Coarsening of Al3Sc precipitates in an Al-0.28 wt pct Sc alloy

The Ostwald ripening of Al3Sc precipitates in an Al-0.28 wt pct Sc alloy during aging at 673, 698, and 723 K has been examined by measuring the average size of precipitates by transmission electron microscopy (TEM) and the reduction in Sc concentration in the Al matrix with aging time, t, by electrical resistivity. The coarsening kinetics of Al3Sc precipitates obey the t1/3 time law, as predicted by the Lifshitz-Slyozov-Wagner (LSW) theory. The kinetics of the reduction of Sc concentration with t are consistent with the predicted t−1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al3Sc interface energy, γ, and volume diffusion coefficient, D, of Sc in Al during coarsening of precipitates. The Gibbs-Thompson equation has been used to give a value of γ using coarsening data obtained from TEM and electrical resistivity measurements. The value of γ estimated from the LSW theory is 218 mJ m−2, which is nearly identical to 230 mJ m−2 from the Gibbs-Thompson equation. The pre-exponential factor and activation energy for diffusion of Sc in Al are determined to be (7.2±6.0)×10−4 m2 s−1 and 176±9 kJ mol−1, respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements.