High-Temperature Structural Stabilities of Ni-Based Single-Crystal Superalloys Ni-Co-Cr-Mo-W-Al-Ti-Ta with Varying Co Contents

doi:10.1007/s40195-017-0678-0

Abstract

Abstract:

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glue-atom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like (\( \overline{\text{Ni}} \)), γ′-forming Cr-like (\( \overline{\text{Cr}}^{{\gamma^{\prime } }} \)) and γ-forming Cr-like (\( \overline{\text{Cr}}^{\gamma } \)), the optimal formula for single-crystal superalloys is established [Al-\( \overline{\text{Ni}} \) ₁₂](Al₁ \( \overline{\text{Cr}}^{{\gamma^{\prime } }}_{0.5} \overline{\text{Cr}}^{\gamma }_{1.5} \)). The Co substitutions for Ni at the shell sites are conducted on the basis of the first-generation single-crystal superalloy AM3, formulated as [Al-Ni_12-_x Co _x ](Al₁Ti_0.25Ta_0.25Cr₁W_0.25Mo_0.25), with x = 1.5, 1.75, 2 and 2.5 (the corresponding weight percents of Co are 9.43, 11.0, 12.57 and 15.71, respectively). The 900 °C long-term aging follows the Lifshitz-Slyozov-Wagner theory (LSW theory), and the Co content does not have noticeable influence on the coarsening rate of γ′. The microstructure and creep behavior of the four (001) single-crystal alloys are investigated. The creep rupture lifetime is reduced as Co increases. The alloy with the lowest Co (9.43 Co) shows the longest lifetime of about 350 h at 1050 °C/120 MPa, and all the samples show N-type rafting after creep tests.

Key words: Nickel-based single-crystal superalloys, Cluster-plus-glue-atom model, Composition formula, Co

Yu Zhang, Qing Wang, Hong-Gang Dong, Chuang Dong, Hong-Yu Zhang, Xiao-Feng Sun. High-Temperature Structural Stabilities of Ni-Based Single-Crystal Superalloys Ni-Co-Cr-Mo-W-Al-Ti-Ta with Varying Co Contents[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(2): 127-133.

Figures/Tables 7

Fig. 1 [Al-Ni12] cluster in L12-γ′. The same cluster also applies in face-centered cubic Ni-Al solid solution γ, as γ′ is an ordered superstructure of γ

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

Alloy names	[Al-Ni_12-_x Co _x ](Al₁Ti_0.25Ta_0.25Cr₁ W_0.25Mo_0.25)		Al	Co	Cr	Ti	Ta	W	Mo	Ni
9.43 Co	[Al-Ni_10.5Co_1.5](Al₁Ti_0.25Ta_0.25Cr₁W_0.25Mo_0.25) x = 1.5	Nominal	5.75	9.43	5.54	1.28	4.82	4.90	2.56	Bal.
		Actual	5.57	9.36	5.43	1.17	4.68	4.91	2.64
		A error	- 0.18	- 0.07	- 0.11	- 0.11	- 0.14	0.01	0.08
		R error%	- 3.21	- 0.71	- 2.07	- 8.31	- 2.98	0.21	3.22
11 Co	[Al-Ni_10.25Co_1.75](Al₁Ti_0.25Ta_0.25Cr₁W_0.25Mo_0.25) x = 1.75	Nominal	5.75	11.00	5.54	1.28	4.82	4.90	2.56
		Actual	5.54	10.92	5.41	1.17	4.70	4.89	2.64
		A error	- 0.21	- 0.08	- 0.13	- 0.11	- 0.12	- 0.01	0.08
		R error%	- 3.72	- 0.70	- 2.42	- 8.31	- 2.56	- 0.20	3.23
12.57 Co	[Al-Ni₁₀Co₂](Al₁Ti_0.25Ta_0.25Cr₁W_0.25Mo_0.25) x = 2	Nominal	5.75	12.57	5.54	1.28	4.82	4.90	2.56
		Actual	5.54	12.48	5.38	1.17	4.72	4.88	2.64
		A error	- 0.21	- 0.09	- 0.16	- 0.11	- 0.10	- 0.02	0.08
		R error%	- 3.71	- 0.69	- 2.96	- 8.30	- 2.14	- 0.39	3.23
15.71 Co	[Al-Ni_9.5Co_2.5](Al₁Ti_0.25Ta_0.25Cr₁W_0.25Mo_0.25) x = 2.5	Nominal	5.75	15.71	5.54	1.28	4.82	4.90	2.56
		Actual	5.52	15.68	5.34	1.08	4.77	4.72	2.72
		A error	- 0.23	- 0.03	- 0.20	- 0.20	- 0.05	- 0.18	0.16
		R error%	- 4.05	- 0.17	- 3.67	- 15.34	- 1.09	- 3.65	6.37

Fig. 2 Microstructures of the alloys 9.43 Co, 11 Co, 12.57 Co and 15.71 Co after standard heat treatment and long-term aging: a, e, i, m for alloys 9.43 Co, 11 Co, 12.57 Co and 15.71 Co, respectively, after standard heat treatment; b, f, j, n for alloys 9.43 Co, 11 Co, 12.57 Co and 15.71 Co, respectively, after 900 °C/300 h; c, g, k, o for alloys 9.43 Co, 11 Co, 12.57 Co and 15.71 Co, respectively, after 900 °C/500 h; d, h, l, p for alloys 9.43 Co, 11 Co, 12.57 Co and 15.71 Co, respectively, after 900 °C/1000 h

Fig. 3 Relationships between the mean particle diameter (a 0 being the initial particle diameter and a t being the instantaneous on) at time during aging at 900 °C

Fig. 4 Creep rupture lifetimes as a function of Co contents

Fig. 5 The [center-shell] interaction and the [glue-shell] interaction about 9.43 Co, the value of the interaction means the value of enthalpies of mixing (kJ/mol) [33]

Fig. 6 Surface morphologies on longitudinal cross section after creep rupture (at 1050 °C/120 MPa): a 9.43 Co, b 11 Co, c 12.57 Co, d 15.71 Co

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