The coarsening kinetics of Ni3 Si(γ') precipitate in a binaryNi-Si alloy containing 6.5 wt. % silicon was studied by magnetic techniques and transmission electronmicroscopy. A calibration curve wasestablished to determine the concentration of silicon in the matrix.The variation of the Si content of the Ni-rich matrix as a function oftime follows Lifshitz and Wagner theory for diffusion controlledcoarsening phenomena. The estimated values of equilibrium solubilityof silicon in the matrix represent the true coherent equilibrium solubilities.
The experimental particle-size distributions and average particlesize were determined from dark field electron micrographs. Theaverage particle size varies linearly with t-1/3 as suggested by Lifshitzand Wagner. The experimental distributions of particle sizes differslightly from the theoretical curve at the early stages of aging, butthe agreement is satisfactory at the later stages. The values of diffusioncoefficient of silicon, interfacial free energy and activation energywere calculated from the results of coarsening kinetics. The experimentalvalue of effective diffusion coefficient is in satisfactory agreementwith the value predicted by the application of irreversible the rmodynamicsto the process of volume constrained growth of coherent precipitateduring coarsening. The coherent γ' particles in Ni-Sialloy unlikethose in Ni-Al and Ni-Ti seem to lose coherency at high temperature.A mechanism for the formation of semi-coherent precipitate is suggested.
