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

Abstract 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 " is="true"> 001 γ ' , " is="true"> 011 γ ' and " is="true"> 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 ' ' .