Microstructural Instability of an Experimental Nickel-Based Single-Crystal Superalloy

doi:10.1007/s40195-020-01057-5

Abstract

Abstract:

Microstructural instability with the precipitation of topologically close-packed (TCP) phases of an experimental nickel-based single-crystal superalloy has been investigated. A significant amount of σ phases are distinguished in the interdendritic region of the as-cast samples after thermal exposure at 900 °C for 1000 h. The σ phases are preferentially precipitated at the periphery of coarse γ/γ′ eutectic, and their morphological evolution from needles to granules is observed. Microstructural analysis suggests that the local segregation of Cr and Ti at the periphery of coarse γ/γ′ eutectic accounts for the formation of σ phases in the as-cast samples. After heat treatment with low solution temperature and short holding time, the dendritic segregation of alloying elements (i.e., W, Re, Ti and Ta) and the volume fraction of γ′ phase in the interdendritic region are similar to that of the as-cast samples. However, no TCP phases are present in the interdendritic region of the heat-treated samples after thermal exposure, which is primarily ascribed to the elimination of local segregation of Cr and Ti near the coarse γ/γ′ eutectic. Moreover, small quantities of μ phases are precipitated in the secondary dendrite arm near the interdendritic region after thermal exposure, due to the increased volume fraction of γ′ phase and the concomitant enrichment of W and Re in the γ matrix.

Key words: Nickel-based single-crystal superalloy, Topologically close-packed (TCP) phases, Segregation, Eutectic

Bin Yin, Guang Xie, Xiangwei Jiang, Shaohua Zhang, Wei Zheng, Langhong Lou. Microstructural Instability of an Experimental Nickel-Based Single-Crystal Superalloy[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(10): 1433-1441.

Figures/Tables 13

Table 1 Nominal compositions of the experimental alloy (wt%)

Cr	Co	Al	Ti	W	Mo	Ta	Re	C	Ni
9	9	3.8	3.8	4.5	1	5.5	2	0.05	Bal.

Table 2 Heat treatment schedule for the experimental alloy

Experiment	Heat treatment schedule
Solution heat treatment	600 °C-1 h-1000 °C-1 h-1260 °C/AC
Aging heat treatment	1130 °C/4 h/AC + 870 °C/24 h/AC

Fig. 1 Scanning electron micrographs of a as-cast sample in b dendrite core, c and d interdendritic region

Fig. 2 Scanning electron micrographs of the sample after solution heat treatment a, b is magnified morphology of the white zone in a

Fig. 3 Scanning electron micrographs of the heat-treated sample: a dendrite core, b, c interdendritic region

Fig. 4 Segregation ratio of as-cast and heat-treated samples

Fig. 5 EPMA element maps of γ/γ′ eutectic in the as-cast sample

Fig. 6 EPMA element maps of γ/γ′ eutectic in the heat-treated sample

Fig. 7 Microstructures in the interdendritic region of the as-cast sample after thermal exposure at 900 °C for a 100 h, b 240 h, c 1000 h

Fig. 8 Morphological evolution of TCPs near the eutectic after thermal exposure at 900 °C for a 240 h, b 1000 h

Fig. 9 TEM micrograph and diffraction pattern associated with a, c σ phase in the as-cast sample and b, d μ phase in the heat-treated sample

Table 3 Compositions (wt%) of σ in the interdendritic region of the as-cast sample and μ in the secondary dendrite arm of the heat-treated sample determined by TEM with EDS

Phase	Cr	Co	W	Mo	Al	Ti	Ta	Re	Ni
σ	25.5	11.2	11.5	6.4	1.1	2.1	2.6	13.1	26.6
μ	11.9	9.4	21.8	3.8	1.3	1.5	2.9	17.7	30.3

Fig. 10 Microstructures of the heat-treated sample after thermal exposure at 900 °C for 1000 h in a dendrite core, b interdendritic region, c secondary dendrite arm

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