Microstructure-property Evolution in a Low-rhenium Second-generation Single Crystal Superalloy Following Long-term Aging

doi:10.11901/1005.3093.2025.254

Chinese Journal of Materials Research

2026, Vol. 40

Issue (4): 241-253 DOI: 10.11901/1005.3093.2025.254

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Microstructure-property Evolution in a Low-rhenium Second-generation Single Crystal Superalloy Following Long-term Aging

FU Yundi¹^,², SHEN Jian²(

), Huang Yaqi², LU Yuzhang², WANG Dong²

^1.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
^2.Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Cite this article:

FU Yundi, SHEN Jian, Huang Yaqi, LU Yuzhang, WANG Dong. Microstructure-property Evolution in a Low-rhenium Second-generation Single Crystal Superalloy Following Long-term Aging. Chinese Journal of Materials Research, 2026, 40(4): 241-253.

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Abstract

Second-generation single-crystal superalloys, developed through the incorporation of approximately 3%Re into first-generation alloys, demonstrate substantially enhanced high-temperature capabilities and have become critical materials for modern aero-engine turbine blades. However, as a strategic rare element, the addition of merely 1%Re doubles the casting cost of single-crystal superalloys while excessive Re content readily induces precipitation of topologically close-packed (TCP) phases that compromise microstructural stability. Therefore, it has become a critical challenge in the present to not only reduce production costs, but also maintain the microstructure stability, while avoiding sacrificing high-temperature mechanical properties. To address this challenge, therefore, a ramp-step heat treatment procedure for low-Re second-generation nickel-based single-crystal superalloys was proposed. Then the effect of long-term aging at 900 ^oC for different durations on the evolution of microstructure and the high-temperature durability performance of the alloy was assessed by means of differential scanning calorimetry (DSC) combined with metallographic analysis. Results revealed that after being aged for 3000 h at 900 ^oC, the γ′-precipitates were coarsened and coalesced with progressive size enlargement, along with precipitation of M₆C carbides in the absence of TCP phase formation or γ′ rafting phenomena, indicating excellent structural stability. Observations for the fractured alloy after durable strength performance tests revealed that a dense γ/γ′ dislocation network was formed in the alloy and shearing super-dislocations a<101> and a<010> emerged within the γ′-phase. Notably, stress rupture properties after 500 h and 1000 h aging at 900 ^oC remain equivalent to the as heat-treated ones. Based on these findings, this work establishes a novel heat treatment procedure for second-generation superalloys while elucidating microstructural stabilization mechanisms and revealing competitive precipitation behavior between M₆C carbides and TCP phases.

Key words: metallic materials single crystal superalloy long-term aging γ′ precipitates microstructure stability stress rupture property

Received: 18 August 2025

ZTFLH:

TG132.32

Fund: National Key Research and Development Program of China(2021YFA1600603);National Key Research and Development Program of China(2021YFB3702900);National Key Research and Development Program of China(2022YFB3705000);Fund of the State Key Laboratory of Solidification Processing in NWPU(SKLSP202402)

Corresponding Authors: SHEN Jian, Tel: 13804984964, E-mail: shenjian@imr.ac.cn

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	FU Yundi
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	Huang Yaqi
	LU Yuzhang
	WANG Dong

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https://www.cjmr.org/EN/10.11901/1005.3093.2025.254 OR https://www.cjmr.org/EN/Y2026/V40/I4/241

Table 1 Nominal composition of the experimental alloy (mass fraction, %)

Fig.1 As-cast microstructure of the DD26G single crystal alloy at cross section (a) and vertical section (b)

Fig.2 DSC heating (a) and cooling (b) curves of the as-cast specimen

Fig.3 Microstructure images of the experimental DD26G alloy after different solution treatment temperatures at 1300 ^oC (a), 1320 ^oC (b), 1340 ^oC (c)

Fig.4 Theme of solution treatment (a) and microstructure images of DD26G alloy after solution treatment at 1260 ^oC/2 h (b), 1270 ^oC/2 h (c), 1280 ^oC/2 h (d), 1290 ^oC/2 h (e) and 1300 ^oC/2 h (f)

Fig.5 Microstructural morphology of γ/γ′ phases after aging 1120 ^oC/2 h (a), 1120 ^oC/4 h (b), 1150 ^oC/4 h (c), 1120 ^oC/2 h, AC + 870 ^oC/24 h, AC (d), 1120 ^oC/4 h, AC + 870 ^oC/24 h, AC (e), 1150 ^oC/4 h, AC + 870 ^oC/24 h, AC (f), statistical results of γ′ precipitates under the aging regime of 1120 ^oC/4 h, AC+ 870 ^oC/24 h, AC (g)

Parameter Type	C	Cr	Co	W	Mo	Nb	Al	Ti	Re	Ni
N_vi	6.66	4.66	1.66	4.66	4.66	5.66	7.66	6.66	3.66	0.66
M_di	-	1.142	0.777	1.655	1.55	2.117	1.9	2.271	1.267	0.717
$N v ¯$ = 2.208, $M d ¯$ = 0.967

Table 2

N v ¯

and the

M d ¯

of DD26G

Fig.6 Morphology of the γ′ phase in the dendritic core of DD26G after long-term aging at 900 ^oC for 0 h (a), 300 h (b), 500 h (c), 1000 h (d), 2000 h (e), and 3000 h (f)

Fig.7 Size and volume fraction of γ′ phase at the dendritic core of DD26G alloy after long-term aging at 900 ^oC for different durations

Fig.8 Morphologies (a, b) and volume fraction (c) of carbide in heat treatment state (a) and long-term aging for 3000 h (b)

Table 3 Compositional results of the precipitated phases

Fig.9 EDS mapping results of the precipitated phases in 900 ^oC/3000 h specimens

Fig.10 TEM (a) and selected area electron diffraction (SAED) (b) analysis results of the precipitate phases in 900 ^oC/3000 h specimens

Fig.11 M₆C carbide volume fraction in the long-term aging process of DD26G alloy

Fig.12 Evolution of stress rupture performance in DD26G alloy during long-term aging under 975 ^oC/255 MPa

Fig.13 Comparative analysis of stress rupture strength in representative single crystal superalloys

Fig.14 Macroscopic fracture surfaces of the alloy after stress rupture testing (a, b) fully heat-treated condition, (c, d) long-term aged for 500 h, (e, f) long-term aged for 1000 h

Fig.15 SEM micrographs of γ/γ′ phase structures in stress-ruptured specimens: (a-c) fully heat-treated alloy vs. (d-f) 500 h and (g-i) 1000 h long-term aged alloy at distances of 0.5 mm (a, d), 2 mm (b, e), and 6 mm (c, f) from fracture surface

Table 4 Degree of topological inversion in γ/γ′ microstructures of DD26G alloy after stress rupture testing: fully heat-treated condition versus 500 h and 1000 h long-term aged condition

Fig.16 TEM micrographs of γ/γ′ phase structures in stress-ruptured specimens (a) fully heat-treated alloy, (b) 500 h long-term aged alloy, (c) 1000 h long-term aged alloy

Fig.17 Effect of long-term aging on average size of γ′ precipitate of the alloy: the correlation between $(a 2) 3 - (a 0 2) 3$ and aging time

Fig.18 Formation process of M₆C phase

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