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-

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E % E%

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O , 0- O , 0-9 O , 9 O , O, O , 0 , 0 , o m 0 oroo o m 0 o m 0 . - ( I N --IN d M N - 4 N 4 0 l m ?IN- U U U m m m & F k h h h c .d m W m m W k Y m H ” I Table XI SUMMARY OF X-RAY DIFFRACTION TESTS Exposure Comments Hours Specimen Material Titanium 0 A Exposure increases the (002) peaks 6A1-4V 20,000 AB3K and decreases the (110) peaks.

There is no significant peak shift.

The amount of fi phase decreases

from an original amount of the order of 1% to about 0.3%.

Titanium 0 B The (010) and (011) peaks decrease 8A1- 1V-1MO 20,000 BE3K and the (002) peak increases on exposure. No significant peak shift or changes in the amount of phase present were observed.

PH15- 7MO 0 C The amount of retained austenite steel 20,000 CA3 decreases from 19.2% to 3% on exposure.

AM 350 D The amount of retained austenite 20,000 steel DB3 increases from 5.7 to 5.9% on expo s u r e.

Inconel 718 0 E Exposure increases the (311) peaks 20,000 EB3 and decreases t h e (111) peaks, but is no change in peak shift.

there Rene 41 0 F Exposure increases t h e (111) peaks 20,000 FC3 and decreases (ZOO), (220) (311) , and (222) peaks without causing significant peak shift.

Table XII SUMMARY OF HARDNESS MEASUREMENTS TAKEN ON Ti-6A1-4V ALLOY THROUGH 20,000 HOURS EXPOSURE Environment Specimen Exposure Ra Hardness Coating Identification Time - Hours Measurements Standard None 69.5, 69.5, 69.5 - A A 1 10,000 70.0, 70.0, 70.0 None A A 2 15,000 70.0, 70.0, 70. 5 AA3 20,000 69.5, 70.0, 70.5 AB1 10,000 71.0, 70. 5, 71.0 Salt AB2 15,000 70.0, 70.0, 70.5 A B3 20,000 70.0, 70.0, 71.0 AC2 9,740 68.5, 68.0, 68.5 Braze AC3 7,124 68.0, 68. 5, 67.0 AC5 9,572 68.0, 68.0, 68. 5 AC6 5,516 5?.0, 69.0, 7o.c AD2 15,000 70.0, 70.0, 69. 5 Braze and AD3 7,124 68.0, 68.0, 69.0 Salt AD5 15,000 70.0, 69.0, 69. 5 AE1 10,000 70. 5, 70.0, 70.0 Salt AE2 15,000 69.5, 70.0, 70.0 Cyclic AE3 20,000 69.5, 70.5, 70.0 Table XI11 SUMMARY O F R, HARDNESS MEASUREMENTS TAKEN ON Ti-8A1-1V-1MO ALLOY THROUGH 20,000 HOURS EXPOSURE E nv iron m e n t Specimen Exposure Ra Hardness Coating Identification Time - Hours Measurements Standard 023 NOM Gage 63. 5, 63. 5, 63. 5 None 050 NOM Gage None 67. 5, 68.0, 68.0 BA 1 10,000 64.0, 64.0, 64. 0 None BA 2 15,000 64.0, 64. 5, 64. 5 BA3 20,000 64.0, 65. 0, 65. 5 BB1 39 80 65. 5, 66.0, 66.

Salt BB2 2640 65.0, 65. 5, 65. 5 BBx2 9928 64.0, 62.0, 62.0 BC 1 10,796 62, 5, 62. 5, 63. 0 BC 2 9,308 61.0, 61.0, 64. 5 Braze 63.0, 63.0, 62.0 BC3 12,572 BC 5 18,480 68.0, 68.0, 69.0 8,876 68.0, 68. 5, 69. 5 BC6 BD1 18,480 65. 5, 66. 5, 64. 5 Braze a n d BD2 15,000 63. 5, 66. 5, 64. 5 Salt BD3 9,644 62. 5, 59. 5, 57. 5 BE 1 10,000 66.0, 65.0, 65.0 Salt BE 2 15,000 65.0, 65.0, 64. 5 Cyclic BE3 20,000 65.0, 65. 0, 65.0 Table XIV SUMMARY OF Ra HARDNESS MEASUREMENTS TAKEN ON PH15-7Mo STAINLESS STEEL THROUGH 20,000 HOURS EXPOSURE Environment Specimen Exposure Ra Hardn-bs

