Distinct atomic structures of the Ni-Nb metallic glasses formed by ion beam mixing

Abstract

Four Ni-Nb metallic glasses are obtained by ion beam mixing and their compositions are measured to be Ni77Nb23, Ni55Nb45, Ni31Nb69, and Ni15Nb85, respectively, suggesting that a composition range of 23–85 at. % of Nb is favored for metallic glass formation in the Ni-Nb system. Interestingly, diffraction analyses show that the structure of the Nb-based Ni31Nb69 metallic glass is distinctly different from the structure of the Nb-based Ni15Nb85 metallic glass, as the respective amorphous halos are located at 2θ≈38 and 39 deg. To explore an atomic scale description of the Ni-Nb metallic glasses, an n-body Ni-Nb potential is first constructed with an aid of the ab initio calculations and then applied to perform the molecular dynamics simulation. Simulation results determine not only the intrinsic glass forming range of the Ni-Nb system to be within 20–85 at. % of Nb, but also the exact atomic positions in the Ni-Nb metallic glasses. Through a statistical analysis of the determined atomic positions, a new dominant local packing unit is found in the Ni15Nb85 metallic glass, i.e., an icositetrahedron with a coordination number to be around 14, while in Ni31Nb69 metallic glasses, the dominant local packing unit is an icosahedron with a coordination number to be around 12, which has been reported for the other metallic glasses. In fact, with increasing the irradiation dose, the Ni31Nb69 metallic glasses are formed through an intermediate state of face-centered-cubic-solid solution, whereas the Ni15Nb85 metallic glass is through an intermediate state of body-centered-cubic-solid solution, suggesting that the structures of the constituent metals play an important role in governing the structural characteristics of the resultant metallic glasses.