Thermodynamic properties and solidification kinetics of intermetallic Ni₇Zr₂ alloy investigated by electrostatic levitation technique and theoretical calculations

The thermodynamic properties, including the density, volume expansion coefficient, ratio of specific heat to emissivity of intermetallic Ni₇Zr₂ alloy, have been measured using the non-contact electrostatic levitation technique. These properties vary linearly with temperature at solid and liquid states, even down to the obtained maximum undercooling of 317 K. The enthalpy, glass transition, diffusion coefficient, shear viscosity, and surface tension were obtained by using molecular dynamics simulations. Ni₇Zr₂ has a relatively poor glass forming ability, and the glass transition temperature is determined as 1026 K. The inter-diffusivity of Ni₇Zr₂ alloy fitted by Vogel-Fulcher-Tammann law yields a fragility parameter of 8.49, which indicates the fragile nature of this alloy. Due to the competition of increased thermodynamic driving force and decreased atomic diffusion, the dendrite growth velocity of Ni₇Zr₂ compound exhibits double-exponential relationship to the undercooling. The maximum growth velocity is predicted to be 0.45 m s^-1 at the undercooling of 335 K. Theoretical analysis reveals that the dendrite growth is a diffusion-controlled process and the atomic diffusion speed is only 2.0 m s^-1.