Coating Identification Time - Hours Measurements

Standard - None 72.5, 72.0, 72.5

None CA1 10,000 75.0, 76.0, 75.5 CA2 15,000 75.5, 75.5, 76.0 CA3 20,000 75.0, 76.0, 76.0 Salt CB 1 10,000 75.0, 75.0, 75.0 CB2 15,000 76.0, 76.0, 75.0 CB3 20,000 75.5, 75.0, 75.0

Braze cc 1 10,000 76.0, 76.0, 76.0

c c 2 15,000 75.0, 75.5, 75.0 c c 3 20,000 76.0, 76.0, 76.0 Eraze ana CD 1 10,000 76.0, 75.0, 76.0 Salt CD2 15,000 74.0, 75.0, 75.5 c D 3 20,000 76.0, 77.0, 76.5 Salt CE 1 10,000 75.0, 74.5, 74.0 Cyclic CE2 15,000 75.5, 76.5, 77.0 CE3 20,000 75.0, 75.0, 75.0 t Table XV SUMMARY O F R a HARDNESS MEASUREMENTS T A m N ON AM 350 STEEL THROUGH 20,000 HOURS EXPOSURE Specimen Exposure Ra Hardness Environment Measurements

Coating Identification Time - Hours

73.5, 73.5, 73.5 Standard None None DA1 10,000 74.5, 74.5, 75.0 75.0, 74.5, 74.0 DA2 15,000 74.5, 74.5, 74.5 Salt DB 1 10,000 Dl32 15,000 74.5, 75.5, 75.0 DB3 20,000 75.0, 75.0, 75.0 Braze DC1 10,000 75.0, 76.0, 75.0 DC2 15,000 74.5, 74.0, 74.0 75.0, 75.0, 75.0 DC3 20,000 75.0, 75.0, 75.0 Braze and DD 1 10,000 Salt DD2 15,000 74.5, 74.0, 75.0 DD3 20,000 74.0, 74.0, 74.5 74.0, 74.5, 74.0 Salt DE 1 3,360 74.5, 74.5, 74.5 Cyclic DE2 3,290 3,290 74.0, 74.5, 74.0 DE3 DE4 2,880 74.0, 74.5, 74.5 Table XVI SUMMARY OF HARDNESS MEASUREMENTS TAKEN ON INCONEL 718 THROUGH 20,000 HOURS EXPOSURE

i

Environment Specimen Exposure Ra Hardness Coating

Identification Time - Hours

Measurements Standard None 69.0, 68.5, 68.5 None EA1 10,000 72.0, 73.0, 72.0 EA2 15,000 72.0, 72.0, 73.0 EA3 20,000 71.5, 72.0, 72.0 Salt EB 1 10,000 72.0, 72.0, 72.0 EB2 15,000 71.0, 71.5, 72.0 EB3 20 y 000 72.0, 72.0, 72.0 Braze EC1 10,000 71.0, 72.5, 72.0 EC2 15,000 71.0, 71.5, 72.0 EC3 20 y 000 71.5, 72.0, 72.0 Braze and ED 1 10,000 72.0, 72.0, 72.0 Salt ED2 15,000 71.0, 72.0, 72.0 ED3 20,000 71.0, 72.0, 72.0 Salt EE 1 10,000 71.0, 71.5, 72.0 Cyclic EE2 15,000 72.0, 72.0, 72.0 EE3 20,000 71.0, 72.0, 72.0

’ !

