Effect of Solidification Conditions on Microstructure Evolution in DD6 Single-Crystal Superalloy

doi:10.11900/0412.1961.2025.00049

Acta Metall Sin

2026, Vol. 62

Issue (4): 550-560 DOI: 10.11900/0412.1961.2025.00049

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Effect of Solidification Conditions on Microstructure Evolution in DD6 Single-Crystal Superalloy

XIE Hongji, LI Jiarong(

), LUO Yushi, ZHENG Sujie, LUO Kailun

Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China

Cite this article:

XIE Hongji, LI Jiarong, LUO Yushi, ZHENG Sujie, LUO Kailun. Effect of Solidification Conditions on Microstructure Evolution in DD6 Single-Crystal Superalloy. Acta Metall Sin, 2026, 62(4): 550-560.

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Abstract

Single-crystal superalloys are widely used in aircraft engines owing to their excellent high-temperature performance. As key factors in controlling the formation of microstructures, solidification conditions directly influence the comprehensive properties of alloys. Therefore, it is of great significance to understand the mechanisms of alloy microstructure evolution under different solidification conditions and to optimize the solidification process to improve the performance of advanced aircraft engine hot-section components. To investigate the effects of the solidification conditions on the evolution of as-cast and heat-treated microstructures in the second-generation single-crystal superalloy DD6, single-crystal bars oriented along the [001] direction were prepared using high-rate solidification (HRS) and liquid metal cooling (LMC) processes. The results showed that in the as-cast state, the γ′ phases in the dendritic core of the HRS alloy exhibited a relatively regular cubic shape, whereas those in the LMC alloy were irregularly cubic; the γ′ phases in the interdendritic regions of the HRS and LMC alloys were irregularly cubic and larger in size than those in the dendritic core. As the pouring temperature increased, the size of γ′ phases in the HRS alloy first increased and then decreased, reaching a maximum at 1560 ^oC. At the same pouring temperature of 1590 ^oC, the size of the γ′ phases in the LMC alloy was smaller. After the heat treatment, the volume fraction of the γ + γ′ eutectic under all conditions decreased significantly. A trend was observed where the smaller the size of the γ + γ′ eutectic in the as-cast alloy, the greater the reduction in its content, regardless of the initial γ + γ′ eutectic content in the as-cast state. In the heat-treated HRS and LMC alloys, the γ′ phases in the dendritic core and interdendritic regions were uniformly distributed and regularly arranged, and they demonstrated good cubicity. Furthermore, the size uniformity of the γ′ particles in the LMC alloy was superior to that in the HRS alloy. The differences in the microstructures of the as-cast alloys were primarily caused by differences in the temperature gradient during directional solidification. Therefore, increasing the pouring temperature or employing LMC to enhance the temperature gradient helped to refine the dendritic structure of the alloy, reduce microsegregation, and this can simplify the alloy's heat treatment process.

Key words: DD6 single-crystal superalloy solidification condition microstructure evolution temperature gradient microsegregation

Received: 24 February 2025

ZTFLH:

TG132.3

Corresponding Authors: LI Jiarong, professor, Tel: (010)62497202, E-mail: jrli126@126.com

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	XIE Hongji
	LI Jiarong
	LUO Yushi
	ZHENG Sujie
	LUO Kailun

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https://www.ams.org.cn/EN/10.11900/0412.1961.2025.00049 OR https://www.ams.org.cn/EN/Y2026/V62/I4/550

Table1 Directional solidification processes under different solidification conditions

Fig.1 Schematic of the sampling location

Fig.2 OM images of the as-cast DD6 alloy under HRS process with pouring temperatures of 1500 ^oC (a, b), 1560 ^oC (c, d), and 1590 ^oC (e, f), and LMC process of 1590 ^oC (g, h)
(a, c, e, g) transverse sections (b, d, f, h) longitudinal sections

Fig.3 Dendrite arm spacing of as-cast DD6 alloy under different solidification conditions (λ₁—primary dendrite arm spacing, λ₂—secondary dendrite arm spacing)

Fig.4 OM images of the γ + γ′ eutectic in as-cast DD6 alloy under HRS process with pouring temperatures of 1500 ^oC (a), 1560 ^oC (b), and 1590 ^oC (c), and LMC process of 1590 ^oC (d)

Fig.5 SEM images of γ′ phases in as-cast DD6 alloy under HRS process with pouring temperatures of 1500 ^oC (a, b), 1560 ^oC (c, d), and 1590 ^oC (e, f), and LMC process of 1590 ^oC (g, h)
(a, c, e, g) dendrite cores (b, d, f, h) interdendritic regions

Fig.6 Element segregation ratios of as-cast DD6 alloy under different solidification conditions
(a) Re, Co, and W (b) Al, Cr, Ta, Nb, and Ni

Fig.7 OM images of the heat-treated DD6 alloy under HRS process with pouring temperatures of 1500 ^oC (a), 1560 ^oC (b), and 1590 ^oC (c), and LMC process of 1590 ^oC (d)

Fig.8 Changes in eutectic content of alloys before and after heat treatment at different solidification conditions

Fig.9 SEM images of γ′ phases in heat-treated DD6 alloys under HRS process with pouring temperatures of 1500 ^oC (a, b), 1560 ^oC (c, d), and 1590 ^oC (e, f), and LMC process of 1590 ^oC (g, h)
(a, c, e, g) dendrite cores (b, d, f, h) interdendritic regions

Fig.10 Changes in γ′ particle size of alloy before and after heat treatment under different solidification conditions

Fig.11 Element segregation ratios of heat-treated DD6 alloy under different solidification conditions
(a) Re, Co, and W
(b) Al, Cr, Ni, Ta, and Nb

Table 2 Segregation coefficients and heat treatment effectiveness coefficients of alloys under different solidification conditions

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