Microstructure and coarsening kinetics of Ni 3Ge precipitates in aged Ni [sbnd]Ge alloys

Abstract

The decomposition of supersaturated Ni Ge alloys containing 14.54, 14.97 or 15.44wt.%Ge was investigated at 700 °C. Transmission electron microscopy was used to measure the average particle sizes and particle size distributions, and magnetic analysis was used to measure the reduction of supersaturation with aging time. Coherent precipitates of Ni 3Ge with the ordered L1 2 crystal structure were present from the earliest aging times (10 min) in all three alloys. The average particle size increases with aging time, t, as t 1/3 within the limits of experimental error, consistent with growth by diffusion-controlled coarsening. The particle size distributions are slightly broader than the theoretical distribution of the Lifshitz-Slyozov-Wagner theory. The kinetics of the depletion of supersaturation with aging time area at least consistent with the predicted t −1/3 time law, although the scatter in the data was quite large. There does not appear to be any effect of volume fraction on the coarsening behavior over the range of f′ γ investigated (0.18–0.27). The values of the interfacial free energy, σ, and diffusion coefficient of Ge in Ni, D, derived from the data were85.2 ± 37.5mJ m −2 and2.00 ± 0.48 × 10 −13cm 2s −1, respectively. The large standard deviations are due to uncertainties in the measurements of the kinetics of solute depletion of the matrix. The equilibrium solubility of Ge in the matrix at 700 °C is12.81 ± 0.07wt.% (10.62 ± 0.06 at.%, which is slightly smaller than previously reported values. The Ni 3Ge precipitates are spherical at small sizes and become increasingly cuboidal as they grow. Spatial correlations that lead to rafting in Ni-base alloys containing misfitting γ′ precipitates are evident in Ni Ge alloys as well. However, the spatial correlations do not appear to be as strong, and the cuboidal morphology not as pronounced, as might be expected solely on the basis of the relatively large lattice misfit (δ ≈ 0.0064) in this alloy. Possible reasons for this behavior are discussed.