Table XVII SUMMARY O F Ra HARDNESS MEASUREMENTS TAKl3N ON RENE’41 THROUGH 20,000 HOURS EXPOSURE Ra Hardness Specimen Exposure Environment Measurements

Identification Time - Hours

Coating 72.5, 71.5, 72.5

Standard - None

73.0, 72.0, 73.0 FA1 10,000 None 73.0, 73.0, 73.0 FA2 15,000 72.0, 73.0, 73.0 FA3 20,000 73.0, 72.5, 73.0 10,000 Salt FB 1 72.5, 73.0, 73.0 FB2 15,000 72.0, 73.0, 73.0

, 000

FB3 20 73.5, 73.5, 73.0 10,000 FC 1 Braze 73.0, 73.0, 73.0 FC2 15,000 72.5, 73.0, 73.0 20,000 FC3 73.0, 73.0, 73.0 FD 1 10,000 Braze and FD 2 15,000 73.5, 73.0, 73.0 Salt 72.5, 72.0, 72.0 FD3 20, 000 73.0, 73.0, 73.0 10,000 Sa It F E 1 72.5, 72.5, 73.5 FE 2 15,000 73.0, 73.0, 73.0 20,000 FE3 Table XVIII SUMMARY OF STRESS CORROSION FRACTURES Notched (N)) o r Failure Unnotched (U) Time H r Specimen No.

Material Treatment N 5,516 Titani urn Braze AC6 U 7,124 6A1-4V 650°F AC3 N 9,572 AC 5 N 9,740 AC2 U 7,124 Braze and Salt AD3 N 15,439 650°F AD5 AD2 N 15,480 Titanium Salt BB8 U 2,592 8Al-lV-lM0 650°F BB2 U 2,640 BB1 U 3,980

BBX2( **) 9,928

U Braze N 8,876 Bc6 650°F U 9,308 B c 2 BC1 U 10,796 13,652 B c 3 U B c 5 N 18, 560 Braze and Salt BD3 9,644 U 650°F BD1 18,560 U Salt BD6 N 22,612 6 50 " F+100" F(*) BE4 U 21,150 AM 350 Salt DB5 N 10,790 650°F DB6 N 15,463 salt 2,880 DE4 U 6 50" F+100" F(*) DE2 U 3,290 DE3 U 3,290 U 3,360 DE1 DE8 U 1,152 (*) Alternating Every 14 Days (**) Sea Salt Coating Table XIX ANALYSIS O F SALT COATING FROM Ti-8A1-1V-1Mo ALLOY, EXPOSED TO 650°F FOR 15,000 HOURS Line Intensity Spacing NaCl NaOH MgO 1 W 3.255 3.258 2. 82 2. 85 2 vvs 2. 805 - 2.106 3 vvw 2.096 4 v s 1.993 1.994 2.03 - 1. 70 5 vvw 1.693 6 S 1.626 1.628 7 f 1.48 8 W 1.409 1.410 9 f 1. 29 - 1. 27 10 f 1. 27 11 m 1.260 1.261 1 2 diff, w 1.151 1.1515 13 W 0.997 0.9969 14 W 0.953 0.9533 15 dblt 0.940 0.9401 dblt 0. 891 0. 8917 17 0.8503 dblt 0.849 - 18 dblt 0.783 dblt 0.781 Table XX MATERIAL PROPERTIES

I

Temperature = 650°F Coeff of Ratio Modulus of Modulus of Poisscn Thermal E

I

Material Elasticity Rigidity Ratio Expansion E G LJ a

Ti-6A1-4V 14.1 x lo6

5.4 x 106** 0.30* 5. x 1.2912 x lo6

i F T U = 160

AM 350 25.6 x lo6 9.9 x l o 6 0.318 6.9 x 10-6 2.373 X lo6

* Estimated

E

** Based on G =

2(1 + Y )

Table XXI CALCULATIONS OF EFFECTIVE BEAM LENGTHS (measured) L* PL*2/B 1 LO Beam (Inches) (Inches) (Inches) (Inches) (Inches) CA2 3.375 9.74 1.1887 9.7 0.00538 DA3 3.625 10.13 1.2334 9. 8 0.28962 F A 1 3.000 10.16 0.957 9.73 0.38862 BA2 4. 500 9.31 1.9329 9.07 0.21093 A A 2 3.152 10.05 1.0302 9.76 0.26093 LOAV = 0.28752 Thickness ‘t3 Titanium 6A1-1V 0.050 ” Titanium 8Al-1V-1Mo 0.048 ”and 0.020 y y PH15-7Mo Steel 0,039 y y AM 350 M Steel 0,040 ” Inconel 718 0.042 ” Rene 41 0.050 y’

Depth of notch 0.010 y y f 0.001 y ’ ; 5

1.6 to 1.8 4L

T h

h cv ‘=;.

