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Influence of Aging Treatment on Precipitation Behavior of η Phase in Ni-Co-Cr Alloy
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ScienceDirect JOURNAL OF IRON AND Sf EEL RESEARCH, INTERNATIONAL. 2010. 17(1): 64-69. 78
Influence of Aging Treatment on Precipitation Behavior of 11 Phase in Ni-Co-Cr Alloy WAN Wen-juan.
HAN Guang-wei ,
DENG Bo
(Department of High Temperature Materials Research. Central Iron and Steel Research Institute. Beijing 100081. China)
Abstract: The influence of aging treatment on precipitation behavior of Tj phase in a new Ni-Co-Cr fastener superalloy. AEREX350. was investigated by means of optical microscopy. scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation behavior of Tj phase with two kinds of morphologies in this material was found to be sensitive to the temperature and time of aging treatment, Time-temperature-transformation curves of these two Tj phase were achieved. Furthermore. diffraction patterns revealed the semi-coherent orientation relationship between Tj phase and y matrix. This approach may be used to optimize the microstructure of the alloy for excellent mechanical properties. Key words: precipitation; Tj phase; aging
AEREX350 alloy is a new Ni-Co-Cr fastener superalloy and also the latest one in the mult iphase (MP) family. Table 1[1] shows chemical composition of three MP alloys. MP35N. the original member of MP family. gains its strength mainly through work hardening'F":", MP159 was strengthened not only by cold working but also aging by addition of Ti and Al to gain y'[4-6 J • AEREX350 has been developed by an empirical selection of alloying element to achieve excellent properties compared with MP159 at elevated temperatures. Owing to possessing high-strength. excellent creep resistance and outstanding corrosion resistance. this alloy can be used up to 730 'C as fastener in high-quality applications including aerospace gas turbine engines. power generating gas turbines and petrochemical industry. A similar coefficient of thermal expansion to that of conventional nickelbased alloys can also make close joint at various temperatures. In addition. this alloy displays good therTable 1 Alloy
mal stability and notch toughnessLI.7-9J. These attractive characteristics suggest that AEREX350 alloy is an excellent material for high temperature fastener applications. Recent investigations: IO - 11 worked on the precipitation behavior of AEREX350 show that the major strengthening phase in this alloy is y' (Ni'J AlTi) with Liz structure. It was also shown that 1j (Ni 3 Ti ) phase. with an ordered GCP hexagonal close-packed D0 21 structure. appeared at grain boundaries or grain interiors during high-temperature aging. Grain boundary 1j precipitated with a discrete. parallel. rod-like morphology plays a role of improving notch toughness; however. Widmanstatten 1j within grains would deteriorate mechanical properties[1.IO-12 1 • Although the morphology of 1j phase has great effect on mechanical properties of AEREX350 superalloy. very limited research has been carried out on the
Nominal composition of AEREX350 and two MP alloys AI
:'>In
2.9
0.2
O. 5
2.2
J. I
1. I
Ni
Co
Cr
Mo
MP35N
35
Balance
20
10
~1
MPI59
Balance
:,6
19
7
AEREX350
Balance
25
17
3
Biography: WAN Wen-juant 1985-), Female. Master:
Ti
(mass percent. Ta
W
Fe ~1
E-mail: wanwenjuanwwjfsyahoo. com. cn ,
9
2
C ~O.
025
~O.O'I ~O.
025
Received Date: August 25, 2008
%) B
~O. ~O. ~O.
01 03 025
Issue I
precipitation behavior of 1j phase. The present work has studied the effect of aging treatment on the precipitation behavior of 1j phase in AEREX350 alloy. The main goal of this investigation was to evaluate the possibility of controlling the final microstructure of .the material by proper selection of aging treatment temperature.
1
65 •
Influence of Aging Treatment on Precipitation Behavior of 1) Phase in Ni-Co-Cr Alloy
Experimental
The material employed in this investigation was prepared via vacuum induction melting followed by vacuum arc remelting. The chemical composition of these hot-rolled bars with a diameter of 15 mm was shown in Table 2. Samples were cut from these bars; solution treated at 1100 °C for 2h and subsequently water quenching. The solution treatment temperature selected according to the earlier work was above the 1j solvus temperature[IO-Jl]. The solution-treated samples were then aged at 780-1080 °C for a time ranging from 5 min to 4 h , and air cooled to room temperature. To study the microstructural features by optical microscopy and scanning electro microscope (SEM), the aged samples were etched in a solution of ethanol (30 ml.) + HCl(20 ml.) +CuCI 2 (3 g) at room temperature after grinding and polishing. Subsequently these samples were tested using the HRC method, at least four indentations were made for each hardness measurement and the average hardness was plotted against aging temperatures for different time. For transmission electron microscopy examination (TEM) studies, slices of O. 4 mm in thickness were cut using an electrical discharge machine (EDM) , and subsequently ground to a thickness of O. 05 mm, Disks of 3 mm in diameter were cut and TEM foils were prepared by electro-polishing using a solution of 6 % HCl04 in ethanol with a voltage of 35 V at - 20 'C.
