As-Cast Microstructure Characteristic and Homogenization of a Newly Developed Hard-Deformed Ni-Based Superalloy GH4975
XIANG Xuemei, JIANG He(), DONG Jianxin, YAO Zhihao
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
XIANG Xuemei, JIANG He, DONG Jianxin, YAO Zhihao. As-Cast Microstructure Characteristic and Homogenization of a Newly Developed Hard-Deformed Ni-Based Superalloy GH4975. Acta Metall Sin, 2020, 56(7): 988-996.
Alloy GH4975 is a newly developed hard-deformed Ni-based superalloy which can keep high performance at elevated temperatures. And it is expected to be applied above 850 ℃. The as-cast microstructure, hot deformation of as-cast alloy, and the microstructural evolution during homogenization of alloy GH4975 were investigated utilizing a combination of FESEM, EBSD and extractive phase analysis. The results show that the γ′ phase, primary MC carbide and eutectic phase are the main precipitates in the as-cast alloy. Alloying elements Ti, Nb and W exhibit severe microsegregation during solidification. Cracking phenomenon can be observed in the hot-deformed samples of as-cast alloy due to the incoordination deformation between matrix and the MC carbide, primary coarse γ′ phase and eutectic phase. Microsegregation of alloying elements is eliminated after heat treated at 1180 ℃ for 50 h. Furthermore, besides of the redissolution of eutectic phase, the morphologies and size of MC carbide also evolved during homogenization process. Thermoplasticity and deformability can be improved obviously after homogenization due to improvement of the coordinated deformation capacity of MC carbide and strengthening phase.
Fig.1 OM image of dendritic morphology in the as-cast GH4975 alloy (a), OM (b) and SEM (c) images of precipitates in the interdendritic region, and XRD spectrum of precipitates powder acquired from extraction phase analysis (d)
Fig.2 SEM images of γ′ phase at dendritic region (a) and interdendritic region (b) in as-cast GH4975 alloy
Fig.3 DSC heating curve of as-cast GH4975 alloy
Fig.4 OM image of cracks (a) and EBSD image of intergranular cracks (b) in as-cast GH4975 samples after hot deformation (1160 ℃, 30%, 0.1 s-1)
Fig.5 The morphologies of MC carbide in the as-cast GH4975 samples after hot deformation (1160 ℃, 30%, 0.1 s-1) (a) cracks caused by MC carbide (b) cracked MC carbide
Fig.6 Cracks propagated along the eutectic phase and primary coarse γ′ phase (a) and cracks propagated along the primary coarse γ′ phase (b) in as-cast GH4975 samples after hot deformation (1160 ℃, 30%, 0.1 s-1)
Element
5 h
10 h
20 h
50 h
Ti
0.38
0.25
0.20
0.15
Nb
0.32
0.23
0.21
0.16
W
0.83
0.68
0.53
0.27
Table 1 Residual segregation indexes of GH4975 alloy after homogenization at 1180 ℃ for different time
Fig.7 Evolutions of dendritic morphology of GH4975 alloy during homogenization at 1180 ℃ for 5 h (a), 10 h (b), 20 h (c) and 50 h (d)
Fig.8 Evolutions of precipitates of GH4975 alloy during homogenization at 1180 ℃ for 5 h (a), 10 h (b), 20 h (c) and 50 h (d)
Fig.9 SEM images of γ′ phase of GH4975 alloy sample homogenized at 1180 ℃ for 5 h (a) and 50 h (b)
Fig.10 Low (a) and locally high (b) magnification SEM images of MC carbide in the GH4975 samples after homogenized at 1180 ℃ for 10 h (1—fine γ′ phase region, 2—cubic shape γ′ phase in the interdendritic region, 3—spherical shape γ′ phase in the dendritic core region)
Fig.11 True stress-true strain curves of hot-deformed (1200 ℃, 50%, 0.1 s-1) GH4975 alloy under different homogenizations (Insets show the macro-morphologies of as-cast and homogenized samples)
Fig.12 EBSD images showing recrystallization of hot-deformed (1200 ℃, 50%, 0.1 s-1) GH4975 samples under different homogenizations at 1180 ℃ for 5 h (a), 10 h (b), 20 h (c) and 50 h (d)
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