A f

n -.-I- cv PORTION 1” CLAMPED I N FRAME

.L

-I-.-

.l - 112’94

+le p -1/29’,

t l e

Unnotched Specimens Notched Specimens Figure 1. Dimensions of Exposure Specimens . - e .

I Figure 2. Exposure Specimens in Test Frame

I

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e i

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II

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!

Figure 3. Exposure Test Frames in Furnace 43 Each specimen is identified by a four-symbol code, as follows: A Titanium 6A1-4V alloy First symbol: Specimen Material B Titanium 8A1-1V-1Mo alloy C PH15-7MO Steel D AM 350 M Steel E Inconel 718 F Rene 41 Second symbol: Surface Treatment A None B Salt Coating C Brazecoating D Salt plus Braze Coating E Cyclic Exposure plus Salt Coating Third symbol: Specimen Serial Number for each material and surface treatment Fourth symbol: Specimen Position Code Letter as illustrated in the diagram below L> f'? I-' I 1 I 1

I ( . I

! : I . L - J

, ' :& t .

e-.--

+

/POSl!I'ION CODE LETTERS UNNOTCHED TENSILE (LONGITUDINAL ) Figure 4. System of Marking and Cutting-up of Exposure Specimens and other Test Specimens 1. Holes to be on centerline of notch root width f . 002.

NOTES: 2. Notch radius 0.002 maximum. $ = 6.0.

3. Notches to be made with U h t finishing cuts or light grinding and must have contour shown.

4. Tool chatter or other tool marks w i l l be cause for reject. DO N O T BUFF.

5. Machine surface o f notch.

6. Nutch and reduced section to be symmetrical about centerline f . 0015.

Figure 5. Notched and Unnotched Miniature Tensile Specimens rn G a, G a E

u

I bo .r( P= m a, E rn

%

E

c

.r( a, a m m a, E a , k s (d .r(

c

CD a , ba

iz

ss W e it h Extenso meter 4?

* ,. , Two stage replica x 2500 TWO stage replica x 1.s,Ooo Figure 8. Microstructure of Titanium 6A1-4V Alloy Prior to Exposure to T e s t Environment Etched x 500 Tvm Stage Replica x 2500 TFlo stage Replica x 15,000 Figure 10. Microstructure of Titanium 6A1-4V Alloy After 20,000 Hours Exposure r Etched x 500 ' / L L M

- 9 C n n

Two stage repiica A L j W V TWO stage replica x V,OW i Microstructure of Titanium 8A1-1Mo-1V Alloy Prior to Fig-xe 11.

E l Exposure to Test Environment r l l

i

15,000 HOUR EXPOSURE I T 0 FAILURE Etched x 500

w o stage r e p l i c a x 15,000

Figure 12. Microstructure of Titanium 8A1-1Mo-1V Alloy A f t e r Exposure in Circulating A i r at 650°F (Specimen BA2) Stage Replica x 2500 Two Stage Replica x 15,000 Figure 13. Microstructure of Titanium 8Al-1V-1Mo Alloy After 20,000 Hours Exposure (BA3) Two stage replica x 2500 TWO stage replica x I5,OOo Figure 14. Microstructure of PH1.5-7Mo Steel Prior to Exposure to Test Environment Ln d a a

?z

E

5!

f- I Lo d .

