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Effect of elevated temperature exposure on cast gamma titanium aluminide (Ti48Al2Cr2Nb)
Pergamon
ScriptaMetallurgicaet Materialia,Vol. 30, No. 9, pp. 1105-1110, 1994 Copyright© 1994ElsevierScienceLtd Printed in the USA.All rights reserved 0956-716X/94 $6.00 + 00
EFFECT OF ELEVATED TEMPERATURE EXPOSURE ON CAST GAMMA T I T A N I U M A L U M I N I D E (Ti-48A1-2Cr-2Nb) T . J . Kelly, C.M. Austin, P . J . Fink, and J. Schaeffer G E A i r c r a ~ Engines, Cincinnati, OH ( R e c e i v e d O c t o b e r 8, 1993) ( R e v i s e d J a n u a r y 4, 1994) Introduction G a m m a titanium aluminide is being considered for use in both commercial and military engines as a replacement for conventional Ni and Ti alloys. Several engine tests are being conducted in 1993, where cast nickel-base components have been replaced by investment cast TiA1. Application temperatures range up to about 800°C. In order to effectively replace the nickel-base alloys currently in use, cast gamma alloys must possess a wide range of mechanical property capabilities, including tensile strength, ductility, fracture toughness, fatigue strength and creep resistance. These properties must be retained after exposure to the high temperature environment of gas turbines. One phenomenon t h a t has surfaced recently is the effect of moisture and hydrogen on tensile ductility. Both Liu[1] and Yamaguchi[2] demonstrated that the ductility of binary TiA1 is reduced in hydrogen-containing environments. Another phenomenon is the post-exposure embrittlement of Ti-48Al-1V-0.2C discovered by Dowling[3]. When exposed in air or a moderate vacuum at 774 or 815°C, a brittle Ti2A1 layer forms on the surface. The fracture of this layer leads to low ductility in tests performed at room temperature; removal of this layer restores full ductility. Similarly, Saitoh[4] found that a thin (1 ~m) oxide scale leads to low ductility tensile failures in bend tests of a binary g a m m a alloy; however, elevated temperature ductility was not affected. His metallographic review of the tested specimens revealed numerous small surface cracks. Prior to the work reported below, it had not been determined if these effects occur in alloys such as Ti-48A1-2Cr-2Nb[5]. The function of the Cr and Nb is to improve strength, ductility, creep resistance and oxidation resistance[6 and 7]. Our results show that both ductility-reducing phenomena do indeed occur in Ti-48A1-2Cr-2Nb, and t h a t they can interact to sharply lower room temperature ductility after surprisingly modest exposures.
In order to reduce compositional and processing effects, all material was produced by Howmet, Ti-cast Division in Whitehall, Michigan as centrifugally cast slabs 13 x 10 x 1.3cm from 70kg double vacuum arc melted electrodes. Analyzed composition
1105
1106
CASTTITANIUMALUMINIDE
Vol. 30, No. 9
appears below, as determined from the ingot by Howmet and from castings by Sherry Laboratory Incorporated of Muncie, Indiana. Ti Ingot 47.9 Casting 48.6
A1 48.0 47.3
Nb 2.0 1.9
Cr 2.1 1.9
Fe 0.007 0.007
Si 0.03 0.03
O 0.1358 0.1797
All material was HIP'ed and heat treated as follows,1260°C/170Mpa/3h HIP plus 1300°C/20h in argon with flowing helium quench. Tensile specimens had 4ram diameter gage sections. R o o m temperature and 200°C tensileductilitybaselines were establishedfor reference purposes prior to any additional testing. All thermal exposures were done in air at 649°C for 16 hours. A Phillips EM420 Transmission Electron Microscope (TEM) was used in the analysis of the surface layers in as-machined and thermally exposed specimens. The specimens were p r e p a r e d by mechanical dimpling and ion milling from the back side of the specimen. Results a n d Discussion The present investigation, initiated after post-creep tensile tests of cast Ti-48A1-2Cr2Nb revealed a reduction in room temperature ductility by more than half. This loss of ductility could not be explained by creep damage. However, if as little as 6prn of the surface was removed by low-stress grinding after the creep exposure, full ductility was restored. Table 1 provides data from several experiments. The first set of data established the as-machined tensile ductility at 1.8% and shows t h a t room temperature ductility after an air exposure at 649°C is reduced by approximately 50%. These results prompted efforts to determine the range of exposure times and temperatures t h a t reduce