Competitive Nucleation and Growth Between the Primary and Peritectic Phases of Rapidly Solidifying Ni–Zr Hypoperitectic Alloy

Abstract

The Ni-16 at. pct Zr hypoperitectic alloy melt was substantially undercooled using an electromagnetic levitator and a drop tube. The undercooling-induced competitive growth between the primary Ni7Zr2 and peritectic Ni5Zr phases was revealed by observing of the recalescence process in situ and confirmed by analyzing the solidified microstructures, X-ray diffraction pattern as well as dendritic growth velocity. When the liquid undercooling is less than a critical value of 106 K, the primary Ni7Zr2 phase initially precipitates from the parent liquid, which is subsequently followed by the nucleation and growth of the peritectic Ni5Zr phase around it. The solidified microstructure consists of the Ni7Zr2 phase, the Ni5Zr phase, and inter-dendritic eutectics. The orientation relationship and interface characteristics of the Ni7Zr2 and Ni5Zr phases were investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The EBSD results clearly demonstrate that the Ni7Zr2 and Ni5Zr phases have the parallel relationship of {111}Ni7Zr2 // {111}Ni5Zr. TEM analysis reveals that a large-scale flat interface exists between the Ni7Zr2 and Ni5Zr phases, indicating good lattice matching of the two phases along the phase boundary. Once the critical undercooling is exceeded, the peritectic Ni5Zr phase preferentially nucleates and grows from the undercooled melt by completely suppressing the formation of the primary Ni7Zr2 phase. The EBSD analysis shows that the peritectic Ni5Zr phase is highly orientated and its growth mode is almost parallel to the 〈110〉 directions. When containerlessly solidified during free fall, typical peritectic microstructures form in large droplets, while only peritectic phase appears in the small droplets. This result further confirms the strong competition between the primary and peritectic phases in the Ni–Zr hypoperitectic alloy induced by large undercoolings.