In 'i K K Lt x Lo I d a k

d

hn PI .r( Frc

E

Two Stage Replica x 2500 1 % Stage Replica x l5,OOO Figure 16. Microstructure of PH15-7Mo Steel A f t e r 20,000 Hours Exposure I (CA3) Two stage replica x 2500 s” * cp” ;P p’wo stage replica x 15,000 Figure 17. Microstructure of AM 350 Steel Prior to Exposure to Test Environment c \ I . o i I I Cn k m V K

t

a

E

u I4 Etched x 500 !ih Stage Replica x 2500 'Pa Stage Zenlica x 15,000 Figure 19. Microstructure of AM 350 Steel After 20,000 Hours Exposure (DB 3) Etched x so0 e - Two stage replica x 2500 TWO stage replica x l5,OOo Figure 20. Microstructure of Inconel 718 Alloy Prior to Exposure to Test Environment m cu x cd 5% a k a to rj r n

E

m

E

Etched x w o o

Two Stage Replica x 2500 Figure 22. Microstructure of hconel 718 A f t e r 20,000 Hours Exposure (EA3K) Two stage replica x 2500 Two stage replica x s , o O O Figure 23. Microstructure of Rene' 41 Alloy Prior to Exposure to Test Environment a- x

*

k a ,

f

-Q w f x

E

E

k m cu

Fi

x x x cd cd

a

k"

" J * .> a M (d d . Q d u3 .

T:m Stage Re7lica x 2500 ? t ~ S t q e ileplica x 15 ,OOO Figure 25. Microstructure of Rene’ 41 A f t e r 20,000 Hours Exposure (FA 3) Unnotched Tensile Test Notched Tensile Test Specimens

r Specimens p K t = 6

h ''Or 1 8 0 w k 1 5 4

A

P

A

a , d

a

-

v

140 '""i

-

-

120 120 l 3 O I 1 I L I I I 1 1 o I ! J 0 5 10 1 5 20 25 30 Time in Test (1000 hours) rl

A

rn a , Specimen Condition

A

k

v v

Exposure i n Removal From ff 120

A

Specimen E m o s u r e Test

a, v

a , Environment Unbroken Broken

v

g 110 "1

None

M

1 0 0 - -

0 5 10 15 20 25 30 Salt

m-a

rn

Time in Test (1000 hours) Braze 9---0 v .

9 G l Braze & Salt

&--+

A

Cyclic Salt D---Q b 1 1 0 5 10 15 20 25 30 Time in Test (1000 hours) Figure 26. Titanium 6A1-4V Alloy Unstressed Exposure Tensile Test Results Notched Tensile Test Unnotched Tensile Test

I

= 6

190 r Specimens

n .A

g, 180

I '

I-

0 I

8 170 -

r ( W '"9 150 ; :

I$ c, Q) 1 3 0 1 tV

cd E

I

'"1 120

.- c, 120

9 - 4 I I I I I I

I 110

0 ' 5 10 ' 1 5 20 25 30

0 5 10 1 5 20 25 30 Time in Test (1000 hours) Time in Test (1000 hours)

I

I

I I

I

Environment unbroken Bi=ukeii a.j 1 1 0 1 None

I

M 0 1 0 0 1 Salt

O--a m 0 5 10 15 20 25 30

I

Time in Test (1000 hours)

W--Q v

Braze

I Braze &Salt &---A A

D-----O m

Cyclic Salt

I

$ 2 0 1 Q l

pc

I

0 5 10 15 20 25 30 d Time in Test (1000 hours)

I

Figure 27. Titanium 6A1-4V Alloy Stressed Exposure Tensile Test Results

I

I

Unnotched Tensile Test Notched Tensile Test

Specimens Kt = 6

190r Specimens

I

n . I 4 180 180

8 170 170}

I

W 160 I) l60L Q, k

A

L)

I

:::I 120 v

I

F, 110 2 110 ( 1 1 1 1 1 1

0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time in Test (1000 hours) Time in Test (1000 hours) n

'z 150

a

I

Specimen Condition k Exposure in Removal F r o m f ; 120, Specimen Exposurc

A

a l Environment Unbroken

v

g 110-

+

None

M

1 0 0 - Salt 0 5 10 15 20 25 30 O--a Time in Test (1000 hours)

