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

The Ni_83.25Zr_16.75 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 ΔT_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.