Correlation Between Phase Stability and Tensile Properties of the Ni-Based Superalloy MAR-M247

doi:10.1007/s40195-020-01139-4

Abstract

Abstract:

Phase stability and its effect on tensile properties of MAR-M247 alloy have been investigated during thermal exposure at 800-900 °C for up to 10,000 h. Detailed investigations reveal that the larger secondary γ′ phase has no obvious growth, but the smaller tertiary γ′ phase obviously coarsens and the coalescence occurs during thermal exposure at 850 °C and below. γ′ coarsening behavior is consistent with the description of Ostwald ripening theory before γ′ coalescence. Hf-rich blocky MC carbide shows excellent thermal stability, but Ta-rich script-type MC carbide gradually degenerates via reaction, MC + γ → M₂₃C₆ + γ′ and finally forms γ׳ film around MC and M₂₃C₆ carbides. With increasing thermal exposure time, the tensile strength decreases. The ductility first increases and then decreases during exposure at 800 °C, but it decreases continuously at 900 °C. In addition, the ductility keeps almost constant when the exposure time is longer than 5000 h.

Key words: Ni-based superalloy, Phase stability, Tensile properties, Fractural behavior

Linxu Li, Xiufang Gong, Changshuai Wang, Yunsheng Wu, Hongyao Yu, Haijun Su, Lanzhang Zhou. Correlation Between Phase Stability and Tensile Properties of the Ni-Based Superalloy MAR-M247[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(6): 872-884.

Figures/Tables 12

Fig. 1 Microstructural characteristics of MAR-M247 alloy after standard heat treatment: OM image of dendrite morphology a; SEM images of γ/γ′ eutectic b and carbides b, e in the interdendritic region, γ′ precipitates in the dendrite core region c and the typical grain boundary morphology d

Table 1 Composition of MC carbides in MAR-M247 alloy (at.%)

MC carbides	C	Ti	Hf	Ta	W	Cr
Script-type MC	56.2	13.8	6.5	19.1	3.4	1.0
Blocky MC	57.4	6.2	21.1	12.9	1.3	1.1

Fig. 2 γ′ characteristics after thermal exposure at 800 °C a-d and 850 °C e for different times: a 1000 h, b 3000 h, c 5000 h d, e 10,000 h

Fig. 3 γ′ characteristics after thermal exposure at 900 °C for a 1000 h, b 3000 h, c 5000 h d 10,000 h

Fig. 4 Curves of the tertiary γ′ size with thermal exposure time

Table 2 Coarsening rate constant k of some cast Ni-based superalloy (μm3/h)

Researchers	800 °C	850 °C
Sudbrack et al. [13]	1.1 × 10^-6	-
Hou et al. [24]	2.3 × 10^-6	5.8 × 10^-6
Yang et al. [25]	-	1.5 × 10^-4
Ou et al. [26]	1.8 × 10^-7	-

Fig. 5 SEM-BSE images of the carbide characteristics after thermal exposure for 10,000 h at a, b 800 °C, c, d, g, h, i 850 °C e, f 900 °C

Table 3 Composition of M23C6 carbide in MAR-M247 alloy (at.%)

C	Ti	Hf	Ta	W	Cr	Ni
26.4	0.3	0.1	0.2	1.3	59.2	12.5

Fig. 6 Grain boundary characteristics after thermal exposure for 10,000 h at a 800 °C, b, d 850 °C c, e, f 900 °C

Table 4 Composition of M6C carbide in MAR-M247 alloy (at.%)

C	Ti	Hf	Ta	W	Mo	Cr	Ni
17.4	0.6	1.2	1.9	55.8	4.2	8.7	10.2

Fig. 7 Tensile strength and ductility at 800 °C of samples after thermal exposure for different times

Fig. 8 Microstructure characteristics of fracture surface after standard heat treatment a, b and thermal exposure for 10,000 h at c, d 800 °C e, f 900 °C

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