ductility. Figure 1 provides the results of this study, showing that exposures as modest as 315°C for 10 hours can reduce ductility. Examination of the exposed tensile specimens after failure revealed numerous small cracks along the gage length of the specimens. Figure 2 is a comparison of an unexposed, ductile specimen and an exposed low ductility specimen, both after tensile testing. Note t h a t the exposed specimen contains numerous small secondary cracks, while the unexposed specimen does not. This suggests t h a t a brittle surface layer forms during exposure, similar to Dowling's[3] findings. However, no Ti2A1 phase nor any other surface layer or feature has been found in the specimens examined in this study. Nevertheless, it is evident that some change in the surface occurs t h a t causes the secondary cracking, but the extent of the embrittlement is less t h a n 6~tm. Auger analysis of electro-polished and exposed samples identified high concentrations of oxygen and nitrogen within about 6~m of the surface. TEM of back-thinned samples, while showing some unusual features in both exposed and unexposed samples, was largely inconclusive. While Saitoh linked a thin oxide layer to secondary cracking and low tensile ductility after thermal exposure, the exposures of the present study did not produce an oxide layer. In a n y event, the embrittled surface layer and the cracks t h a t form during tensile testing are not sufficient to cause ductility loss. As shown in Table 1, specimens were tested in argon with no loss in ductility for exposed samples, even though the same secondary cracking occurred, see Figure 3. Similar behavior was found in tests conducted in pure oxygen.
Vol. 30, No. 9
CASTTITANIUMALUM]NIDE
1107
Table 1. Room Temperature Tensile Test Results. Snecimen Preoaration
Exposure
UTS. MPa
Y$, MPa
Baseline, as-machined Baseline, as-machined As-machined As-machined
N N 650°C/16h 650°C/16h
474 470 399 415
357 361 340 361
1.9 1.8 0.9 0.9
As-machined, As-machined, As-machined, As-machined,
N N 650°C/16h 650°C/16h
479 510 454 479
365 359 368 368
1.8 2.6 1.4 2.0
Electro-polish 75pm Electro-polish 75pm
650°C/16h 650°C/16h
445 418
368 359
1.2 0.8
Electro-polish 125pm Electro-polish 125~tm
N 650°C/16h
448 476
367 374
1.2 1.8
Electro-polish 250pm Electro-polish 250pm
N 650°C/16h
495 442
385 357
1.0
Chem-Milled 125pm Chem-Milled 125pm Chem-Milled 125pm
N N 650°C/16h
456 435 457
361 359 371
1.7 1.2 1.5
Chem-Milled 250~m Chem-Milled 250~m Chem-Milled 250~tm
N N 650°C/16h
436 426 437
371 353 364
1.0 1.3 1.1
Ar Ar Ar Ar
test test test test
Elon~.%(Dlastic)
1.4
Figure 4 is a plot of tensile elongation versus test temperature for thermally exposed specimens. Although there is some scatter, the general trend is toward increasing ductility with increasing test temperature up to 200°C, at which point the as-machined ductility is obtained. Again, secondary cracks were observed in all specimens, even those tested under conditions that provided normal ductility. Tests of exposed specimens conducted at a higher strain rate also produced normal levels of ductility. The Ar tests, oxygen tests, 200°C tests and the high strain rate tests all represent conditions where atmospheric moisture is excluded or where hydrogen effects are reduced[2]. Therefore, fracture in exposed specimens is caused by the formation of an embrittled layer that prematurely forms cracks during tensile testing and grow in the presence of atmospheric moisture. A third aspect of the problem is also revealed in Table 1. After either electropolishing or chemical milling 125~m from the surface of specimens prior to exposure, the ductility loss was not observed. The gage sections contained numerous secondary cracks similar to those found in other exposed specimens. Although there is scatter in the data, it appears that removal of 125~m prevented post-exposure ductility loss; however, 75~m removal by chemical means did not prevent post-exposure ductility loss. This result indicates that a residual machining stress m a y be involved in the post-exposure ductility loss of cast Ti°48Al2Cr-2Nb. The fact that ductility again declines with chemical removal of 250~m m a y indicate some critical level of hydrogen charging is occurring with continued chemical processing. Clearly, the 125~tm removal by either electro-polishing or chemical milling produces a much higher average post exposure ductility than do either the 75 or 250pm removal.
