1. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China 2. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
HUANG Taiwen,LU Jing,XU Yao,WANG Dong,ZHANG Jian,ZHANG Jiachen,ZHANG Jun,LIU Lin. Effects of Rhenium and Tantalum on Microstructural Stability of Hot-Corrosion Resistant Single Crystal Superalloys Aged at 900 ℃. Acta Metall Sin, 2019, 55(11): 1427-1436.
The development of gas turbines urgently requires the development of new single crystal superalloys with capacity to service for long time at higher temperature and under hot corrosion environment. In order to take into account both the strength and structural stability of the alloy, it is an effective way to increase the content of key strengthening elements Re and Ta reasonably, and control the mismatch of γ/γ' to delay the growth kinetics of γ'. In this work, the long-term thermal exposure (LTTE) experiments of single crystal superalloys with different Re and Ta contents at 900 ℃ were carried out. The evolution of morphology and size of γ', and the precipitation of topological close-packed (TCP) phase during 0~7500 h ageing process were quantitatively analyzed. The results show that the size and the coarsening rate of γ' phase decreases with the increase of Ta and Re content in 2Ta2Re, 5Ta0Re, 5Ta2Re, 8Ta0Re and 8Ta2Re alloys. The coarsening rates are 1.445×10-5, 1.569×10-5, 1.390×10-5, 1.465×10-5 and 1.384×10-5 μm3/h respectively. With the increase of Ta and Re content, the effective diffusion coefficient decreases and the diffusion activation energy increases, thus of the coarsening rate of precipitated phase decreases. After ageing for 2000 h, TCP phase was precipitated in turn in alloys containing Re, and the precipitation of TCP phase was more serious in alloys containing higher content of Ta. The interaction between Ta and Re affects the atomic distribution behavior of elements in γ and γ', in which 8Ta2Re alloy Ta enters the γ' phase to increase its lattice constant, and at the same time promotes the distribution of elements such as Re, W and Cr in the γ matrix, which results in more negative γ/γ' mismatch and promotes the precipitation of TCP phase.
Fund: National Key Research and Development Program No(2016YFB0701400);National Natural Science Foundation of China(51331005);National Natural Science Foundation of China(51631008)
Fig.1 Atomic distribution maps of elements in γ and γ′ phases of 2Ta2Re
Fig.2 Concentration distribution of elements in the region near γ/γ′ interface in 2Ta2Re
Fig.3 Partition ratios of alloying elements between γ and γ′ phases (kiγ/γ′) of the five alloys
Fig.4 Microstructure morphologies at dendritic core of 2Ta2Re (a1~a4), 5Ta0Re (b1~b4), 5Ta2Re (c1~c4), 8Ta0Re (d1~d4) and 8Ta2Re (e1~e4) alloys during long-term thermal exposure (LTTE) at 900 ℃ for 100 h (a1~e1), 1000 h (a2~e2), 2000 h (a3~e3) and 5000 h (a4~e4)
Alloy
100 h
200 h
500 h
1000 h
2000 h
5000 h
2Ta2Re
1.41±0.12
1.23±0.13
1.26±0.22
1.27±0.20
1.35±0.15
1.20±0.24
5Ta0Re
1.37±0.15
1.35±0.13
1.38±0.14
1.41±0.23
1.37±0.16
1.42±0.23
5Ta2Re
1.45±0.20
1.37±0.20
1.25±0.23
1.26±0.29
1.29±0.21
1.29±0.29
8Ta0Re
1.36±0.20
1.33±0.15
1.30±0.12
1.26±0.28
1.28±0.14
1.22±0.23
8Ta2Re
1.47±0.24
1.39±0.27
1.47±0.33
1.38±0.26
1.40±0.17
1.39±0.23
Table1 Feret Ratio of the γ' precipitates during LTTE at 900 ℃ of five alloys
Fig.5 Size of γ′ precipitates at different thermal exposure time (a) and coarsening rate of γ′ phase (b) for five alloys during LTTE at 900 ℃ (r—size of γ′)
Fig.6 Area fraction of TCP precipitates of 5Ta2Re and 8Ta2Re alloys after LTTE for different time
Fig.7 Morphologies of TCP phase precipitated after LTTE at 900 ℃ for different time(a) 5Ta2Re, 7500 h (b) 8Ta2Re, 2000 h (c) 8Ta2Re, 7500 h (d) 8Ta2Re, 10000 h
Alloy
aγ / nm
aγ' / nm
δ / %
2Ta2Re
0.3580
0.3584
0.1087
5Ta0Re
0.3578
0.3590
0.2956
5Ta2Re
0.3590
0.3588
-0.0730
8Ta0Re
0.3579
0.3594
0.4210
8Ta2Re
0.3598
0.3592
-0.1521
Table 2 Lattice parameter of γ and γ′ phases and γ/γ′ misfit at room temperature of five alloys after standard heat treatment
Alloy
γ' precipitates morphology
Correlation startup / h
Misfit / %
2Ta2Re
Nearly cuboidal → nearly round
1000
-0.0113
5Ta0Re
Nearly cuboidal → nearly cuboidal
-
0.1760
5Ta2Re
Cuboidal → nearly cuboidal → nearly round
500
-0.1873
8Ta0Re
Cuboidal → nearly cuboidal → nearly round
500
0.2997
8Ta2Re
Cuboidal → nearly cuboidal
1000
-0.2666
Table 3 A qualitative listing of the γ' precipitates morphology during LTTE at 900 ℃ and misfit of the five alloys
Element
/ (m2·s-1)
/ (kJ·mol-1)
Cr
3×10-6
170.7
Co
7.5×10-5
285.1
Mo
1.15×10-4
281.3
Re
8.2×10-7
255.0
W
8.0×10-6
264.0
Ni
1.9×10-4
284.0
Table 4 Pre-exponential factor () and diffusion activation energy () for solute atoms in Ni[23]
Alloy
/ (m2·s-1)
/ (kJ·mol-1)
2Ta2Re
9.98×10-6
368.63
5Ta0Re
1.15×10-5
369.83
5Ta2Re
8.67×10-6
378.76
8Ta0Re
9.29×10-6
385.05
8Ta2Re
7.66×10-6
387.89
Table 5 Calculated effective diffusivity () and diffusion activation energy () of five alloys
Alloy
Tendency
Tendency
Tendency
VTCP / %
STCP / %
2Ta2Re
1.78
No
0.954
No
-4.47
No
0.79
0.1
5Ta0Re
1.85
No
0.966
No
-4.29
No
0
0
5Ta2Re
1.95
No
0.973
No
0.73
Yes
2.42
1.8
8Ta0Re
2.05
No
0.986
Yes
3.44
Yes
0
0
8Ta2Re
2.18
No
0.993
Yes
5.58
Yes
4.28
5.6
Table 6 Values of 、、 and TCP phase volume fraction calculated by JMatPro，and measured value of TCP phase area fraction of five kinds of experimental alloys
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