Fig. 2
Table 2
2
Ti
Al
Nb
Ta
% w
2.18
I. 08
I. 20
3.97
I. 96
Chemical composition of tested AEREX350
Ni
Co
Balance
24.79
Cr
Mo
16.89 3.08
Results and Analysis
Fig. 1 shows the result of hardness testing performed on samples aged at different temperatures for different times. It is clear that the hardness is decreased first and then increased with an increase in temperatures when aging for O. 5, 1, 1. 5, 2, 2. 5, 3, 3. 5, 4 h. It is also apparent that the temperature toward minimum hardness is about 980 - 1040 ·C. Fig. 2 shows the micrographs of samples aged at different temperatures for 1 h. As seen in Fig. 2, the volume percent of 1j phase increased first but decreased then while aging temperatures are increasing. This precipitating behavior has the reverse trend compared with the hardness. It is suggest that 44r---------------~
40
36
Aging time • 0.5 h .1 h ... 1.5 h '" 2 h
.2.5h ~ 3.5h
.. 3h .4h
G
~ 32 ~
~
:z::
28 24 20 '-'-_ _'__ __'_ _~ _ ~_ _'__ __'_ ____J 800 840 880 920 960 1 000 1 040 1 080
Temperature/'\:::
Fig. 1
Variation of hardness with aging temperature for aged AEREX350 samples
Micrographs of samples aged at 890°C (a). 1020 t: (b) and 1050 t: (c) for 1 h (arrows indicated the positions of 1) phase)
• 66 •
Journal of Iron and Steel Research. International
the hardness IS relative to the precipitation of 11 phase in AEREX350. For further investigate the effect of aging treatments on 11 precipitation behavior, micrographs of samples solution-treated and aged at 1020'( for different times are shown in Fig. 3. Fig. 3 (a) shows that no evidence is found for the precipitation of 11 phase in solution-treated grain structure with a uniform grain size distribution as 80-120 flm, just with some annealing twins and carbide particles. And samples aged at temperatures respectively below 870'C and above 1080'C have the same microstructures as solution-treated with no 11 phase.
Ca) As-solution treated sample; (c) Samples aged at 1020 C for O. 5 h;
Fig. 3
Vol. 17
It is found that the shortest pregnant time for 11 precipitation at grain boundaries in a parallel rodlike morphology is 10 min when aging at 1020 'C [Fig. 3 (b) J. With prolonged exposure 11 phase increases in amount and size and grows into grain interiors. Fig. 3 (c) shows that Widmanstatten 11 precipitates within grains along specific orientation when aging time increased up to 30 min. Continuing to exposure, 11 platelets are elongated and increased in amount, moreover, precipitated along more orientations, as shown in Fig. 3 (d). Fig. 3 has shown the precipitation process of 11 phase in AEREX350 durmg aging.
(b) Samples aged at 1020 C for IOmin; (d) Samples aged at 1020(' for I. 5 h.
