Role of Ru on the Microstructural Evolution During Long-Term Aging of Ni-Based Single Crystal Superalloys

doi:10.1007/s40195-020-01110-3

Abstract

Abstract:

Two experimental alloys containing different contents of Ru were investigated to study the effect of Ru on the microstructural evolution during long-term thermal exposure. The increase in Ru promoted the formation of cubical, tiny, and even γ′ phase after full heat treatment. Moreover, the samples after full heat treatment were exposed at 1100 °C for different time. Based on the classical model by Lifshitz, Slyozov, and Wagner, the coarsening of γ′ phase of the alloy containing 2.5 and 3.5 wt.% Ru during the long-term aging was controlled by the interface reaction and diffusion, respectively. The γ/γ′ lattice misfit was more negative with the increment of Ru addition, which induced the formation of stable rafted γ′ phase in the alloy containing 3.5 wt.% Ru at the initiation of long-term aging. Besides, the increase in Ru reduced the diffusion coefficient, which could restrain the γ′ phase coarsening. The lower γ/γ′ lattice misfit of the alloy containing 2.5 wt.% Ru promoted the interface reaction, which induced the rapid coarsening of γ′ phase. Therefore, the increase in Ru improved the microstructural stability of the alloys. On the other hand, the raise of Ru induced “reverse partitioning” behavior, which was effective in suppressing the emergence of the topologically close-packed phase (TCP phase). The TCP phase occasionally occurred in the alloy containing 2.5 wt.% Ru, which was attributed to the high temperature and the supersaturation of the γ matrix. Moreover, the TCP phase was determined as μ phase, which had a high concentration of Co, Re, Mo, and W. A sketch map describing the evolution of the TCP phase was also constructed.

Key words: Ni-based superalloys, Ruthenium, Coarsening, TCP phase

Wei Song, Xin-Guang Wang, Jin-Guo Li, Ye-Shun Huang, Jie Meng, Yan-Hong Yang, Jin-Lai Liu, Ji-De Liu, Yi-Zhou Zhou, Xiao-Feng Sun. Role of Ru on the Microstructural Evolution During Long-Term Aging of Ni-Based Single Crystal Superalloys[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(12): 1689-1698.

Figures/Tables 13

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

Alloy	Co	Cr	W+Mo+Ta	Re	Ru	Al	Hf	Ni
2.5 Ru	11.7	4	15.4	5	2.5	6.2	0.1	Bal
3.5 Ru	11.7	4	15.4	5	3.5	6.2	0.1	Bal

Fig. 1 OM images of the as-cast microstructures of the two alloys: a alloy 2.5 Ru, b alloy 3.5 Ru

Fig. 2 Microstructures after full heat treatment and average size of the γ′ phase: a, b alloy 2.5 Ru, c, d alloy 3.5 Ru [20]

Fig. 3 The γ/γ′ partitioning ratio of each alloying element in the two alloys

Fig. 4 SEM images of alloy 2.5 Ru after long-term aging at 1100 °C: a 100 h, b 200 h, c 500 h, d 1000 h

Fig. 5 SEM images of alloy 3.5 Ru after long-term aging at 1100 °C: a 100 h, b 200 h, c 500 h, d 1000 h

Fig. 6 Average size of the γ′ phase after long-term aging at 1100 °C with different time: a alloy 2.5 Ru, b alloy 3.5 Ru

Fig. 7 Fitting straight line of the second and third power of γ′ size with time: a third power, b second power

Fig. 8 Microstructures of the two alloys after long-term aging at 1100 °C: a 100 h, b 200 h, c 500 h , d 1000 h of alloy 2.5 Ru; e 100 h, f 200 h, g 500 h, h 1000 h of alloy 3.5 Ru

Fig. 9 TEM images and corresponding selected area diffraction patterns (insets) of the μ phase of alloy 2.5 Ru after long-term aging of 1000 h at 1100 °C: a rod-like, b bulk-like

Table 2 Chemical compositions obtained by EPMA (at.%) and the accordingly associated partitioning ratios of the two phases after 1000-h aging at 1100 °C

Alloy		Re	Ru	Co	Cr	W	Mo
2.5 Ru	γ	3.4±0.3	2.7±0.1	18.1±0.3	6.2±0.3	2.1±0.1	0.9±0.4
	γ′	0.9±0.1	1.3±0.2	11.9±0.2	2.7±0.2	2.0±0.2	0.4±0.2
	K_i	3.7	2.1	1.5	2.3	1.0	2.1
3.5 Ru	γ	3.2±0.2	3.1±0.3	21.9±0.5	9.6±0.4	3.2±0.1	1.8±0.3
	γ′	1.1±0.2	1.7±0.1	14.6±0.3	4.7±0.3	2.2±0.1	0.8±0.5
	K_i	2.9	1.8	1.5	2.1	1.2	2.1

Table 3 Compositions of TCP phase in the alloy 2.5 Ru (wt.%)

Cr	Co	W	Mo	Ru	Re	Ta	Ni
6.3	11.4	19.1	6.3	2.7	33.4	5.4	Bal

Fig. 10 Sketch map of possible microstructural evolution of the μ phase from 500 to 1000 h in alloy 2.5 Ru

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