Influence of cross correlations on interdiffusion in Al-rich Al-Ni melts

The relation of self- and interdiffusion in a liquid metal, particularly the influence of cross correlations at low concentrations, is studied experimentally. Accurate interdiffusion data are obtained by a combination of x-ray radiography with the shear-cell method on the ground and on the sounding rocket MAPHEUS under microgravity conditions. Self-diffusion coefficients, measured by quasielastic neutron scattering, increase with decreasing Ni concentration, whereas interdiffusion coefficients are about constant. We show that cross correlations influence interdiffusion even at concentrations as low as 2 at. % Ni. Consequently, Darken's equation is not valid in this case.

Figure 1

Two-dimensional projection of the concentration distribution of the diffusion couple Al-Ni 4 at. %–Al-Ni 16 at. %. Images are obtained under microgravity 1, 5, and 30 s after starting the diffusion process through a shear movement. The dimension of each cutout is $3.5\times 1.2$ mm. The color scale indicates the Ni concentration in at. %. A clear shear-induced convection can be seen, but after 30 s the concentration distribution approaches parallel concentration lines.

Figure 2

Concentration profile of two experiments with a different spread around the mean concentration $c_{\mathrm{Ni}}=10\text{at}.\%$ 200 s after the start of diffusion. Inset: $4D_{\mathrm{AlNi}}t$ from the fit with Eq. (2) for these two experiments showing a linear time dependence with coinciding slope.

Figure 3

Top: Interdiffusion coefficients with mean value shown as a solid line as a function of Ni concentration. Bottom: Self-diffusion coefficients for nickel as a function of Ni concentration. $D_{\mathrm{Ni}}$ at 10 at. % Ni is taken from [24]. The dashed line is a guide to the eye.

Figure 4

Thermodynamic factor $\mathrm{\Phi }$ derived from [42] and cross-correlation term $S$ as calculated from experimental data as a function of Ni concentration.

Figure 5

Self- and interdiffusion coefficients of ${\mathrm{Al}}_{95}{\mathrm{Ni}}_5$ as a function of temperature. Solid lines are fits with the Arrhenius function. Also shown are the thermodynamic factor $\mathrm{\Phi }$ calculated from [42] and the cross-correlation term $S$ resulting from experimental values.