!z5

B r a z e Ip-0 Braze & Sal1 &-- 4

Cyclic Salt o-----o

d 0 5 10 15 20 25 30 Time in Test (1000 hours) Figure 28. Titanium 8Al-lMo-lV Alloy Unstressed Exposure Tensile Test Results

I

Unnotched Tensile Test Notched Tensile Test 190r Specimens 190 Specimens $ = 6 n .A

g, 180

1 8 0 1

8 170

W l ' O t a, 1 6 0 1 k fs 15 a , s-4 -r( rn F : 14 a , E - Q) 1 3 0 - A .c,

v

1 2 0 - s-4

3 v

I I I 1 I 110, 1 1 0 7 0 5 10 1 5 20 25 30 Time in Test (1000 hours) n 'B 150 a

i i

Specimen Condition

I

None

I

M

a

1 0 0 1 Salt 0 5 10 1 5 20 25 30

0--43

m

I Time i n Test (1000 hours)

v---*o

Braze

v

Brazf: &Salt &---A

I

A

Cyclic Salt O-----O

I

3 2

pc 0 5 10 1 5 20 25 30 d

I

Time in Test (1000 hours)

I Figure 29. Titanium 8A1-1Mo-1V Alloy Stressed Exposure Tensile

Test Results

I

I

Unnotched Tensile Test Notched Tensile Test 2501 Specimens = 6 k i 3 180 d :::I . . .

. , 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time in Test (1000 hours) Time in Test (1000 hours) Specimen Condition Exposure on Removal From Specimen Exposure 'est hvironment Unbroken Broke] None

M 0

Salt

&-a

Time in Test (1000 hours)

Braze P--O

v

Braze & Salt &--A

A

2 0 1 301

Cyclic Salt D----O

-

0 0 5 10 15 20 25 30 Time in Test (1000 hours) Figure 30. PH15-7Mo Steel Unstressed Exposure Tensile Test Results Notched Tensile Test Unnotched Tensile Test A

'""t

. rn . 1

"I 170

1701 . .

0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time in Test (1000 hours) Time in Test (1000 hours) Specimen Condition

Exposure I on Removal From

Salt

P--Q 8

G G Time in Test (1000 hours) 2 GI

Braze w--o v

Braze & Salt

&--A A

Cyclic Salt B----O

d 0 5 10 15 20 25 30

Time in Test (1000 hours) Figure 31. PH15-7Mo Steel Stressed Exposure Tensile Test Results Unnotched Tensile Test Notched Tensile Test 250r Specimens

250 Specimens Kt = 6

.r(

240 240 1

- t 230

2 190

E d 3 180

DD 170

0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time in T e s t (1000 hours) Time in T e s t (1000 hours) Exposure Specimen Unbroken ' Broker Environment None

M a

150 0 5 10 15 20 25 30 Salt

m--O

T i m e in T e s t (1000 hours) B r a z e o---V

v

Braze & Sal1

&--a A

Cyclic Salt D---O - Time in T e s t (1000 hours) Figure 32. AM 350 Steel Unstressed Exposure Tensile T e s t Results Unnotched Tensile Test Notched Tensile Test

250 -

250r Specimens Specimens = 6 .r( 240 240

- t 0

230.

220.

{ 190

4 3 E 180 I

=3 170

0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time in Test (1000 hours) Time in Test (1000 hours)

I Specimen Condition

I Exposure on Removal From

I I n----..-- mAn+ I Specimen lbApUDUt; A C U L Environment Unbroken Broke?

None

o----o 0

I,.,...

