Coarsening of Al 3 Sc precipitates in an Al-0.28 wt pct Sc alloy
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7/1/04
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Coarsening of Al3Sc Precipitates in an Al-0.28 Wt Pct Sc Alloy C. WATANABE, T. KONDO, and R. MONZEN 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 t1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al3Sc interface energy, , 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 using coarsening data obtained from TEM and electrical resistivity measurements. The value of estimated from the LSW theory is 218 mJ m2, which is nearly identical to 230 mJ m2 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) 104 m2 s1 and 176 9 kJ mol1, respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements.
I. INTRODUCTION
ALUMINUM alloys containing scandium have excellent mechanical properties at room temperature, owing to the presence of coherent, finely dispersed Al3Sc precipitates that can be obtained at a high number density, thus preventing the dislocation motion. Using conventional age-hardened Al alloys at elevated temperatures is generally limited by the nonequilibrium intermediate precipitates. On the contrary, the Al3Sc precipitates are thermodynamically stable[1] and have modest coarsening rates at elevated temperatures.[2] This attractive feature of Al3Sc precipitates leads to the effective suppression of recrystallization and the stabilization of fine grains, at high temperatures. Thus, Sc-containing Al alloys can be used at significantly higher temperatures compared with conventional age-hardened Al alloys. It is therefore technologically important to understand the coarsening characteristics of Al3Sc precipitates since this governs the stability of the mechanical properties at elevated temperatures. Some experiments have been carried out on the coarsening behavior of Al3Sc precipitates. Jo and Fujikawa[3] have investigated the precipitation kinetics of Al3Sc phases in Al-0.15 wt pct Sc and Al-0.25 wt pct Sc alloys aged at 533 to 733 K by electrical resistivity measurements, and showed that the kinetics of the depletion of supersaturation with aging time, t, for the Al-0.25 wt pct Sc alloy obey the t1/3 law, as deduced from the theory of Lifshitz and Slyozov[4] and Wagner[5] (LSW) modified by Ardell.[6] The calculated values of the Al/Al3Sc interface energy, , using the impurity diffusivity of Sc in Al[7] range fr
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