The Kinetics of Precipitate Dissolution in a Nickel-Base Superalloy

The effect of microstructure and soft impingement on the kinetics of static dissolution of gamma' precipitates during supersolvus solution treatment of the powder-metallurgy nickel-base superalloy LSHR were determined experimentally and interpreted in the context of a simple diffusion model. The starting material had either a fine duplex structure of gamma grains and gamma' precipitates (each of which were -2 mu m in diameter) or a structure comprising -2-mu m-diameter gamma' precipitates lying within large (similar to 15 mu m) gamma grains. Using a direct-resistance-heated Gleeble((R)) machine, the temperature of each sample was first equilibrated at a fixed subsolvus temperature (at which the microstructure was also stabilized) and then quickly raised to and held for a predetermined time in the single-phase gamma field. The dissolution of gamma' was found to be approximately five times as fast for the duplex structure in comparison to the kinetics for the intragranular precipitates. The results were interpreted using numerical simulations of dissolution based on the Whelan model modified to account for a distribution of precipitates (rather than a single, isolated spherical particle) and soft impingement of diffusion fields. Using independently-measured values for diffusivity, precipitate composition, and phase equilibria as input, simulation predictions showed good agreement with the observations in terms of the temporal variation of volume fraction and average particle radius. By this means, insight into the retarding influence of soft impingement and the accelerating effect of pipe diffusion (due to stored dislocations) on the rate of dissolution was obtained. (C) The Minerals, Metals & Materials Society and ASM International 2017