Phase transformation sequences in Refractory Metal High Entropy Superalloys

Refractory metal high entropy superalloys (RSA) look set to offer a step change in temperature capability over existing high temperature alloys. However, the transformation sequences that lead to their two-phase A2-B2 microstructures are yet to be fully understood. Current hypotheses are based on a high temperature spinodal decomposition and subsequent ordering reaction but there is no conclusive evidence as to the veracity of this mechanism. Direct characterisation of these transformation sequences, along with the temperatures at which they occur is imperative to developing our understanding of these materials and guiding future alloy design. Here we propose to make a series of neutron diffraction measurements at elevated temperatures to conclusively establish, for the first time, the microstructural evolution pathway of an exemplar RSA and a simplified quaternary analogue. These results will provide crucial and high impact information to the global RSA community and help determine the direction of future alloy design.