5 10 15 20 25 30 Salt 0---0 Time in Test (1000 hours) Braze

F---0 v

Braze &Salt &-A 30 I

A

Cyclic Salt

e---a

' o ~ : ~ . , 0 5 10 15 20 25 30 Time in Test (1000 hours) Figure 33. AM 350 Steel Stressed Exposure Tensile Test Results Unnotched Tensile Test Notched Tensile T e s t rl W

I

$ 160 16 170 0 t

a , k ff 150 a , d .r( I40 Q, €+ Q, 130 130 *- 4 120 110 110 0 5 10 15 20 25 30 0 5 10 1 5 20 25 30 Time in T e s t (1000 hours) Time in Test (1000 hours) n a lpecimen Condition Exposure n Removal From

g 1201

Specimen Exposure 'est a, a , h v i r onment Unbroken Broke] None

M 0

t

I 1 I 1 100 1 Salt 0 5 10 15 20 25 30 O--a Time in Test (1000 hours)

o--9

B r a z e v

&---a

Braze & Salt

A

P----o

Cyclic Salt

-

0 "

k : 0

pc 0 5 10 15 20 25 30 d Time in Test (1000 hours) Figure 34. Inconel 718 Alloy Unstressed Exposure Tensile Test Results Unnotched Tensile Test Notched Tensile Test F 4

- I

160t 1 1 0 1 0 5 10 15 20 25 30 Time in Test (1000 hours) Time in Test (1000 hours)

160 r

n ~ ~~

I Specimen Condition

Exposure on Removal From

Specimen I Exposure rest

- Environment I Unbroken Broker - None

o----o a

100 ( 1 1 1 1 1 1 0 5 10 15 20 25 30 Salt

D--Q m

Time in Test (1000 hours) c-c --e, Braze

P---V v

Braze &Salt

A

&-a

Cyclic Salt

~ - - - o

d 0 5 10 15 20 25 30 Time in Test (1000 hours) Figure 35. Inconel 718 Alloy Stressed Exposure Tensile Test Results ~

I

Unnotched Tensile Test Notched Tensile Test Specimens Specimens Kt = 6

I

n 240r

240r

.A

I

VI a r( W VI rn

210 I

a , k

ff

a , d .A rn

2ool 190

a , 180c c , d Ei .A

s

1 1 a 1 1 1 1 6 0 1 0 5 10 1 5 20 25 30 0 5 10 15 20 25 30 T i m e in Test (1000 hours) Time in T e s t (1000 hours) 170 r Specimen Condition Exposure In Removal From Specimen Exposure 'est Q) Environment Unbroken Broken

s 120

None

t

110 -

0 5 10 1 5 20 25 30 Salt Time i n Test (1000 hours) B r a z e

v

Braze & Salt A

Cyclic Salt U "

$ 5 0

I...,,.

PI 0 5 10 1 5 20 25 30 d Time in Test (1000 hours) Figure 36. Rene' 4 1 Alloy Unstressed Exposure Tensile Test Results

I

Notched Tensile Test Unnotched Tensile Test * Specimens Specimens Kt = 6

240i

n . d 240r

I

220 -

I

-

I

-

I

-

I I c ,

I

2 180t 180 t

5 170t 1 6 0 n 160 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time in Test (1000 hours) Time in Test (1000 hours)

I

170p

I n

. d

I

I Specimen Condition

Exposure on Removal From "u Specimen

I Q)

Environment $ 120 I None

t

I

1 1 0 - i Salt 0 5 10 15 20 25 30

=-a

i Time in Test (1000 hours) Braze

9---9 v

i

3or

Braze & S a l t &--A A

_----

Cyclic Salt O-----D

2F lo

I

0 2 0 5 10 15 20 25 30 I Time in Test (1000 hours) Figure 37. Rene' 41 Alloy Stressed Exposure Tensile Test Results w

i

w

I

x 2500 x 2500 x 15,000 x 15,000 Braze and S a l t Coat Braze coat F a i l m t i m e 15,439 hrs Failure time 5516 hrs (AD 5) (AC 6 ) Figure 39. Fractographs of Stress Corrosion Cracks in Titanium 6A1-4V Alloy x 2500 x. 2500 F a i l u r e t i n e 11,560 h r s Failure t i m e 13>6,52 hrs.

(!X 5) (RC 3 ) Figure 40. Fractographs of Stress Corrosion Cracks in Titanium 8A1-1V-1Mo Alloy (Braze Coated)

I -

x 2500 x 15,000 Figure 41. Fractographs of Surface of Stress Corrosion Cracks in AM 350 Steel A f t e r Alternate Exposure at 650°F and 1 0 0 ' F .

