Diffusion in Al-Cu Melts Studied by Time-Resolved X-Ray Radiography

We use time-resolved x-ray radiography in order to monitor interdiffusion processes in situ in liquid alloys. We measure temperature and composition dependent interdiffusion coefficients ($D_{\mathrm{AlCu}}$) in Al-rich Al-Cu melts. At constant temperature, $D_{\mathrm{AlCu}}$ only exhibits a weak dependence of the alloy composition. As compared to self-diffusion, interdiffusion is enhanced by a factor of about 3. Our results demonstrate that it is not possible to express interdiffusion in terms of self-diffusion and thermodynamic driving forces.

Figure 1

X-ray image of the sample during diffusion annealing. Dark lines represent absorption by the Mo heating wires. Cu concentration profiles [Fig. 2a] are derived from the gray values along the sample in the center of the graphite holder. For a better representation the image is rotated by 90°.

Figure 2

(a) Concentration profiles for the $\mathrm{Al}\mathrm{\text{−}}{\mathrm{AlCu}}_{4.5}$ diffusion couple at two different annealing times. Lines are fits with Eq. (1). (b) Diffusion length square, $L^2=4D_{\mathrm{AlCu}}t$ (open circles), as a function of annealing time at 983 K and resulting interdiffusion coefficients $D_{\mathrm{AlCu}}$ (closed circles).

Figure 3

Interdiffusion coefficients $D_{\mathrm{AlCu}}$ (closed circles) and thermodynamic factor $\Phi$ (open circles) as a function of mean alloy composition of the diffusion couple at 983 K.

Figure 4

Temperature dependent diffusion coefficients in Al-Cu melts determined by XRR and QNS. The dashed line represents interdiffusion coefficients derived via the Darken equation [Eq. (2)] under the assumption that $D_{\mathrm{Al}}=D_{\mathrm{Cu}}$. The solid lines are guides to the eye.