Growth behavior of γ’/γ” coprecipitates in Ni-Base superalloys

Precipitation of the γ” phase on {100} facets of preceding γ’ 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 γ” shells, growth of the γ’ core along the 001_γ’, 011_γ’ and 111_γ’ directions, hard impingement between γ’ and γ” precipitates, and soft impingement among γ” 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 γ’ core in a coprecipitate is controlled by the interplay among: (1) partial removal of supersaturated γ matrix surrounding the γ’ core by coprecipitation of γ” shells, (2) cooperative growth of γ’ and γ” in the coprecipitates and (3) atomic mobility of γ’-formers in the γ” phase. To maximize the effect of coprecipitation on preventing γ’ from overaging upon slow cooling, the alloy composition and heat treatment schedule should be optimized to minimize the size of γ’ cores at which coprecipitation of γ” shells occurs and to reduce diffusion of γ’-formers through γ’’.