Structural transition and glass-forming ability of the Ni–Hf system studied by molecular dynamics simulation

A tight-binding Ni–Hf potential is constructed by fitting some of the ground-state properties, such as the cohesive energy, lattice constants, and the elastic constants of some Ni–Hf alloys. The constructed potential is verified to be realistic by reproducing some static and dynamic properties of the system, such as the melting points and thermal expansion coefficients for the pure Ni and Hf as well as some of the equilibrium compounds, through molecular dynamics simulation. Applying the constructed potential, molecular dynamics simulations are performed to compare the relative stability of the face-centered-cubic (fcc)/hexagonal close-packed (hcp) solid solutions to their disordered counterparts as a function of solute concentration. It is found that the solid solutions become unstable and transform into the disordered states spontaneously, when the solute concentrations exceed the two critical solid solubilities, i.e., 25 at.% Ni for hcp Hf-rich solid solution and 18 at.% Hf for fcc Ni-based solid solution, respectively. This allows us to determine that the glass-forming ability/range of the Ni–Hf system is within 25–82 at.% Ni. Interestingly, simulations also reveal for the first time, that two mixed regions exist in which an amorphous phase coexists with a crystalline phase, and at about 18 at.% Ni, the hcp lattice turns into a new metastable phase identified to be face-centered orthorhombic structure.