Effect of melt thermal history on solidification behavior and microstructural characteristics of a third-generation Ni-based single crystal superalloy

The melt superheating treatment is performed on a third-generation Ni-based single crystal superalloy during directional solidification, aiming to investigate the influence of melt thermal history (melt superheating temperature, superheating time and cooling rate) on solidification behavior and microstructural characteristics. The results show that the nucleation undercooling increases nonlinearly with the increase of melt superheating temperature from 1450 to 1780 °C, but decreases when the melt superheating temperature further increases up to 1800 °C. The nucleation undercooling first has little change and then increases sharply with increasing the melt superheating time. The critical superheating time is obtained to be about 15 min. Moreover, it is found that the higher the cooling rate is, the larger the nucleation undercooling and liquidus-solidus range are. Additionally, the segregation is reduced, and the dendrite and γ′ phase are obviously refined when the superheating temperature increases from 1500 to 1700 °C. The influence mechanism of melt superheating treatment on the nucleation undercooling and solidification microstructure is discussed, which is helpful for optimizing the process parameters and improving the metallurgical quality of Ni-based single crystal superalloy.