Nickel self-diffusion in a liquid and undercooled NiSi alloy

Abstract

The nickel self-diffusion coefficient was measured in a ${\mathrm{Ni}}_{75}{\mathrm{Si}}_{25}$ alloy in the liquid and undercooled state by quasielastic neutron scattering. The undercooled state was achieved by applying aerodynamic levitation. That containerless technique allowed an undercooling of $\mathrm{\ensuremath{\Delta}}T\ensuremath{\approx}90\phantom{\rule{0.16em}{0ex}}\text{K}$ over 4 h of measurement time. The temperature dependence of the derived diffusion coefficients follows an Arrhenius-type behavior. The activation energy for the diffusion process is about 10% larger than in pure nickel and is probably the reason for the slower self-diffusion coefficient compared to pure Ni. With increasing Si content more covalent bonding is formed, which might be the origin for the reduced mobility. Molecular dynamics simulations predicted a change in dynamics from an Arrhenius-type behavior to a power-law for temperatures as high as twice the glass transition temperature. Our data are compatible with a power-law behavior for the Ni self-diffusion.