The Intrinsic Relationship Between Microstructure Evolution and Thermal Fatigue Behavior of a Single-Crystal Cobalt-Base Superalloy

doi:10.1007/s40195-017-0646-8

Abstract

Abstract:

The intrinsic relationship between the microstructure evolution and thermal fatigue behavior of a single-crystal cobalt-base superalloy has been investigated. The thermal fatigue tests are performed cyclically between room temperature and 1050 °C using V-notch plate specimens. Three states of thermal fatigue specimens are selected: the as-cast, solutionized as well as aged states. The solution treatment is carried out at 1260 °C for 24 h, which results in the dissolution of most of interdendritic continuous primary carbides. The subsequent aging treatment is carried out at 1100 °C for 100 h after solution treatment, resulting in the precipitation of a profusion of chain- and point-like M ₂₃C₆ carbides in the matrix. The results indicate that the heat treatment can improve the thermal fatigue properties of the alloy and the effect of the solution treatment is more prominent than that of the aging treatment. The coarse and continuously distributed primary carbides in the as-cast state are changed into small and discontinuous distribution by heat treatment, which is the dominant factor in the improvement of thermal fatigue property. Additionally, the effect of oxidation behavior during thermal fatigue test on the thermal fatigue behavior is also studied.

Key words: Cobalt-base superalloys, Heat treatment, Thermal fatigue, Carbide, Oxidation

Wei-Min Gui, Hong-Yu Zhang, Min Yang, Tao Jin, Xiao-Feng Sun, Qi Zheng. The Intrinsic Relationship Between Microstructure Evolution and Thermal Fatigue Behavior of a Single-Crystal Cobalt-Base Superalloy[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(12): 1192-1200.

Figures/Tables 13

Table 1 Chemical composition of the test alloy (wt%)

Cr	Ni	W	C	Ta	Ti	Zr	Al	Co
25.5	10.7	7.81	0.42	0.34	0.19	0.16	0.97	Bal.

Fig. 1 Schematic diagram of the thermal fatigue specimen (mm)

Fig. 2 Optical micrographs showing the dendritic structures of the three states of the specimens: a the as-cast, b solutionized, c aged

Fig. 3 SEM micrographs showing microstructures of the three states of the specimens: a the as-cast, b solutionized, c aged

Fig. 4 Curves of the crack length as a function of cycle during thermal fatigue test for the specimens at the three states

Fig. 5 Crack initiation in three-state specimens after 40 cycles: a the as-cast, b solutionized, c aged

Fig. 6 Propagation morphologies near the crack tip in three-state specimens after 160 cycles: a the as-cast, b solutionized, c aged

Fig. 7 Propagation morphologies of thermal fatigue cracks in three-state specimens after 300 cycles: a the as-cast, b solutionized, c aged

Fig. 8 Microstructures, XRD pattern and EDS results of the oxidation products in the as-cast specimen after 300 cycles: a BSE micrograph of the crack surface, b BSE micrograph of the fracture surface, c XRD pattern, d-f EDS results of CoO, CoCr2O4 and Cr2WO6, respectively

Fig. 9 Microstructures of crack surface and fracture surface in the solutionized and aged specimens after 300 cycles: a, b solutionized specimen, c, d aged specimen

Fig. 10 a Morphology near the crack tip, b the corresponding EPMA map of the distribution of O element in the as-cast specimen

Fig. 11 Schematic illustration of the different initiation and propagation paths of thermal fatigue cracks for the three-state specimens: a the as-cast, b solutionized, c aged

Fig. 12 Schematic diagram for the formation process of oxidation products in the crack

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