Prospects for BCC Refractory High-Entropy Alloys Given Their Phase Stability and Mechanical Properties

Refractory high-entropy alloys (RHEAs) are high-entropy alloys in which the constituent elements are refractory elements. Their high melting points make them attractive as next-generation high-temperature materials because of their potential for applications beyond state-of-the-art Ni-base superalloys. However, before that potential is realized, a few critical aspects of RHEAs need to be clarified, including the following: (1) are their tensile creep properties in fact superior to those of leading superalloys, (2) do they possess a minimal level of tensile ductility at room temperature to withstand normal handling and accidental drops, and (3) do they undergo phase transformations (microstructural instabilities) and the associated degradation of mechanical properties under expected service conditions. In this talk I will address these points by presenting our experimental and thermodynamic simulation results on model RHEAs of the VNbMoTaW and TiZrHfNbTa systems and their lower-order subsystems (quaternary, ternaries, etc.). Our results show that, the latter alloys, although relatively ductile at room temperature, have a long way to go before they can be creep-competitive with Ni-base superalloys at elevated temperatures. The former alloys are brittle at room temperature, and there is insufficient tensile creep data to judge whether they will be creep-competitive with Ni-base superalloys. Additionally, many of these RHEAs undergo phase decompositions when subjected to the expected thermal cycles of engine components such as turbine blades, which would further deteriorate the already marginal ductility and toughness of these alloys. By systemic investigation of pseudo-binary subsystems of these model quinary alloys, we attempt to understand the key factors responsible for the observed phenomena and provide suggestions for improvement of their physical and mechanical properties.