SEM images of samples showing the precipitation process of 11 phase during aging
Above or below 1020"C, the pregnant times of 11 phase precipitated at grain boundaries and within grains both increase while aging temperatures increasing or decreasing, respectively. The precipitation temperature of grain boundary 11 ranges from 870'C to 1080·C. as shown in Fig. 4 (a) and Fig. 5 (a); and that for Widmanstat ten type 11 ranges from 970 'C to 1070 'C, Fig. 4 (b). Fig. 6 shows the time-temperature-transformation (TTT) curves obtained from microstructural examination of all aged samples. These C-shape curves represent the aging temperatures and times of beginning precipitation of 11 phase at grain boundaries and within grains, respectively. Compared Fig. 1 with Fig. 6, it has been found that the aging temperatures corresponding to minimum hardness are around the sharp C-nose temperature, 1020 C, which presents the shortest pregnant precipitation time of 11 phase. To further investigate the origin of this phenomenon, TEM examination was carried out on aged alloy. TEM foils were prepared from samples aged at 870'C and 1 060'C for 4 h. Fig. 7 (a) and (b) shows grain boundary 11 precipitated along a specific
direction at grain boundaries during aging are parallel and rod-like. Fig. 7 (c) shows plenty of fine y' . particles with a size of 20 - 30 nm uniformly distributed throughout y matrix, and it is apparent that there is a y' -Iree zone around grain boundary 11 as shown in Fig. 7 (a). Fig. 8 (a) shows Widrnanstatten 11 platelets precipitate within grains aligned along {Ill} crystallographic planes of y matrix, and fewer v' phase with uneven size distributed in this sample. The y'free zone around 11 and y'-rich zone far away from 11 platelets are respectively shown in Fig. 8 (a) and (b), y' particles in this sample precipitated with a bigger size of 30-150 nm than the former sample. It means that 11 phase may precipitates with an expense of v' in AEREX350 alloy. Electron diffraction patterns, as shown in Fig. 7 (d) and Fig. 8 (C), have further revealed that the orientation relationship between y matrix and 11 ( Ni 3 Ti ) phase with an ordered hexagonal closepacked 002 1 structure is {ll1}y// {000l}~, [OllJy// [21 10 J~. A semi-coherent lattice relationship has been found between 11 phase and y matrix, and y' is completely coherent with y matrix. In order to keep
Issue 1
Influence of Aging Treatment on Precipitation Behavior of Tj Phase in Ni-Co-Cr Alloy
Fig. 4
Fig. S
• 67 •
SEM images of 1) phase precipitated with different morphologies in samples aged at 970 t: for 4S min (a) and 4 h (b)
SEM images of 1) phase precipitated with different morphologies in samples aged at 1060 t: for 2 h (a) and 4 h (b)
this orien tation relationship during precipitation and growth. 'T/ phase appears parallel to each other. as shown in Fig. 7 and Fig. 8.
1100 1050
P
Ql
~
1000
3
Discussion
J
950
~
900
llO
60
Fig. 6
120 Agingtime/min
TIT curves of
180
240
1) phase precipitation in
AEREX3S0 alloy
The results presented in this investigation re~ veal important aspects of the influence of aging on AEREX350 alloy. Hardness-testing result has shown that. for similar aging times. samples with aging temperatures around 1020 ·C. the sharp tip of TTT curves. are generally weaker than the other ones. The probably reason is that the nucleation and growth of 'T/ phase consume Ti atoms so that y' phase is difficult
Journal of Iron and Stccl Rc~carch. International
• 68 •
(a) Bright field image of '1 and (c)
t' phasl';
Bright field image of y'phase;
Fig. 7
( h) Dark field image of '1 phase, (d) Diffraction pattern.
TEM images of sample aged at 870"c for 4 h
(ill Bright field image of '1 plat elct s , (c)
(h) Bright [ield image of y'phas,,;
Diffraction pattern (H=[OOI],=[OII], =[2110:", =[211
Fig. 8
Vol. 17
).
TEM images of sample aged at I 060 'C for 4 h
to nucleate. Titanium is the main composition of y'
(Ni 3AITi) phase and Tj ( Ni.Ti ) phase. y' is the main strengthening phase of AEREX350 alloy and TEM images have shown y'-free zones around Tj phase. More Tj and fewer y' will precipitate in a sample aging at a closer temperature to 1020 C for a certain time. As a result, the strengthening ability will be lower and hardness will be decreased. There are two possible reasons. high energy and element segregation, in favor of Tj (Nil Ti ) phase earlier nucleation at grain boundaries than within grains during aging. Moreover, water quenching after solution treated has produced supersaturated vacancies at room temperature, these supersaturated vacancies can help titanium atoms segregating to
grain boundaries during aging at elevated temperat ures to achieve an atom ratio of Ni : Ti = 3 : 1. Precipitation and growth of Tj (Ni- Ti ) phase is a diffusion-type phase transformation process, which largely depends on enough titanium and driving force. The amount of Tj phase precipitated in AEREX:350 alloy depends on the diffusion rate of titanium atoms in matrix and driving force for phase transformation. The relationship between diffusion rate of titanium atoms and temperature can be explained as follows: Q D=D"expl kT (1) where, D is the diffusion coefficient; Q is the activation energy for diffusion; D" is the frequency factor
