The growth of gamma prime precipitates in aged Ni−Ti alloys

Abstract

The kinetics of growth of the γ′ precipitate in a Ni-8.74 wt pct Ti alloy were studied by magnetic analysis and transmission electron microscopy. The variation of the titanium content of the nickel-rich matrix as a function of aging time was studied by measuring the ferromagnetic Curie temperature of alloys aged at 692°, 593°, and 525°C. The kinetics of this process accurately obeyed the predictions of the Lifshitz-Wagner theory of diffusion controlled coarsening after relatively short aging times at all aging temperatures. Dark-field transmission electron microscopy was used to measure the particle-size distributions and the average particle sizes of samples aged for various times at 692°C. The kinetics of particle growth also obeyed the time law predicted by the Lifshitz-Wagner theory within the limits of experimental error. Additional analysis of the data provided a value of approximately 21 erg per sq cm for the interfacial free energy of the γ′-matrix interface, and a value for the diffusion coefficient of titanium in nickel which is in very good agreement with an independently determined value. The distribution of γ′ particle sizes was found to be significantly broader than the theoretical distribution of the Lifshitz-Wagner theory. It is suggested that this is due to the relatively large lattice parameter mismatch between γ′ and the Ni−Ti matrix. The results and conclusions of this study are critically compared with those of other investigations.