Nickel self-diffusion in a liquid and undercooled NiSi alloy

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 $\mathrm{\Delta }T\approx 90\text{K}$ 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.

Figure 1

Freezing curves of two runs at the end of the measurement that demonstrate that one run was performed in the undercooled state.

Figure 2

Plot (a) displays a spectrum at $Q=0.6{\text{Å}}^{-1}$ for a temperature of $T=1535$ K and includes the empty levitator measurement (line) and one run with the vanadium sphere (dashed line) on a logarithmic scale. Plot (b) shows 2 spectra at $Q=0.5{\text{Å}}^{-1}$ for $T=1420$ K and 1920 K. The lines depict the fit with a Lorentzian curve.

Figure 3

The deduced HWHM from the Lorentzian fits are plotted. The lines depict a fit with ${\mathrm{\Gamma }}_{1/2}\left(Q\right)=\hslash D{Q}^2$

Figure 4

The derived diffusion coefficients are plotted in comparison with values for liquid and supercooled pure Ni [12]. The error bars for the diffusion coefficients are smaller than the symbol size. The lines are Arrhenius law fits to the temperature dependence and the dashed line is a fit with a power law (see text for more details). Included are also values from the MD simulation for the nickel diffusion coefficients [8] and relaxation rates $1/\tau (Q=1.8{\text{Å}}^{-1})$ (hexagons) from our data.