Coarsening of Ni3Ge in binary Ni–Ge alloys: microstructures and volume fraction dependence of kinetics Original Research Article

Coarsening of Ni3Ge precipitates in binary Ni–Ge alloys containing 12.15, 13.01 and 14.03 at.% Ge, aged at 724 °C (equilibrium volume fractions, fe, equaling 0.022, 0.105 and 0.202, respectively) was investigated using transmission electron microscopy and magnetic analysis. The rate constants for the kinetics of particle growth and depletion of supersaturation depend anomalously on fe, i.e. they both decrease slightly as fe increases. Average values of the chemical diffusion coefficient and the Ni3Ge/matrix interfacial free energy, derived from analysis of the data, are in reasonable agreement with previously reported values. Alignment of the Ni3Ge precipitates parallel to cube directions is strong, but coalescence into plate shapes is never observed. Precipitates of the solid solution γ phase nucleated in large, coherent, concave-cuboidal Ni3Ge precipitates. This behavior is expected, considering the phase boundary between the two-phase and Ni3Ge regions in a recently published phase diagram.