Growth behavior of γ′/γ coprecipitates in Ni-Base superalloys

Precipitation of the gamma '' phase on {100} facets of preceding gamma' precipitates is found to prevent the latter from overaging upon slow cooling from solution treatment in Ni-base superalloys based on the composition of alloy 718. By computer simulation using a multi-phase-field model, we find that the growth of a coprecipitate involves several concurrent and closely coupled processes, including thickening and lengthening of gamma '' shells, growth of the gamma' core along the < 001 >(gamma') < 011 >(gamma'), and < 111 >(gamma'), directions, hard impingement between gamma' and gamma '' precipitates, and soft impingement among gamma '' precipitates of different variants. These processes at different stages of growth are analyzed systematically as a function of coprecipitate size and configuration, and the results show that the growth kinetics of the gamma' core in a coprecipitate is controlled by the interplay among: (1) partial removal of supersaturated gamma matrix surrounding the gamma' core by coprecipitation of gamma '' shells, (2) cooperative growth of gamma' and gamma '' in the coprecipitates and (3) atomic mobility of gamma'-formers in the gamma '' phase. To maximize the effect of coprecipitation on preventing gamma' from overaging upon slow cooling, the alloy composition and heat treatment schedule should be optimized to minimize the size of gamma' cores at which coprecipitation of gamma '' shells occurs and to reduce diffusion of gamma'-formers through gamma ''. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.