Optimum microstructure combination for maximizing tensile strength in a polycrystalline superalloy with a two-phase structure

Different microstructural features were obtained under various heat treatment conditions, which provided insight into the factors controlling the critical strength in a polycrystalline Ni–Co-based disk superalloy (TMW-4M3 alloy) with a two-phase structure. The contribution of each microstructural feature, namely, the grain size, annealing twin boundary and distribution of γ′ precipitates, to the total strength was analyzed quantitatively by measuring the Vickers hardness over the nanometer to micron size range. Grain boundary strengthening decreased to nearly zero with increasing solution heat treatment temperature, while the secondary and tertiary γ′ precipitation hardening increased. Therefore, there is an optimum combination of microstructural features for achieving the highest tensile strength in such superalloys, the key factors being the temperature and time used for the solution heat treatment and the subsequent aging treatment. A method for determining the optimum factors for TMW-4M3 is proposed.