Enhancing the High-Temperature Strength of a Co-Base Superalloy by Optimizing the g/g¢ Microstructure

Compositionally complex polycrystalline g/g¢ CoNi-base superalloys, such as CoWAlloy2 (Co41-Ni32-Cr12-Al9-W5-Ti0.3-Ta0.2-Si0.4-Hf0.1-C-B-Zr) are interesting candidates for new high-temperature materials. To maximize their high-temperature strength, the g/g¢ microstructure has to be optimized by adjusting the multi-step heat treatments. Various microstructures after different heat treatments were analyzed by scanning and transmission electron microscopy and especially in-situ small-angle neutron scattering during heat treatment experiments. The corresponding mechanical properties were determined by compression tests and hardness measurements. From this, an optimum g¢ precipitate size was determined that is adjusted mainly in the first precipitation heat treatment step. This is discussed on the basis of the theory of shearing of g¢ precipitates by weak and strong pair-couplings of dislocations. A second age hardening step leads to a further increase in the g¢ volume fraction above 70% and the formation of tertiary g¢ precipitates in the g channels, resulting in an increased hardness and yield strength. A comparison between two different three-step heat treatments revealed an increase in strength of 75 MPa for the optimized heat treatment.