Structure transitions near liquidus and the nucleation of undercooled melt of Ni-Cr-W superalloy

The liquid structure of Ni-Cr-W superalloy at 1405 °C and 1420 °C is characterized by high temperature X-ray diffractometer, and the effect of liquid structure on exothermic reaction and solidification microstructure has been investigated. The effect of melt structure on the nucleation is used for elucidating the mechanism of grain refinement. It demonstrates that the formation of atomic clusters near liquidus of the Ni-Cr-W alloy can significantly refine the microstructures of the alloys and decrease the degree of undercooling. A pre-peak in front of the main peak of the structure factor S(Q) curve is observed when the melt is cooled down to 1405 °C, which is closely related to atomic clusters. The size of the atomic clusters in the melt decreased from 11.05 Å to 8.8 Å as the temperature increased from 1405 °C to 1420 °C. Meanwhile, compared with the melt directly cooled down, the degree of undercooling of the melt held at 1405 °C becomes smaller. Fine equiaxed and non-dendritic grains with an average size of 60 μm and the circularity shape factor (CSF) values of 20 are obtained when the melt is kept at 1405 °C for 20 min before solidification. The formation of equiaxed and non-dendritic grains is due to the formation of clusters and the high rate of nucleation in the entire undercooled melt.