Effect of thermal exposure on the microstructure and creep properties of a fourth-generation Ni-based single crystal superalloy

doi:10.1016/j.jmst.2020.07.008

Abstract

Abstract:

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickel-based single crystal superalloy was investigated. The thermal exposure of samples after the full heat treatment was carried out at 1000 °C, 1100 °C and 1140 °C for 100 h and 200 h. The γ′ coarsening, γ′ rafting and γ channel widening were observed in samples after thermal exposure. When the thermal exposure time was constant, the morphology of γ′ phase in the alloy evolved significantly with increasing aging temperature. The interfacial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature. When the higher exposure temperature was used, enlargement of the defect pores was observed in samples, the microcracks were more likely to initiate and propagate at the corner of these pores. After aging at 1000 °C for 100 h, the creep life at 1140 °C/137 MPa was slightly longer than that of heat-treated sample, which could be attributed to the slightly coarsened γ′ phase, homogenization of refractor elements. In contrast, the creep life of sample exposed at 1140 °C for 100 h was greatly decreased. The decrease of creep life was dominated by the rafting of γ′ phase, the irregular interfacial dislocation networks as well as the enlargement of homogenization pores.

Key words: Ni-based single crystal superalloys, Thermal exposure, Microstructural evolution, Creep properties, Porosity

Yeshun Huang, Xinguang Wang, Chuanyong Cui, Zihao Tan, Jinguo Li, Yanhong Yang, Jinlai Liu, Yizhou Zhou, Xiaofeng Sun. Effect of thermal exposure on the microstructure and creep properties of a fourth-generation Ni-based single crystal superalloy[J]. J. Mater. Sci. Technol., 2021, 69: 180-187.

Figures/Tables 14

Table 1 Chemical compositions (wt.%) of the experimental alloys.

	Cr	Co	Al	Mo + W + Ta	Re	Ru	Ni
Nominal	4	12	5.9	15	5.4	3	Bal.
Measured	3.97	12.02	5.81	15.03	5.44	3.01	Bal.

Fig. 1. Photograph of sample for creep test.

Fig. 2. (a) SEM micrograph of sample after standard heat treatment; (b) the average size and distribution of the γ′ precipitate in sample after standard heat treatment.

Fig. 3. Microstructure of samples exposed at different conditions: (a) 1000 °C, 100 h; (b) 1000 °C, 200 h; (c) 1100 °C, 100 h; (d) 1100 °C, 200 h; (e) 1140 °C, 100 h; (f) 1140 °C, 200 h.

Fig. 4. The creep life of samples at 1140 °C/137 MPa after full heat treatment and thermal exposure at different temperature for 100 h.

Table 2 Mechanical properties of the sample of different conditions.

Samples	Heat-treated	1000 °C/100 h	1100 °C/100 h	1140 °C/100 h
Creep life (h)	124	144	80	31

Fig. 5. The γ/γ′ microstructure in samples aged at different temperatures for 100 h after creep rupture at 1140 °C/137 MPa. (a) 1000 °C; (b) 1100 °C; (c) 1140 °C.

Fig. 6. The dislocation configurations of creep rupture samples after (a) full heat treatment and after thermal exposure at different temperatures for 100 h: (b) 1000 °C; (c) 1100 °C; (d) 1140 °C.

Fig. 7. SEM micrographs of aged samples ruptured after creep deformation at 1140 °C/137 MPa: (a) fracture surface, (b) distribution of the pores, enlarged images of (c, d) homogenization pores and (e, f) creep pores.

Fig. 8. Schematic illustration of initiation and propagation of microcracks at the corner of pores.

Fig. 9. The homogenization pores in samples after thermal exposure at different temperature for 100 h: (a) 1000 °C, (b) 1100 °C, (c) 1140 °C.

Fig. 10. Schematic shown the evolution of microstructure during thermal exposure.

Fig. 11. The model describing the process of γ′ forming raft.

Fig. 12. Schematic illustration of the morphological evolution of the interfacial dislocation networks in samples aged with different temperatures for 100 h ruptured after creep deformation: (i) the full heat treatment; (ii) 1000 °C; (iii) 1100 °C; (iv) 1140 °C.

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