Thermal-Stress Coupling Effect on Microstructure Evolution of a Fourth-Generation Nickel-Based Single-Crystal Superalloy at 1100oC
XU Jinghui1, LI Longfei1(), LIU Xingang2, LI Hui2, FENG Qiang1
1.Beijing Innovation Center for Materials Genome Engineering, State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China 2.Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
XU Jinghui, LI Longfei, LIU Xingang, LI Hui, FENG Qiang. Thermal-Stress Coupling Effect on Microstructure Evolution of a Fourth-Generation Nickel-Based Single-Crystal Superalloy at 1100oC. Acta Metall Sin, 2021, 57(2): 205-214.
The mechanism of microstructure evolution and its effect on the mechanical properties of nickel-based single-crystal superalloys during creep at high temperatures and low stresses are critical to the development of advanced single-crystal superalloys for aeroengines with high thrust: weight ratios. In this work, the microstructural evolution of a fourth-generation nickel-based single-crystal superalloy during creep at 1100oC for 200 h at various stress levels was investigated using a specially designed sample with variable cross-sections, with the aim of obtaining different applied stresses synchronously on a single sample. The effects of applied stress on γ/γ' microstructure, interfacial dislocation configuration, alloy element partitioning behavior, and lattice misfit of γ/γ' phases of the used single-crystal superalloy were also studied, as were the effects on room temperature Vickers hardness. The results indicated that the typical rafting microstructure was formed during creep over the 200 h period at 1100oC under various stress levels. With increasing applied stress, the volume fraction and rafted thickness of the γ' phase gradually decreased, while the rafting degree of the γ' phase and the channel width of the γ phase gradually increased. A dense interfacial dislocation network was formed at the γ/γ' interface, and interfacial dislocation spacing decreased with increasing applied stress. Simultaneously, increased partitioning of solution-strengthening elements Re, Mo, and Cr to the γ phase and increased partitioning of γ'-strengthening element Ta to the γ' phase resulted in a larger absolute value of γ/γ' lattice misfit at higher stress. In addition to the decreases in volume fraction and rafted thickness of the γ' phase and the increase in channel width of the γ phase, another important factor in the strength decline of the single-crystal superalloy was the pile-up of dislocations at bent γ/γ' interface boundaries, mainly caused by the dissolution of the γ' phase and promotion of dislocation shear into the γ' phase. This work provides a basis for quickly establishing the relationship between creep conditions and microstructure evolution of nickel-based single-crystal superalloys.
Fig.1 Schematic of the variable section creep (VSC) specimens (unit: mm)
Fig.2 Microstructure of the nickel-based single-crystal superalloy after the standard heat treatment
Fig.3 Creep curves of the nickel-based single-crystal superalloy at 1100oC under 130 MPa
Fig.4 SEM images of longitudinal section at different zones of the nickel-based single-crystal superalloy after creep rupture at 1100oC under 130 MPa
Fig.5 SEM images of dendrite region at cross sections of the nickel-based single-crystal superalloy after creep for 200 h at 1100oC and various stresses
Specimen
Vf / %
Ω
D / nm
W / nm
SHT
68.0±1.3
0
356±18
57±11
VSC 43 MPa, 200 h
58.5±1.7
0.284±0.061
518±15
442±45
VSC 54 MPa, 200 h
57.9±0.8
0.334±0.048
512±26
453±25
VSC 71 MPa, 200 h
56.7±1.4
0.518±0.032
484±35
481±32
VSC 96 MPa, 200 h
55.9±1.5
0.602±0.053
458±28
488±21
Creep rupture 130 MPa, 231 h
49.7±1.1
0.403±0.037
365±46
583±33
Table 1 Quantitative statistics of microstructure parameters of the nickel-based single-crystal superalloy after various thermal-stress coupling conditions
Fig.6 SEM images of dendrite region at longitudinal sections of the nickel-based single-crystal superalloy after creep for 200 h at 1100oC and various stresses
Fig.7 Typical dislocation configurations of the nickel-based single-crystal superalloy after creep for 200 h at 1100oC and 43 MPa (a), 54 MPa (b), 71 MPa (c), and 96 MPa (d), and interfacial dislocation space (e)
Fig.8 Synchrotro radiation XRD spectrum and (400) peak splitting results of the nickel-based single-crystal superalloy after creep for 200 h at 1100oC under 96 MPa
Specimen
aγ
aγ'
δ
SHT
0.3576
0.3568
-0.22
43 MPa
0.3583
0.3573
-0.26
54 MPa
0.3589
0.3577
-0.33
71 MPa
0.3596
0.3582
-0.37
96 MPa
0.3608
0.3589
-0.42
Table 2 Changing of γ/γ' lattice parameter and misfit of the nickel-based single-crystal superalloy after various thermal-stress coupling conditions
Fig.9 Element partitioning coefficients of the nickel-based single-crystal superalloy after creep for 200 h at 1100oC and different stresses
Fig.10 Room temperature Vickers-hardness of the nickel-based single-crystal superalloy after creep for 200 h at 1100oC and different stresses
Fig.11 Lattice parameters (a) and lattice misfits (b) of γ/γ' phases calculated using γ/γ' compositions and experimental results of the nickel-based single-crystal superalloy after creep for 200 h at 1100oC and various stresses
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