Effect of composition and coherency strains on coarsening in nickel-aluminum-silicon alloys

Abstract

Recently, significant advances have been made in the theoretical understanding of coarsening in coherently stressed solids. Experimental verification of the theoretical predictions are difficult. Interpretation of coarsening rate data obtained in multi-component systems is difficult, due to the presence of a gradually varying, composition-dependent coarsening rates, which has to be subtracted from the measured coarsening rates. Further, ternary systems are often plagued by the lack of diffusivity data.

In this study, several alloys were selected from the Ni-Al-Si system. These alloys had a constant Ni content of 88 at. % while the proportions of Si and Al were varied. This resulted in alloys with two phases in equilibrium, $\gamma$ and coherently precipitated $\gamma\sp\prime,$ with their lattice parameters spanning a large range. Coarsening rates of $\gamma\sp\prime$ were measured in these alloys using Small-angle neutron scattering. Diffusion measurements were made in the binary Ni-Al and Ni-Si systems and the ternary Ni-Al-Si system; diffusion coefficients at the coarsening temperatures were calculated using available values for the activation energies. Room temperature lattice parameters and the thermal expansion coefficients of $\gamma$ and $\gamma\sp\prime$ were measured using neutron powder diffraction.

Using the measured diffusion coefficients and composition of $\gamma\sp\prime$ obtained from the lattice parameter measurements, the chemical effect was calculated and subtracted from the measured coarsening rate variation. The remaining variation with composition was correlated with the values of the misfit parameter. It was observed that the coarsening rates increased with the magnitude of the misfit parameter, reached a maximum and then decreased again. This behavior is explained on the basis of interparticle elastic interactions. It is argued that the increase in coarsening rates is due to the particles being elastically harder than the matrix and increasing the misfit resulted in increasing interparticle interactions, and the decrease in coarsening rates is attributed to the relaxation of the interaction energy.