Issue 1
Influence of Aging Treatment on Precipitation Behavior of 1) Phase in Ni-Co-Cr Alloy
which is a constant in interstitial diffusion; k is the Boltzmann's constant; and T is the temperature. It is clear that a linear relation between InD and liT with a slope of - QI k. It means that the diffusion rate of titanium atoms will increase with an increase in aging temperature. However, as we know, higher aging temperature would result in decreased driving force of this kind of phase transformation. The C-shape TTT curves for 1j phase precipitation, as shown in Fig. 6, are resulted from the interaction between these two factors. It is known that 1j phase tends to precipitate in superalloys with Til Al ratio larger than 1. Til Al ratio is about 2 for AEREX350 alloy, therefore leading to precipitation of 1j phase in this alloy during aging. Some studies[13] have shown that the presence of a few of fine and discontinuous 1j phase precipitated at grain boundaries would enhance the notch toughness of alloys at elevated temperature, also for AEREX350 alloy[J]. The increase in notch toughness at elevated temperature by this kind of 1j phase may be attributed to pinning grain boundary and resulting in serrated grain boundary, as shown in Fig. 3 (b), Fig. 4 (a) and Fig. 5 (a). The former could prevent grain boundary sliding at elevated temperature, and the latter could inhibit crack propagation along grain boundaries. That is, grain boundary 1j with proper morphology is considered to be able to reinforce grain boundaries and improve notch toughness effectively at elevated temperature. However, further investigation of appropriate size, morphology and amount of grain boundary 1j in AEREX350 alloy is needed. Widmanstatten 1j precipitated within grains would lead to deterioration in ductility of superalloys[],IO-IZ]. Widrnanstatten 1j is weaker than y matrix, the net-like 1j platelets will be the propagated channels of cracks during deforming. Accordingly, an optimizing microstructure corresponding to excellent mechanical properties of AEREX350 fastener superalloy can be obtained employing an aging treatment selected by virtue of the TTT curves gained in this investigation.
4
O. 5 to 4 h. The temperature range corresponding to the minimum values of hardness curves was 9801040 ·C. 2) The precipitation behavior of 1j phase in AEREX350 superalloy can be explained by time-temperature-transformation (TTT) curves gained in this investigation. The shortest pregnant times of 1j phase precipitated at grain boundaries and within grains were 10 min and 30 min when aged at 1020 'C, respectively. 3) y' -Iree zones around 1j phase in samples aged 870'C and 1060'C for 4 hours showed that 1j phase precipitated in AEREX350 alloy at the expense of y' phase. This may explain the relationship between hardness and 1j phase: precipitation of 1j phase decreased the amount of y' phase so that the strengthening ability of AEREX350 alloy decreased. 4) The orientation relationship between the ordered hexagonal close-packed 1j phase with a DO Z4 structure and face-centered cubic y matrix in AEREX350 alloy was {llI}111 {000l}~, [01lJ111 [2IIoJ~.
The authors want to express their deep gratitude to CHEN Hui-xia , GAO Ling, WANG Chang and FENG Tian-you for help in experimental techniques. References: [IJ
[2J [3J
[4J
[5J [6J
[7J
Conclusions [8J
The results presented in this paper are summarized as follows: 1) The hardness test result showed that the hardness was generally decreased first but increased with an increase in aging temperatures when samples aged at 780-1080'C for a certain time ranging from
• 69 •
[9J [10J
SPS Technologies Aerospace Fasteners Group. Superalloys Developed by SPS Technologies for Aerospace Fasteners [RJ. Jenkitown , SPS Technologies Inc. 1998. Raghavan M. Berkowitz B J. Strain Induced Transformation in MP Alloys [1]. Scripta Metallurgica , 1980. 14: 1009. Raghavan M. Berkowitz B J. Kane R D. A Transaction Electron Microscopic Investigation of Phase Transformations in MP35N [1]. Metallurgical Transactions A. 1980. n A, 203. Slaney J S. Nebiolo R A. Development of MULTIPHASE Alloy MP159 Using Experimental Statistics [1]. Metallography. 1983. 16: 137. ASM Internation. Cobalt Alloy [1]. Alloy Digest. 1974: Co68. LU Shi-qiang , SHANG Bao-zhong , LUa Zi-jian, Investigation on the Cold Deformation Strengthening Mechanism in MP159 Alloy [1]. Metall Mater Trans A. 2000. 31A: 5. Buzolits S. New High Temperature Alloy Characterized by Superior Alloy Properties at Temperatures to 1350 "F [1]. Industrial Heating. 1994. 6H 12): 34. Buzolits S R. Kline L A. Bolting Alloy Fills High Temperature Gap [1]. Advanced Materials and Processes. 1995. 147(2): 33. ASM International. Nickel Alloy [1]. Alloy Digest. 1995 (5): Ni-479. Asgari S. Age-Hardening Behavior and Phase Identification in Solution-Treated AEREX350 Superalloy [1]. Metall Mater Trans A. 2006. 37A: 2051.
