Trans-interface-diffusion-controlled coarsening of γ′ particles in Ni–Al alloys: commentaries and analyses of recent data

Recently published data on coarsening of γ′ precipitates in a binary Ni–Al alloy are critically reviewed within the framework of the trans-interface diffusion-controlled theory of particle coarsening. These data are shown to be remarkably consistent in every respect with the predictions of theory using the temporal exponent n = 2.2, which was arrived at by fitting experimental histograms and experimental cumulative distribution functions to their theoretical counterparts. This is the best procedure for evaluating the temporal exponent n, but plotting the average radius, 〈r〉, as 〈r〉n versus aging time t is a suitable alternative. Semiquantitative agreement is obtained with all the data, including the kinetics of solute depletion as well as the temporal dependencies of the volume fraction and number density, Nv. The inverse time dependency of Nv is shown once again to be incorrect, failing the simplest test of internal consistency, specifically constancy of the product Nvt. The notion that there exists a “quasi-stationary” regime of γ′ precipitate coarsening is seriously questioned and shown to be untenable. Analysis of the data enables quantitative prediction of the interfacial free energy, σ. In combination with previous work, this provides the first concrete experimental evidence for a linear decrease of σ with increasing temperature.