Evolution of microstructure of nickel base superalloy at high temperatures

Abstract

The evolution of primary and secondary γ′ precipitates in the high γ′ volume fraction Rene 80 Ni based superalloy has been examined during aging at elevated temperatures for periods up to 1750 h. While the increase in average dimension of particles followed the cube rate Lifshitz, Slypzof and Wagner (LSW) law, r³_t – r³₀=Kt, there were significant discrepancies between the experimental and theoretical particle size distributions (PSDs) and inconsistency with the kinetic constants associated with the two populations of particles. These differences are attributed to the influence of elastic coherency strains which have not been considered in conventional capillarity driven coarsening models. During thermal exposure at 871°C, coalescence of primary cuboidal γ′ was predominant in early stages of aging, while the microstructure was relatively stable at longer aging times. The stability of the microstructure at longer aging times is attributed to the formation of the network of closely spaced dislocations at the γ/γ′ interface which would cause the loss of internal misfit stresses associated with the growth. Secondary spheroidal γ′ particles were initially coarsened and their volume fraction gradually decreased until they completely dissolved after 500 h at 871°C or 1 h at 982°C.