1108
CAST TITANIUM ALUMINIDE
Vol. 30, No. 9
A common feature to all of the results is that the yield strength of the material is always reached prior to failure, i.e., some ductility is retained. Most manufacturing operations can be completed before any exposure that might affect them. However, assembly may involve parts that have been exposed to embrittling coating cycles. Exposures affect ductility only below 200°C and at low strain rates, a regime of limited concern during operation of aircraft engines. Therefore, postexposure ductility loss is mostly an issue during assembly and especially during overhaul.
1. The loss of ductility after thermal exposure is due to the formation during thermal exposure of a brittle surface layer, which produces small cracks that propagate in the presence of atmospheric moisture. 2. Residual machining stresses may play a role in the formation of the brittle surface layer. 3. Post-exposure ductility loss only occurs below 200°C and at low strain rates, a regime of little concern in service, but a potential problem during assembly and overhaul. References [1]. [2]. [3]. [4].
[5]. [6]. [7].
Liu, C. T. and Kim, Y-W, Room-Temperature Environmental Embrittlement in TiA1 Alloy, Scripta Metallurgica, Vol. 27, No. 5, 1992, pp. 599-603. Yamaguchi, Masaharu, Microstructures and Mechanical Behavior of Gamma Titanium Aluminide, Proceedings of the Seventh World Conference on Titanium, June 28°July, 1992, San Diego. Dowling, W. E. Jr., and Donlon, W. T., The Effect of Surface Film Formation From Thermal Exposure on the Ductility of Ti-48Al-lV-0.2C (at%), Scripta Metallurgica, Vol. 27, No. 11, pp. 1663-1668, 1992. Saitoh Y., Asakawa, K., and Mino, K., Embrittlement of Titanium Aluminides by Slight Oxydation, to be presented at the JIMIS-7,"Aspects of Deformation and Fracture at High Temperature in Crystalline Materials", Japan Inst. of Metals, 28-31 July 1993. Huang, S. C., Titanium Aluminum Alloys Modified by Chromium and Niobium and Method of Preparation, U. S. Patent Number 4,879,092, Nov. 7, 1989 Huang, S-C. and Hall, E. L., The Effects of Cr Additions to Binary TiA1-Base Alloys, Met. Trans. A, Vol. 22A, Nov., 1991, pp. 2619-2627 Wukusick, C. S., Oxidation Behavior of Intermetallic Compounds in the NbTi-Al System, GEMP-218, July 31, 1963, Contract No. AT (40-1)-2847
Vol. 30, No. 9
CAST TITANIUM ALUMINIDE
1109
Effect of Exposure Time end Temperature on Cast Ti-48AI-2Cr-2Nb Tensile DucUlity 2" t,O
u
o •
c O
•
W
[]
•
•
_u
•
•
,
g
Unexposed Material I 650°C Air ExDosure 315°C Air Exposure
$
_a a.
o~ . . . . . .
-i
. . . . . .
1
-i
10
. . . . . .
,-I
100
Time (h.}
. . . . . .
-I
. . . . . .
7
100010000
Figure 1. Plot of room t e m p e r a t u r e tensile ductility a f a r thermal exposure in air for various times at t e m p e r a t u r e , showing t h a t after only 10 hours exposure to 315°C the ductility loss occurs.
(a)
(b)
Figure 2. The features of two room t e m p e r a t u r e tested tensile specimen along their gauge section (a) the smooth uncracked appearance of a specimen tested in the asmachined condition and (b) the numerous microcracks in the specimen tested after thermal exposure.
1110
CAST TITANIUM ALUMINIDE
Vol. 30, No. 9
Figure 3. A cross-section of a thermally exposed tensile test specimen that was tested in argon and achieved full recovery of room temperature ductility even though it contained the numerous microcracks typical of the embrittled specimens tested in air.
The Effect of Test Temperature on the Ductility of Ex )osed Cast Ti-48AI-2Cr-2Nb 2.0• •
c O m
B
4~
(0 O)
•
¢= 1.5 2
•
IXl
2
;
1.0 I
~'==
•
I• •
I 0.S
...... 0
650°C/16h Air Exoosure Unexposed Matedal
• ....................... 50
100
150
200
250
Test Temperature (°C)
Figure 4. Plot of tensile elongation versus test temperature for thermally exposed specimens illustrating the recovery of room temperature as-machined ductility st 204°F.
