Key Takeaways & Executive Findings
- •• Increasing cooling rate refines dendritic structure and reduces secondary dendrite arm spacing, following λ2=216.78·R^-0.42. • Higher cooling rates enlarge the solidification range by raising liquidus and lowering solidus temperatures, affecting micro-segregation. • Cooling rate significantly influences the size and distribution of MC carbides and eutectic (γ+γ′), with higher rates reducing their sizes but increasing eutectic area fraction. • A moderate cooling rate of 30 °C·min-1 is recommended for optimal solidification of GH4975 alloy to balance microstructural refinement and segregation control.
Abstract
The high-alloyed wrought superalloy GH4975 tends to form coarse MC carbides and eutectic (γ+γ′) phases, which adversely affect the cogging and homogenization process. To provide theoretical guidance for control of MC carbides and eutectic (γ+γ′) formation, differential thermal analysis (DTA) was utilized to investigate the effect of cooing rate (10-90 °C·min-1) on solidification behavior and micro-segregation of GH4975 alloy. According to the thermodynamic calculation and distribution characteristics of precipitates, the MC carbides can act as nucleation sites for γ dendrites, but the nucleation of γ dendrites becomes less dependent on the MC carbide primers at higher cooling rates. As the γ dendrites grow, the elements including Ti and Nb gradually accumulate in the residual liquid and leads to the formation of more MC carbides near the interdendritic region. Finally, the solidification is terminated with the formation of eutectic (γ+γ′). With an increase in cooling rate, the liquidus temperature rises, but the solidus temperature decreases, and thus the solidification range is obviously enlarged. The dendritic structure is significantly refined by the increase of cooling rate. The secondary dendrite arm spacing, λ2, as a function of cooling rate, , can be expressed as λ2=216.78 -0.42. Moreover, the increasing cooling rate weakens the back diffusion of Al, Ti, and Nb, increases the undercooling, and limits the growth of precipitates. Consequently, the sizes of MC carbides, eutectic (γ+γ′), and primary γ′ significantly decrease, but the area fraction of eutectic (γ+γ′) linerly increases as the cooling rate rises. Thus moderate cooling rate (such as 30 °C·min-1) should be selected during the solidification process of GH4975 alloy.
1. Introduction
Ni-based GH4975 (Russian ЭП975) represents a new γ′ precipitation-hardened wrought superalloy with superior high-temperature strength and creep resistance. Predominantly, it is utilized in the fabrication of high-performance aeroengine turbine discs [1]. The alloying degree of GH4975 alloy is extremely high, and the total content of (Al+Ti+Nb) is approximately 9.2wt.%. Therefore, the mass fraction of γ′ in the aging state can reach up to 64%, and the long-term service temperature of this alloy is as high as 850-950 °C [2]. However, the high alloying degree significantly increases the deformation resistance and worsens the hot ductility. It is currently recognized as the highest alloyed disk alloy that can be processed using the cast and wrought route [3].
It has been reported that the Nb, Ti, Al, and W exhibit severe segregation during solidification of GH4975 alloy. The Nb, Ti, and Al are segregated in interdendritic regions (positive segregation), while W is segregated in dendrite arms (negative segregation). The MC carbides, eutectic (γ+γ′), and γ′ are the main precipitates in as-cast GH4975 alloy [4, 5]. During the hot deformation of as-cast GH4975 alloy, MC carbides are prone to become the crack initiation source due to their cracking. Furthermore, the stress concentration around MC carbides can lead to cracking of γ matrix [5]. Our previous study has found that the coarse eutectic (γ+γ′) is the most effective crack initiation site in the similar high-alloyed superalloy U720Li, and high temperature homogenization treatment is essential to dissolve the eutectic (γ+γ′) [6]. However, coarse eutectic (γ+γ′) is very difficult to dissolve [7, 8], and MC carbides usually remain stable during the homogenization process [9-11]. Therefore, it is necessary to understand how to control the formation of eutectic (γ+γ′) and MC carbides during solidification of GH4975 alloy.
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Guang-di Zhao, Xi-min Zang, Yi-xuan Sun, Xiao-yu Yao (2026). Effect of cooling rate on solidification behavior and micro-segregation of high-alloyed wrought superalloy GH4975. China Foundry. https://doi.org/10.1007/s41230-025-4083-4
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Frequently Asked Questions
What is the effect of cooling rate on the solidification behavior of GH4975 superalloy?
Increasing cooling rate raises the liquidus temperature and lowers the solidus temperature, thereby enlarging the solidification range. It also refines the dendritic structure and reduces secondary dendrite arm spacing, following the relationship λ2=216.78·R^-0.42.
How does cooling rate influence micro-segregation in GH4975 alloy?
Higher cooling rates weaken back diffusion of Al, Ti, and Nb, increase undercooling, and limit precipitate growth. This results in smaller MC carbides, eutectic (γ+γ′), and primary γ′ sizes, but the area fraction of eutectic (γ+γ′) increases linearly with cooling rate.
What is the recommended cooling rate for solidification of GH4975 alloy?
A moderate cooling rate of 30 °C·min-1 is recommended to balance microstructural refinement and segregation control, as it provides a good compromise between reducing precipitate sizes and managing eutectic formation.
Why is it important to control MC carbides and eutectic (γ+γ′) in GH4975 alloy?
Coarse MC carbides and eutectic (γ+γ′) are detrimental to hot workability and mechanical properties. They can act as crack initiation sites during hot deformation and are difficult to dissolve during homogenization, so controlling their formation during solidification is crucial.
What methods were used to study the solidification behavior of GH4975 alloy?
Differential thermal analysis (DTA) was utilized to investigate the effect of cooling rate on solidification behavior and micro-segregation. Thermodynamic calculations and distribution characteristics of precipitates were also analyzed.
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