The microstructure of a single crystal superalloy after different aging heat treatments

Abstract

To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single crystal superalloy containing 22 wt% refractory elements was investigated. Results show that for the experimental alloy, even the homogenization–solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ′ phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ′ cubes. For specimens with primary aging heat treated at 1120 °C, γ′ morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 °C, γ′ phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ′ phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ′ phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ′ phase. As a result, the γ′-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ′ size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then, high-temperature properties can be improved.