Precipitation of Co from supersaturated Au90Co10: microstructure and kinetics

Abstract

Abstract The kinetics of the precipitation of Co from a supersaturated solid solution of Au-10.12 at.% Co was investigated by differential scanning calorimetry upon isothermal annealing at temperatures in the range from 567 K to 612 K and upon isochronal annealing at heating rates between 5 K min−1 and 40 K min−1. The microstructural evolution during the course of precipitation was traced by (high-resolution) transmission electron microscopy, scanning electron microscopy, X-ray diffraction and hardness measurements. First, the exothermic formation of 0.5 – 1 nm thin plate-like bcc Co precipitates takes place parallel to {100} habit planes of the Au-matrix. Upon continued annealing, equiaxed fcc Co particles in epitaxial orientation with the Au lattice occur by a Bain-type transformation of the Co particles from the bcc to the fcc lattice structure associated with loss of coherency. Fitting of a modular model of transformation kinetics (Liu et al., Int. Mat. Rev. 52 (2007) 193) simultaneously to the first peak of all isothermal differential scanning calorimetry runs and simultaneously to the first peak of all isochronal differential scanning calorimetry runs demonstrated that the nucleation of bcc Co plates is governed by site saturation in the case of isothermal annealing and continuous nucleation in the case of isochronal annealing, and by linear growth for both isothermal and isochronal annealing. The activation energy for growth is remarkably small due to the presence of quenched-in vacancies. Recovery prior to precipitation leads to a change of the nucleation mechanism from site saturation to continuous nucleation as well as an increase in the activation energy for growth.