Phase-field simulation of γ' precipitates rafting and creep property of Co-base superalloys

Abstract

Co-base superalloys show attractive properties for the strengthening of γ'-Co3(Al,W) precipitates. Directional coarsening of γ' precipitates under external stress forms the rafting morphology, which plays a crucial role in the creep property. The microelastic theory, coupled with the crystal plasticity model, is utilized to study the rafting behavior during the early stage of creep in Co-Al-W superalloys. High W content and large eigenstrain can produce a lower creep strain of early stage, for the subdued plastic deformation in the γ matrix. Driving forces of element redistribution during rafting are compared among the chemical, interfacial and elastic contribution, in which the elastic contribution is dominant in rafting. With the change in creep stress from 197 MPa to 297 MPa, the element redistribution is strengthened, and the rafting is accelerated in Co-10 Al-9 W (at.%) alloy. Elements redistribution in the rafting process widens the γ/γ' interface. In contrast, the width of γ/γ' interface along the rafting direction begins to decrease when the coalescence of γ' precipitates happens. The straight-forward insight on the element diffusion during rafting and the related creep properties guides the composition design and rafting behavior prediction of this novel Co-Al-W superalloy.