(Continued on Page 78)
• 78 •
[5J
[6J
Journal of Iron and Steel Research. International
Institute. 2008. Uranga P. Fernandez A I. Lopez B. et al. Modeling of Austenite Grain Size Distribution in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling (TSDRJ Route [JJ. lSI] Int. 2004. 44(8): 1416. Uranga P. Fernandez A I. Lopez B. et al. Transition Between Static and Metadynarnic Recrystallization Kinetics in Coarse Nb Microalloyed Austenite [1]. Materials Science and Engineering A. 2003. 345: 319.
[7J
[8J
[9J
VoLl7
Siciliano F. Leduc L L. Modeling of the Microstructural Evolution and Mean Flow Stress During Thin Slab Casting/Direct Rolling of Niobium Microalloyed Steels [JJ. Materials Science Forum. 2005(500/501): 221. Medina SF. Hernandez C A. Modelling of the Dynamic Recrystallization of Austenite in Low Alloy and Microalloyed Steels [JJ. Acta Mater. 1996. 440): 165. Irvine K 1. Pickering F B. Gladman T. Grain Refined C-Mn Steels [n. JISI. 1967. 205: 16 I.
(Continued From Page 69) [I I J
[I2J
Tomasello C M. Pettit F S. Birks N. et al. Precipitation Behavior in AEREX350 [CJ II Kissinger RD. Deye D 1. Anton D L. et al , eds, Superalloys 1996. Warrendale: TMS. 1996: 145. Shibata T. Shudo Y. Takahashi T. et al. Effect of Stabilizing Treatment on Precipitation Behavior of Alloy 706 [CJ IIKissinger R D. Deye D 1. Anton D L. et al , eds. Superalloys
[I3J
1996. Warrendale: TMS. 1996: 627. Danflou H L. Marty M. Walder A. Formations of Serrated Grain Boundaries and Their Effect on the Mechanical Properties in a P/M Nickel Base Superalloy [C] II Antolovich S D. St usrud R W. MacKay R A. et al , eds, Superalloys 1992. Warrendale: TMS. 1992: 63.
ScienceDirect JOURNAL OF IRON AND Sf EEL RESEARCH, INTERNATIONAL. 2010. 17(1): 64-69. 78
Influence of Aging Treatment on Precipitation Behavior of 11 Phase in Ni-Co-Cr Alloy WAN Wen-juan.
HAN Guang-wei ,
DENG Bo
(Department of High Temperature Materials Research. Central Iron and Steel Research Institute. Beijing 100081. China)
Abstract: The influence of aging treatment on precipitation behavior of Tj phase in a new Ni-Co-Cr fastener superalloy. AEREX350. was investigated by means of optical microscopy. scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation behavior of Tj phase with two kinds of morphologies in this material was found to be sensitive to the temperature and time of aging treatment, Time-temperature-transformation curves of these two Tj phase were achieved. Furthermore. diffraction patterns revealed the semi-coherent orientation relationship between Tj phase and y matrix. This approach may be used to optimize the microstructure of the alloy for excellent mechanical properties. Key words: precipitation; Tj phase; aging
AEREX350 alloy is a new Ni-Co-Cr fastener superalloy and also the latest one in the mult iphase (MP) family. Table 1[1] shows chemical composition of three MP alloys. MP35N. the original member of MP family. gains its strength mainly through work hardening'F":", MP159 was strengthened not only by cold working but also aging by addition of Ti and Al to gain y'[4-6 J • AEREX350 has been developed by an empirical selection of alloying element to achieve excellent properties compared with MP159 at elevated temperatures. Owing to possessing high-strength. excellent creep resistance and outstanding corrosion resistance. this alloy can be used up to 730 'C as fastener in high-quality applications including aerospace gas turbine engines. power generating gas turbines and petrochemical industry. A similar coefficient of thermal expansion to that of conventional nickelbased alloys can also make close joint at various temperatures. In addition. this alloy displays good therTable 1 Alloy
mal stability and notch toughnessLI.7-9J. These attractive characteristics suggest that AEREX350 alloy is an excellent material for high temperature fastener applications. Recent investigations: IO - 11 worked on the precipitation behavior of AEREX350 show that the major strengthening phase in this alloy is y' (Ni'J AlTi) with Liz structure. It was also shown that 1j (Ni 3 Ti ) phase. with an ordered GCP hexagonal close-packed D0 21 structure. appeared at grain boundaries or grain interiors during high-temperature aging. Grain boundary 1j precipitated with a discrete. parallel. rod-like morphology plays a role of improving notch toughness; however. Widmanstatten 1j within grains would deteriorate mechanical properties[1.IO-12 1 • Although the morphology of 1j phase has great effect on mechanical properties of AEREX350 superalloy. very limited research has been carried out on the
Nominal composition of AEREX350 and two MP alloys AI
:'>In
2.9
0.2
O. 5
2.2
J. I
1. I
Ni
Co
Cr
Mo
MP35N
35
Balance
20
10
~1
MPI59
Balance
:,6
19
7
AEREX350
Balance
25
17
3
Biography: WAN Wen-juant 1985-), Female. Master:
Ti
(mass percent. Ta
W
Fe ~1
E-mail: wanwenjuanwwjfsyahoo. com. cn ,
9
2
C ~O.
