The coarsening behavior of the γ′ precipitate in nickel-silicon alloys

Abstract

The coarsening kinetics of the γ′ (Ni 3Si) precipitate in a binary Ni-6.5 wt. % Si alloy were investigated by a magnetic technique and transmission electron microscopy. The magnetic technique was used to determine the variation of the Si content of the matrix as a function of aging time over the temperature range 625–775°C. This process accurately followed t − 1 3 kinetic behavior predicted by the Lifshitz-Slyozov-Wagner (LSW) theory of diffusion-controlled coarsening. However, after about 16 hr at 775°C, a pronounced departure from t − 1 3 kinetics was observed. Transmission electron microscopy showed that this departure was associated with partial loss of coherency of the γ′ precipitates. The kinetics of γ′ particle growth were determined at 775°C by dark-field electron microscopy, as were the particle size distributions. Prior to becoming semicoherent, the average γ′ particle size increased as t 1 3 , in agreement with the LSW theory. The experimentally determined histograms agreed with the theoretical histograms after about 5 hr of aging. Quantitative analysis of our data, combined with other data in the literature, yielded reasonable values for the interfacial free energy of the γ′-matrix interface (between 10 and 13 erg/cm 2). However, values of the diffusion coefficient obtained from our data were about an order of magnitude larger than independently determined diffusivities of Si in dilute Ni-Si aloys. The application of the equations for diffusion under conditions of volume constrained growth of coherent precipitates yielded “effective” diffusion coefficients that agreed extremely well with those determined experimentally.