Evidence for the transition from primary to peritectic phase growth during solidification of undercooled Ni-Zr alloy levitated by electromagnetic field

The Ni83.25Zr16.75 peritectic alloy was undercooled by electromagnetic levitation method up to 198 K. The measured dendritic growth velocity shows a steep acceleration at a critical undercooling of Delta T-crit = 124 K, which provides an evidence of the transition of the primary growth mode from Ni7Zr2 phase to peritectic phase Ni5Zr. This is ascertained by combining the temperature- time profile and the evolution of the solidified microstructures. Below the critical undercooling, the solidified microstructure is composed of coarse Ni7Zr2 dendrites, peritectic phase Ni5Zr and eutectic structure. However, beyond the critical undercooling, only a small amount of Ni7Zr2 phase appears in the solidified microstructure. The dendritic growth mechanism of Ni7Zr2 phase is mainly governed by solute diffusion. While, the dendritic growth mechanism of Ni5Zr phase is mainly controlled by thermal diffusion and liquid-solid interface atomic attachment kinetics.