strengths at elevated temperatures. Indeed, compression tests to 1200 °C have confirmed that the yield strength of the VNbMoTaW RHEA is superior to that of Inconel 718 above 800 °C. RHEAs such as VNbMoTaW and TiZrNbHfTa, with estimated melting temperatures (Tm) of 2700°C (rule of mixtures) and 2000°C (CALPHAD calculation), respectively, are being explored. However, the phase stability and phase separation in RHEAs is historically difficult to study because of the limitation of experimental capability at high temperatures and sluggish decomposition kinetics to reach equilibrium at low temperatures. For example, investigations into the phase stability of the Ti-Zr-Hf-Nb-Ta system have shown that TiZrNbHfTa has a single-phase BCC structure after 2 hours at 1000°C and 3 hours at 1200°C, but transformed into a BCC1+BCC2 structure with ZrHf-enriched and NbTa-enriched phases after 2 hours at 800°C . However, additional work on TiZrNbHfTa by showed that after high-pressure torsion (HPT) at room temperature, the alloy consisted of a single BCC phase and subsequent annealing for 1 hour at 300°C did not produce a phase transformation . The phase separation at high temperature does not seem consistent with the single phase observed at low temperature.