Failure time 1152 Hours (DE8)

8 0

v\ v\ cu 4 K X b( X I (d

a a

Q, k Q) a t9 M cd UJ rn P

E

-.

rn N

2-

X X cu

d- E i

a , c d

!d!

k O H 4 ha .r( I% al M t d L 3 i n

E

E

I

Etched x 500 / Figure 43.

xostructure of Braze and Salt-coated Titanium 6A1-4V Alloy Notch Specimen AD5 Showing Surface Crack. Specimen Failed After 15,000 H o u r s Exposure in Circulating A i r at 650°F 0 9 l J 3 9,308 HOUR EXPOSURE FAIIED Two stage replica x 2500

w o stage replica x 3-5,OOO

Figure 44. Microstructure of Braze-coated Titanium 8A1-1Mo-1V Alloy After Exposure in Circulating A i r at 650°F (Specimen BC 2) 9,644 HOUR EXPOSURE FAILED !Fwo stage replica x 2500 Two stage replica x 15,000 Figure 45. Microstructure of Braze and Salt-coated Titanium 8A1-1Mo-1V Alloy After Exposure in Circulating A i r at 650°F (Specimen BD3) O K V J 3,980 HOUR EXPOSURE FAILED x 2500 Two stage replica Figure 46. Microstructure of Salt-coated Titanium 8A1-1Mo-1V Alloy After Exposure in Circulating A i r at 650°F (Specimen BB1) 10,OOO HOUR MPC6URE NO FAILURE Two stage replica x 2500 Two stage replica x l5,ooO Figure 47. Microstructure of Salt-coated Titanium 8A1-1Mo-1V Alloy After Alternating 14-day Exposure in Circulating A i r at 650°F and in Humidity Cabinet at 100°F 1 (Specimen BE 1) x 500 Etched Figure 48. Microstructure of Salt-coated AM 350 Steel Notch Specimen DB6 at Fracture in Notch Showing Old and New Crack Areas. Specimen Failed After 15,000 Hours Exposure in Circulating A i r at 650" F 10,796 HOUR EXPOSURE FAILED Two stage replica x 2500 Two stage replica x 15,ooO Figure 49. Microstructure of Salt-coated AM 350 Steel After Exposure in Circulating A i r at 650°F (Specimen DB5)

2NaCl + Ti + 1 / 2 0 2 + H20 = Tic12 + 2NaOH

FREE ENERGY KCALS

- 30

- 4 c

- 5 (

400 5 00 600 700 "K 260 440 6 20 800 O F Figure 50. Free Energy Change i n Titanium Corrosion Reaction Backscatter limge X888 Unexposed Alloy Backscatter Image X888 Salt Coated Specimen # BBlK Failure Time 3980 H o u r s S w l e Current Image X888 Specimen as above Figure 51. Electron Microprobe Analysis of Titanium 8A1-1Mo-1V Alloy 3:aI.l Crack near L a r s Crack x 886

3 ~ k - s ~ 5 t m hs

# AD3K Figure 52. Electron Microprobe Analysis of Titanium 6A1-4V x 2500 x 2500 x 15,000 Structure near f r e e Structure near stress edie of s~pechnm corrosion crack.

Figure 53. Comparison of Fine Structure of Titanium 8A1-1Mo-1V Alloy Fractured Under Salt Coating After 2640 Hours Exposure (Specimen BB2) P 0 0.2 0.4 0.8 0.6

Figure 54. Deflection and Moment Arm Versus Load - Stiffness

P a r a m e t e r s P L*-A 1.0

A S/L 0.1

. 8 h I

v S/L 0 . 3

x

I - 6 I I4 W S / L 0 . 5 .c,

2 . 4

ra S/L 0.7 . 2 S/L 0.9 PL*/B Figure 55.

Moment Coefficients Versus Load - Stiffness Parameters

for Intermediate Seam Locations