025
~O.O'I ~O.
025
Received Date: August 25, 2008
%) B
~O. ~O. ~O.
01 03 025
Issue I
precipitation behavior of 1j phase. The present work has studied the effect of aging treatment on the precipitation behavior of 1j phase in AEREX350 alloy. The main goal of this investigation was to evaluate the possibility of controlling the final microstructure of .the material by proper selection of aging treatment temperature.
1
65 •
Influence of Aging Treatment on Precipitation Behavior of 1) Phase in Ni-Co-Cr Alloy
Experimental
The material employed in this investigation was prepared via vacuum induction melting followed by vacuum arc remelting. The chemical composition of these hot-rolled bars with a diameter of 15 mm was shown in Table 2. Samples were cut from these bars; solution treated at 1100 °C for 2h and subsequently water quenching. The solution treatment temperature selected according to the earlier work was above the 1j solvus temperature[IO-Jl]. The solution-treated samples were then aged at 780-1080 °C for a time ranging from 5 min to 4 h , and air cooled to room temperature. To study the microstructural features by optical microscopy and scanning electro microscope (SEM), the aged samples were etched in a solution of ethanol (30 ml.) + HCl(20 ml.) +CuCI 2 (3 g) at room temperature after grinding and polishing. Subsequently these samples were tested using the HRC method, at least four indentations were made for each hardness measurement and the average hardness was plotted against aging temperatures for different time. For transmission electron microscopy examination (TEM) studies, slices of O. 4 mm in thickness were cut using an electrical discharge machine (EDM) , and subsequently ground to a thickness of O. 05 mm, Disks of 3 mm in diameter were cut and TEM foils were prepared by electro-polishing using a solution of 6 % HCl04 in ethanol with a voltage of 35 V at - 20 'C.
Fig. 2
Table 2
2
Ti
Al
Nb
Ta
% w
2.18
I. 08
I. 20
3.97
I. 96
Chemical composition of tested AEREX350
Ni
Co
Balance
24.79
Cr
Mo
16.89 3.08
Results and Analysis
Fig. 1 shows the result of hardness testing performed on samples aged at different temperatures for different times. It is clear that the hardness is decreased first and then increased with an increase in temperatures when aging for O. 5, 1, 1. 5, 2, 2. 5, 3, 3. 5, 4 h. It is also apparent that the temperature toward minimum hardness is about 980 - 1040 ·C. Fig. 2 shows the micrographs of samples aged at different temperatures for 1 h. As seen in Fig. 2, the volume percent of 1j phase increased first but decreased then while aging temperatures are increasing. This precipitating behavior has the reverse trend compared with the hardness. It is suggest that 44r---------------~
40
36
Aging time • 0.5 h .1 h ... 1.5 h '" 2 h
.2.5h ~ 3.5h
.. 3h .4h
G
~ 32 ~
~
:z::
28 24 20 '-'-_ _'__ __'_ _~ _ ~_ _'__ __'_ ____J 800 840 880 920 960 1 000 1 040 1 080
Temperature/'\:::
Fig. 1
Variation of hardness with aging temperature for aged AEREX350 samples
Micrographs of samples aged at 890°C (a). 1020 t: (b) and 1050 t: (c) for 1 h (arrows indicated the positions of 1) phase)
• 66 •
Journal of Iron and Steel Research. International
the hardness IS relative to the precipitation of 11 phase in AEREX350. For further investigate the effect of aging treatments on 11 precipitation behavior, micrographs of samples solution-treated and aged at 1020'( for different times are shown in Fig. 3. Fig. 3 (a) shows that no evidence is found for the precipitation of 11 phase in solution-treated grain structure with a uniform grain size distribution as 80-120 flm, just with some annealing twins and carbide particles. And samples aged at temperatures respectively below 870'C and above 1080'C have the same microstructures as solution-treated with no 11 phase.