ScriptaMetallurgicaet Materialia,Vol. 30, No. 9, pp. 1105-1110, 1994 Copyright© 1994ElsevierScienceLtd Printed in the USA.All rights reserved 0956-716X/94 $6.00 + 00
EFFECT OF ELEVATED TEMPERATURE EXPOSURE ON CAST GAMMA T I T A N I U M A L U M I N I D E (Ti-48A1-2Cr-2Nb) T . J . Kelly, C.M. Austin, P . J . Fink, and J. Schaeffer G E A i r c r a ~ Engines, Cincinnati, OH ( R e c e i v e d O c t o b e r 8, 1993) ( R e v i s e d J a n u a r y 4, 1994) Introduction G a m m a titanium aluminide is being considered for use in both commercial and military engines as a replacement for conventional Ni and Ti alloys. Several engine tests are being conducted in 1993, where cast nickel-base components have been replaced by investment cast TiA1. Application temperatures range up to about 800°C. In order to effectively replace the nickel-base alloys currently in use, cast gamma alloys must possess a wide range of mechanical property capabilities, including tensile strength, ductility, fracture toughness, fatigue strength and creep resistance. These properties must be retained after exposure to the high temperature environment of gas turbines. One phenomenon t h a t has surfaced recently is the effect of moisture and hydrogen on tensile ductility. Both Liu[1] and Yamaguchi[2] demonstrated that the ductility of binary TiA1 is reduced in hydrogen-containing environments. Another phenomenon is the post-exposure embrittlement of Ti-48Al-1V-0.2C discovered by Dowling[3]. When exposed in air or a moderate vacuum at 774 or 815°C, a brittle Ti2A1 layer forms on the surface. The fracture of this layer leads to low ductility in tests performed at room temperature; removal of this layer restores full ductility. Similarly, Saitoh[4] found that a thin (1 ~m) oxide scale leads to low ductility tensile failures in bend tests of a binary g a m m a alloy; however, elevated temperature ductility was not affected. His metallographic review of the tested specimens revealed numerous small surface cracks. Prior to the work reported below, it had not been determined if these effects occur in alloys such as Ti-48A1-2Cr-2Nb[5]. The function of the Cr and Nb is to improve strength, ductility, creep resistance and oxidation resistance[6 and 7]. Our results show that both ductility-reducing phenomena do indeed occur in Ti-48A1-2Cr-2Nb, and t h a t they can interact to sharply lower room temperature ductility after surprisingly modest exposures.
In order to reduce compositional and processing effects, all material was produced by Howmet, Ti-cast Division in Whitehall, Michigan as centrifugally cast slabs 13 x 10 x 1.3cm from 70kg double vacuum arc melted electrodes. Analyzed composition
1105
1106
CASTTITANIUMALUMINIDE
Vol. 30, No. 9
appears below, as determined from the ingot by Howmet and from castings by Sherry Laboratory Incorporated of Muncie, Indiana. Ti Ingot 47.9 Casting 48.6
A1 48.0 47.3
Nb 2.0 1.9
Cr 2.1 1.9
Fe 0.007 0.007
Si 0.03 0.03
O 0.1358 0.1797
All material was HIP'ed and heat treated as follows,1260°C/170Mpa/3h HIP plus 1300°C/20h in argon with flowing helium quench. Tensile specimens had 4ram diameter gage sections. R o o m temperature and 200°C tensileductilitybaselines were establishedfor reference purposes prior to any additional testing. All thermal exposures were done in air at 649°C for 16 hours. A Phillips EM420 Transmission Electron Microscope (TEM) was used in the analysis of the surface layers in as-machined and thermally exposed specimens. The specimens were p r e p a r e d by mechanical dimpling and ion milling from the back side of the specimen. Results a n d Discussion The present investigation, initiated after post-creep tensile tests of cast Ti-48A1-2Cr2Nb revealed a reduction in room temperature ductility by more than half. This loss of ductility could not be explained by creep damage. However, if as little as 6prn of the surface was removed by low-stress grinding after the creep exposure, full ductility was restored. Table 1 provides data from several experiments. The first set of data established the as-machined tensile ductility at 1.8% and shows t h a t room temperature ductility after an air exposure at 649°C is reduced by approximately 50%. These results prompted efforts to determine the range of exposure times and temperatures t h a t reduce ductility. Figure 1 provides the results of this study, showing that