Precipitation Behavior of W-Rich Phases in a High W-Containing Ni-Based Superalloys K416B
ZHU Yuping1,2, Naicheng SHENG1,3(), XIE Jun1, WANG Zhenjiang1, XUN Shuling1, YU Jinjiang1, LI Jinguo1, YANG Lin2, HOU Guichen1, ZHOU Yizhou1, SUN Xiaofeng1
1.Shi -Changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Technology, Shenyang University of Technology, Shenyang 110870, China 3.Innovation Academy for Light-Duty Gas Turbine, Chinese Academy of Sciences, Beijing 100190, China
Cite this article:
ZHU Yuping, Naicheng SHENG, XIE Jun, WANG Zhenjiang, XUN Shuling, YU Jinjiang, LI Jinguo, YANG Lin, HOU Guichen, ZHOU Yizhou, SUN Xiaofeng. Precipitation Behavior of W-Rich Phases in a High W-Containing Ni-Based Superalloys K416B. Acta Metall Sin, 2021, 57(2): 215-223.
The high temperature strength of Ni-based cast superalloys can be significantly improved by adding tungsten (W), a solid solution strengthening element. Hence, superalloys with high W content have been developed as key materials for the preparation of aircraft engine blades. However, the high segregation coefficient of W results in inconsistent composition and microstructure during the solidification process, which can be difficult to eliminate via heat treatment leading to deteriorated mechanical properties. The Ni-based superalloy K416B contains approximately 16.5%W (mass fraction) and exhibits a high tendency to precipitate W-rich phases, such as the α-W and M6C phases, which not only consume a large amount of W in the matrix but also reduce the solid solution strengthening ability of the alloy. W-rich precipitates also become the origin and propagation paths of cracks during stress-rupture testing. Much research on high W-containing, Ni-based superalloys has focused on the effects of W content on W-rich phase formation and mechanical properties. However, the roles of casting temperature and cooling rate on the formation of the W-rich phase are still unclear. In this work, five groups of K416B alloy test bars with the same composition were prepared with different processes. Three casting temperatures were chosen, and the cooling rate was controlled by burying sand in the thick shell and single shell, respectively. The relationship between the precipitation behavior of the W-rich phase in the K416B alloy and casting temperature, and solidification rate under different casting processes were analyzed using SEM and EDS. When the casting temperature is lowered from 1500°C to 1450°C, the grain size is significantly reduced. Results show massive α-W phases in the residual eutectic of the alloy at different casting temperatures, and the morphology of the α-W phase show few differences. The larger M6C phase in the alloy exists with residual eutectic, and the small M6C phase is embedded at the edge of the residual eutectic. At a high solidification rate, the precipitation of the W-rich phase is inhibited, which is primarily manifested by the decreased number and size of the W-rich phase in the alloy. When casting high W-containing Ni-based superalloys, choosing an appropriate casting temperature and adopting an appropriate heat preservation system to accelerate the cooling rate during solidification will affect the precipitation and transformation of W-rich phases, and optimize the properties of the alloy.
Fund: National Natural Science Foundation of China(51971214);Talents Program of Chinese Academy of Sciences(51771191);Innovation Science Foundation from Innovation Academy for Light-Duty Gas Turbine, Chinese Academy of Sciences(CXYJJ20-QN-02)
Fig.1 Microstructures of the K416B alloys with various casting temperatures
Fig.2 Typical microstructure of the K416B alloy prepared by single shell at a casting temperature of 1550oC
Phase
C
W
Nb
Hf
Ni
MC
16.9
11.7
37.5
24.9
Bal.
M6C
9.2
70.9
0
0
Bal.
α-W
5.4
91.2
0
0
Bal.
Table 1 EDS results of typical precipitates
Fig.3 DCS cooling curve of K416B alloy (Tγ', Tγ/γ', TMC, and TL indicate γ' solution temperature, residual eutectic solution temperature, MC carbide solution temperature, and liquidus temperature, respectively)
Fig.4 Microstructure of the K416B alloy after aging at 1320oC for 30 min and quenching
Fig.5 Low (a-c) and high (d-f) SEM images showing typical microstructures of the K416B alloys cast at various temperatures
Temperature / oC
Residual eutectic area fraction / %
Secondary dendrite arm spacing / μm
1550
10.92
16.19
1500
12.14
15.48
1450
14.22
11.31
Table 2 Area fractions of residual eutectic and secondary dendrite arm spacing of the K416B alloys at various casting temperatures
Fig.6 Low (a, b) and high (c, d) magnified microstructures of the K416B alloy cooled by thick shell buried sand (a, c) and single shell (b, d) at a casting temperature of 1550oC
Process
Area fraction of residual eutectic
Area fraction of W-rich phase
Thick shell buried sand
16.0
1.0
Single shell
23.5
8.0
Table 3 Area fractions of residual eutectic and W-rich phase of K416B alloy cooled by thick shell buried sand and single shell at a casting temperature of 1550oC
Fig.7 Relationship between homogeneous temperature and mold shell mass at a casting temperature of 1550oC
Fig.8 Relationship between homogeneous temperature and casting temperature when the mold shell mass is 1.5 kg
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