Effect of cooling rate on solidification behavior and micro-segregation of high-alloyed wrought superalloy GH4975

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, Ṫ, can be expressed as λ2=216.78Ṫ−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.