Ostwald ripening of precipitates during two successive heat treatments performed at different temperatures

Two-stage Ostwald ripening of precipitates has been investigated experimentally and theoretically. A nickel-base alloy in which spherical nano-scale precipitates of the gamma'-phase form served as a model system. Each specimen was heat treated at two successive temperatures T-1 and T-2 in the range 949-1119 K. Thus, monodal as well as bimodal precipitate dispersions were produced. On the basis of an equation published by Wagner, Lifshitz, and Slyozov for isothermal ageing, the function <(r(t(1), T-1, t(2), T-2))over bar> is modeled, where <(r(t(1), T-1, t(2), T-2))over bar> is the average precipitate radius after double ageing and t(i) is the ageing time at T-i. In the case of bimodal gamma'-precipitate dispersions, two average radii are considered. The agreement between measured and calculated values for <(r(t(1), T-1, t(2), T-2))over bar> is within 7%, which is about the same accuracy as that of experimental data determined by transmission electron microscopy. The quoted agreement holds for monodal gamma'-precipitate dispersions as well as for each gamma'-population of bimodal dispersions. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.