Local atomic structure correlating to phase selection in undercooled liquid Ni-Zr peritectic alloy

Abstract

The local atomic structure and its correlation with phase selection during rapid solidification of undercooled liquid Ni5Zr peritectic alloy has been investigated by combining the electrostatic levitation technique and first-principles molecular dynamics simulations. The calculated density agrees well with the present experimental measurements. The partial pair distribution functions indicate that liquid Ni5Zr alloy exhibits a remarkable chemical short range order, which leads to a preferred association of Ni and Zr atoms. Atomic three-dimensional structure analyses reveal that a high fraction of atom pairs locates in perfect or distorted icosahedral environments (>36.9%) although the number of the fully developed icosahedra or distorted icosahedra is rather rare (<2.5%), implying that this liquid contains a lot of fragmented clusters with perfect or distorted pentagonal faces. Moreover, we demonstrate that the short range orders of the undercooled liquid differ topologically from those in the ideal Ni5Zr crystal but are similar to those in the ideal Ni7Zr2 crystal in the temperature range from the liquidus temperature of 1612 K to 1400 K, which indicates that the nucleation of primary phase Ni7Zr2 from the undercooled liquid is structurally favored due to the low free energy barrier. However, the nucleation of primary Ni7Zr2 phase is presumed to be replaced by the direct formation of the peritectic Ni5Zr phase if a sufficient undercooling is achieved, which is ascertained by the solidified microstructures and X-ray diffraction patterns. These results shed light on the relationship between the local atomic structure and phase selection during the rapid solidification of the undercooled peritectic alloys.