Towards understanding the role of Re in microstructural and mechanical enhancement of CoNi-based superalloys

Abstract

Rhenium (Re) is known to enhance the microstructural stability of superalloys, enabling components such as turbine blades to withstand extreme thermal stresses. In this study, we investigate the influence of Re additions (0–6 at.%) on the microstructure, lattice misfit, thermophysical properties, and high temperature mechanical properties of a low density Co-30Ni-7Al-12Cr-4Ti-2Nb γ/γ′ alloy. Alloys with up to 3 at.% Re exhibit refined γ′ precipitate size, morphological transitioning from cuboidal to rounded cuboids, while additions beyond 4 at.% promote the formation of detrimental topologically close-packed (TCP) phases. HR-XRD reveals that Re reduces γ/γ′ lattice misfit and causes a misfit sign reversal from positive to negative as temperature increases from 900 °C to 1000 °C. APT confirms partitioning of Re in the γ-matrix without interfacial segregation. Differential scanning calorimetry (DSC) was used to construct a quasi-binary Co-Re phase diagram, highlighting key phase transition temperatures. Re addition also causes atomic volumetric shrinkage, increasing the alloy density beyond the predictions of the rule of mixtures. Notably, the 3 at.% Re alloy displays enhanced yield strength at both room and high temperatures, exhibiting a yield strength anomaly.