Kinetics of growth of γ′- precipitates in Ni–6·75AI alloy

The size distributions of γ precipitates in a Ni-based alloy containing 6·75 wt–% aluminium have been measured, using dark-field electron microscopy, at five temperatures ranging from 600 to 825°C and the corresponding volume fractions have been assessed. From these distributions, the average particle sizes were obtained and plotted against time according to the kinetics laws proposed by theory for diffusion–controlled growth, interface-controlled growth, and growth in the transition region where neither process dominates. The particle-size distributions were reconstructed in terms of the ratio of the actual particle size/average size, and the resulting distributions were compared with those predicted by theory for the three controlling conditions. Although the values of activation energy obtained from the plots of particle size against annealing time are equally consistent for the three processes, the agreement between the predicted distributions and the experimental distribution is distinctly better for growth in the transition region than for the two alternative processes.