Ca) As-solution treated sample; (c) Samples aged at 1020 C for O. 5 h;
Fig. 3
Vol. 17
It is found that the shortest pregnant time for 11 precipitation at grain boundaries in a parallel rodlike morphology is 10 min when aging at 1020 'C [Fig. 3 (b) J. With prolonged exposure 11 phase increases in amount and size and grows into grain interiors. Fig. 3 (c) shows that Widmanstatten 11 precipitates within grains along specific orientation when aging time increased up to 30 min. Continuing to exposure, 11 platelets are elongated and increased in amount, moreover, precipitated along more orientations, as shown in Fig. 3 (d). Fig. 3 has shown the precipitation process of 11 phase in AEREX350 durmg aging.
(b) Samples aged at 1020 C for IOmin; (d) Samples aged at 1020(' for I. 5 h.
SEM images of samples showing the precipitation process of 11 phase during aging
Above or below 1020"C, the pregnant times of 11 phase precipitated at grain boundaries and within grains both increase while aging temperatures increasing or decreasing, respectively. The precipitation temperature of grain boundary 11 ranges from 870'C to 1080·C. as shown in Fig. 4 (a) and Fig. 5 (a); and that for Widmanstat ten type 11 ranges from 970 'C to 1070 'C, Fig. 4 (b). Fig. 6 shows the time-temperature-transformation (TTT) curves obtained from microstructural examination of all aged samples. These C-shape curves represent the aging temperatures and times of beginning precipitation of 11 phase at grain boundaries and within grains, respectively. Compared Fig. 1 with Fig. 6, it has been found that the aging temperatures corresponding to minimum hardness are around the sharp C-nose temperature, 1020 C, which presents the shortest pregnant precipitation time of 11 phase. To further investigate the origin of this phenomenon, TEM examination was carried out on aged alloy. TEM foils were prepared from samples aged at 870'C and 1 060'C for 4 h. Fig. 7 (a) and (b) shows grain boundary 11 precipitated along a specific
direction at grain boundaries during aging are parallel and rod-like. Fig. 7 (c) shows plenty of fine y' . particles with a size of 20 - 30 nm uniformly distributed throughout y matrix, and it is apparent that there is a y' -Iree zone around grain boundary 11 as shown in Fig. 7 (a). Fig. 8 (a) shows Widrnanstatten 11 platelets precipitate within grains aligned along {Ill} crystallographic planes of y matrix, and fewer v' phase with uneven size distributed in this sample. The y'free zone around 11 and y'-rich zone far away from 11 platelets are respectively shown in Fig. 8 (a) and (b), y' particles in this sample precipitated with a bigger size of 30-150 nm than the former sample. It means that 11 phase may precipitates with an expense of v' in AEREX350 alloy. Electron diffraction patterns, as shown in Fig. 7 (d) and Fig. 8 (C), have further revealed that the orientation relationship between y matrix and 11 ( Ni 3 Ti ) phase with an ordered hexagonal closepacked 002 1 structure is {ll1}y// {000l}~, [OllJy// [21 10 J~. A semi-coherent lattice relationship has been found between 11 phase and y matrix, and y' is completely coherent with y matrix. In order to keep
Issue 1
Influence of Aging Treatment on Precipitation Behavior of Tj Phase in Ni-Co-Cr Alloy
Fig. 4
Fig. S
• 67 •
SEM images of 1) phase precipitated with different morphologies in samples aged at 970 t: for 4S min (a) and 4 h (b)
SEM images of 1) phase precipitated with different morphologies in samples aged at 1060 t: for 2 h (a) and 4 h (b)
this orien tation relationship during precipitation and growth. 'T/ phase appears parallel to each other. as shown in Fig. 7 and Fig. 8.
1100 1050
P
Ql
~
1000
3
Discussion
J
950
~
900
llO
60
Fig. 6
120 Agingtime/min
TIT curves of
180
240
1) phase precipitation in
AEREX3S0 alloy
The results presented in this investigation re~ veal important aspects of the influence of aging on AEREX350 alloy. Hardness-testing result has shown that. for similar aging times. samples with aging temperatures around 1020 ·C. the sharp tip of TTT curves. are generally weaker than the other ones. The probably reason is that the nucleation and growth of 'T/ phase consume Ti atoms so that y' phase is difficult
Journal of Iron and Stccl Rc~carch. International
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(a) Bright field image of '1 and (c)
t' phasl';
Bright field image of y'phase;
Fig. 7
( h) Dark field image of '1 phase, (d) Diffraction pattern.
TEM images of sample aged at 870"c for 4 h
(ill Bright field image of '1 plat elct s , (c)
(h) Bright [ield image of y'phas,,;
Diffraction pattern (H=[OOI],=[OII], =[2110:", =[211
Fig. 8
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).