exposures as modest as 315°C for 10 hours can reduce ductility. Examination of the exposed tensile specimens after failure revealed numerous small cracks along the gage length of the specimens. Figure 2 is a comparison of an unexposed, ductile specimen and an exposed low ductility specimen, both after tensile testing. Note t h a t the exposed specimen contains numerous small secondary cracks, while the unexposed specimen does not. This suggests t h a t a brittle surface layer forms during exposure, similar to Dowling's[3] findings. However, no Ti2A1 phase nor any other surface layer or feature has been found in the specimens examined in this study. Nevertheless, it is evident that some change in the surface occurs t h a t causes the secondary cracking, but the extent of the embrittlement is less t h a n 6~tm. Auger analysis of electro-polished and exposed samples identified high concentrations of oxygen and nitrogen within about 6~m of the surface. TEM of back-thinned samples, while showing some unusual features in both exposed and unexposed samples, was largely inconclusive. While Saitoh linked a thin oxide layer to secondary cracking and low tensile ductility after thermal exposure, the exposures of the present study did not produce an oxide layer. In a n y event, the embrittled surface layer and the cracks t h a t form during tensile testing are not sufficient to cause ductility loss. As shown in Table 1, specimens were tested in argon with no loss in ductility for exposed samples, even though the same secondary cracking occurred, see Figure 3. Similar behavior was found in tests conducted in pure oxygen.
Vol. 30, No. 9
CASTTITANIUMALUM]NIDE
1107
Table 1. Room Temperature Tensile Test Results. Snecimen Preoaration
Exposure
UTS. MPa
Y$, MPa
Baseline, as-machined Baseline, as-machined As-machined As-machined
N N 650°C/16h 650°C/16h
474 470 399 415
357 361 340 361
1.9 1.8 0.9 0.9
As-machined, As-machined, As-machined, As-machined,
N N 650°C/16h 650°C/16h
479 510 454 479
365 359 368 368
1.8 2.6 1.4 2.0
Electro-polish 75pm Electro-polish 75pm
650°C/16h 650°C/16h
445 418
368 359
1.2 0.8
Electro-polish 125pm Electro-polish 125~tm
N 650°C/16h
448 476
367 374
1.2 1.8
Electro-polish 250pm Electro-polish 250pm
N 650°C/16h
495 442
385 357
1.0
Chem-Milled 125pm Chem-Milled 125pm Chem-Milled 125pm
N N 650°C/16h
456 435 457
361 359 371
1.7 1.2 1.5
Chem-Milled 250~m Chem-Milled 250~m Chem-Milled 250~tm
N N 650°C/16h
436 426 437
371 353 364
1.0 1.3 1.1
Ar Ar Ar Ar
test test test test
Elon~.%(Dlastic)
1.4
Figure 4 is a plot of tensile elongation versus test temperature for thermally exposed specimens. Although there is some scatter, the general trend is toward increasing ductility with increasing test temperature up to 200°C, at which point the as-machined ductility is obtained. Again, secondary cracks were observed in all specimens, even those tested under conditions that provided normal ductility. Tests of exposed specimens conducted at a higher strain rate also produced normal levels of ductility. The Ar tests, oxygen tests, 200°C tests and the high strain rate tests all represent conditions where atmospheric moisture is excluded or where hydrogen effects are reduced[2]. Therefore, fracture in exposed specimens is caused by the formation of an embrittled layer that prematurely forms cracks during tensile testing and grow in the presence of atmospheric moisture. A third aspect of the problem is also revealed in Table 1. After either electropolishing or chemical milling 125~m from the surface of specimens prior to exposure, the ductility loss was not observed. The gage sections contained numerous secondary cracks similar to those found in other exposed specimens. Although there is scatter in the data, it appears that removal of 125~m prevented post-exposure ductility loss; however, 75~m removal by chemical means did not prevent post-exposure ductility loss. This result indicates that a residual machining stress m a y be involved in the post-exposure ductility loss of cast Ti°48Al2Cr-2Nb. The fact that ductility again declines with chemical removal of 250~m m a y indicate some critical level of hydrogen charging is occurring with continued chemical processing. Clearly, the 125~tm removal by either electro-polishing or chemical milling produces a much higher average post exposure ductility than do either the 75 or 250pm removal.
1108
CAST TITANIUM ALUMINIDE
Vol. 30, No. 9
A common feature to all of the results is that the yield strength of the material is always reached prior to failure, i.e., some ductility is retained. Most manufacturing operations can be completed before any exposure that might affect them. However, assembly may involve parts that have been exposed to embrittling coating cycles. Exposures affect ductility only below 200°C and at low strain rates, a regime of limited concern during operation of aircraft engines. Therefore, postexposure ductility loss is mostly an issue during assembly and especially during overhaul.
1. The loss of ductility after thermal exposure is due to the formation during thermal exposure of a brittle surface layer, which produces small cracks that propagate in the presence of atmospheric moisture. 2. Residual machining stresses may play a role in the formation of the brittle surface layer. 3. Post-exposure ductility loss only occurs below 200°C and at low strain rates, a regime of little concern in service, but a potential problem during assembly and overhaul. References [1]. [2]. [3]. [4].
[5]. [6]. [7].
Liu, C. T. and Kim, Y-W, Room-Temperature Environmental Embrittlement in TiA1 Alloy, Scripta Metallurgica, Vol. 27, No. 5, 1992, pp. 599-603. Yamaguchi, Masaharu, Microstructures and Mechanical Behavior of Gamma Titanium Aluminide, Proceedings of the Seventh World Conference on Titanium, June 28°July, 1992, San Diego. Dowling, W. E. Jr., and Donlon, W. T., The Effect of Surface Film Formation From Thermal Exposure on the Ductility of Ti-48Al-lV-0.2C (at%), Scripta Metallurgica, Vol. 27, No. 11, pp. 1663-1668, 1992. Saitoh Y., Asakawa, K., and Mino, K., Embrittlement of Titanium Aluminides by Slight Oxydation, to be presented at the JIMIS-7,"Aspects of Deformation and Fracture at High Temperature in Crystalline Materials", Japan Inst. of Metals, 28-31 July 1993. Huang, S. C., Titanium Aluminum Alloys Modified by Chromium and Niobium and Method of Preparation, U. S. Patent Number 4,879,092, Nov. 7, 1989 Huang, S-C. and Hall, E. L., The Effects of Cr Additions to Binary TiA1-Base Alloys, Met. Trans. A, Vol. 22A, Nov., 1991, pp. 2619-2627 Wukusick, C. S., Oxidation Behavior of Intermetallic Compounds in the NbTi-Al System, GEMP-218, July 31, 1963, Contract No. AT (40-1)-2847
Vol. 30, No. 9
CAST TITANIUM ALUMINIDE
1109
Effect of Exposure Time end Temperature on Cast Ti-48AI-2Cr-2Nb Tensile DucUlity 2" t,O
u
o •
c O
•
W
[]
•
•
_u
•
•
,
g
Unexposed Material I 650°C Air ExDosure 315°C Air Exposure
$
_a a.
o~ . . . . . .
-i
. . . . . .
1
-i
10
. . . . . .
,-I
100
Time (h.}
. . . . . .
-I
. . . . . .
7
100010000
Figure 1. Plot of room t e m p e r a t u r e tensile ductility a f a r thermal exposure in air for various times at t e m p e r a t u r e , showing t h a t after only 10 hours exposure to 315°C the ductility loss occurs.
(a)
(b)
Figure 2. The features of two room t e m p e r a t u r e tested tensile specimen along their gauge section (a) the smooth uncracked appearance of a specimen tested in the asmachined condition and (b) the numerous microcracks in the specimen tested after thermal exposure.
1110
CAST TITANIUM ALUMINIDE
Vol. 30, No. 9
Figure 3. A cross-section of a thermally exposed tensile test specimen that was tested in argon and achieved full recovery of room temperature ductility even though it contained the numerous microcracks typical of the embrittled specimens tested in air.
The Effect of Test Temperature on the Ductility of Ex )osed Cast Ti-48AI-2Cr-2Nb 2.0• •
c O m
B
4~
(0 O)
•
¢= 1.5 2
•
IXl
2
;
1.0 I
~'==
•
I• •
I 0.S
...... 0
650°C/16h Air Exoosure Unexposed Matedal
• ....................... 50
100
150
200
250
Test Temperature (°C)
Figure 4. Plot of tensile elongation versus test temperature for thermally exposed specimens illustrating the recovery of room temperature as-machined ductility st 204°F.