TEM images of sample aged at I 060 'C for 4 h
to nucleate. Titanium is the main composition of y'
(Ni 3AITi) phase and Tj ( Ni.Ti ) phase. y' is the main strengthening phase of AEREX350 alloy and TEM images have shown y'-free zones around Tj phase. More Tj and fewer y' will precipitate in a sample aging at a closer temperature to 1020 C for a certain time. As a result, the strengthening ability will be lower and hardness will be decreased. There are two possible reasons. high energy and element segregation, in favor of Tj (Nil Ti ) phase earlier nucleation at grain boundaries than within grains during aging. Moreover, water quenching after solution treated has produced supersaturated vacancies at room temperature, these supersaturated vacancies can help titanium atoms segregating to
grain boundaries during aging at elevated temperat ures to achieve an atom ratio of Ni : Ti = 3 : 1. Precipitation and growth of Tj (Ni- Ti ) phase is a diffusion-type phase transformation process, which largely depends on enough titanium and driving force. The amount of Tj phase precipitated in AEREX:350 alloy depends on the diffusion rate of titanium atoms in matrix and driving force for phase transformation. The relationship between diffusion rate of titanium atoms and temperature can be explained as follows: Q D=D"expl kT (1) where, D is the diffusion coefficient; Q is the activation energy for diffusion; D" is the frequency factor
Issue 1
Influence of Aging Treatment on Precipitation Behavior of 1) Phase in Ni-Co-Cr Alloy
which is a constant in interstitial diffusion; k is the Boltzmann's constant; and T is the temperature. It is clear that a linear relation between InD and liT with a slope of - QI k. It means that the diffusion rate of titanium atoms will increase with an increase in aging temperature. However, as we know, higher aging temperature would result in decreased driving force of this kind of phase transformation. The C-shape TTT curves for 1j phase precipitation, as shown in Fig. 6, are resulted from the interaction between these two factors. It is known that 1j phase tends to precipitate in superalloys with Til Al ratio larger than 1. Til Al ratio is about 2 for AEREX350 alloy, therefore leading to precipitation of 1j phase in this alloy during aging. Some studies[13] have shown that the presence of a few of fine and discontinuous 1j phase precipitated at grain boundaries would enhance the notch toughness of alloys at elevated temperature, also for AEREX350 alloy[J]. The increase in notch toughness at elevated temperature by this kind of 1j phase may be attributed to pinning grain boundary and resulting in serrated grain boundary, as shown in Fig. 3 (b), Fig. 4 (a) and Fig. 5 (a). The former could prevent grain boundary sliding at elevated temperature, and the latter could inhibit crack propagation along grain boundaries. That is, grain boundary 1j with proper morphology is considered to be able to reinforce grain boundaries and improve notch toughness effectively at elevated temperature. However, further investigation of appropriate size, morphology and amount of grain boundary 1j in AEREX350 alloy is needed. Widmanstatten 1j precipitated within grains would lead to deterioration in ductility of superalloys[],IO-IZ]. Widrnanstatten 1j is weaker than y matrix, the net-like 1j platelets will be the propagated channels of cracks during deforming. Accordingly, an optimizing microstructure corresponding to excellent mechanical properties of AEREX350 fastener superalloy can be obtained employing an aging treatment selected by virtue of the TTT curves gained in this investigation.
4
O. 5 to 4 h. The temperature range corresponding to the minimum values of hardness curves was 9801040 ·C. 2) The precipitation behavior of 1j phase in AEREX350 superalloy can be explained by time-temperature-transformation (TTT) curves gained in this investigation. The shortest pregnant times of 1j phase precipitated at grain boundaries and within grains were 10 min and 30 min when aged at 1020 'C, respectively. 3) y' -Iree zones around 1j phase in samples aged 870'C and 1060'C for 4 hours showed that 1j phase precipitated in AEREX350 alloy at the expense of y' phase. This may explain the relationship between hardness and 1j phase: precipitation of 1j phase decreased the amount of y' phase so that the strengthening ability of AEREX350 alloy decreased. 4) The orientation relationship between the ordered hexagonal close-packed 1j phase with a DO Z4 structure and face-centered cubic y matrix in AEREX350 alloy was {llI}111 {000l}~, [01lJ111 [2IIoJ~.
The authors want to express their deep gratitude to CHEN Hui-xia , GAO Ling, WANG Chang and FENG Tian-you for help in experimental techniques. References: [IJ
[2J [3J
[4J
[5J [6J
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Conclusions [8J
The results presented in this paper are summarized as follows: 1) The hardness test result showed that the hardness was generally decreased first but increased with an increase in aging temperatures when samples aged at 780-1080'C for a certain time ranging from
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[9J [10J
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