TiZrHfNbTa (RHEA-Ti) and VNbMoTaW (RHEA-V) alloys, which are expected to be used as new refractory alloys over Ni-based superalloys, have been widely studied. Our experiments found that these two alloys have different strength and ductility properties, but the reason and mechanism are still unclear. The present study aims to clarify the underlying factors that cause the differences in mechanical properties by electronic structure calculations.
 In metals, lattice distortion is known to correlate closely with the strength. Therefore, we evaluated the lattice distortion using an effective parameter, mean square atomic displacement (MSAD), which is directly evaluated using atomic displacement (Fig. 1a). The results show that the MSAD of RHEA-Ti is three times larger than RHEA-V's, indicating a significant increase in strength.
 The dislocation structure, which determines ductility was analyzed (Fig. 1b). The dislocation core of RHEA-Ti is heterogeneously spread and the dislocation energy becomes significantly low. This finding indicates that dislocations are easily introduced into the matrix, contributing to the excellent ductility in RHEA-Ti at low temperatures.
 Electronic structure calculations showed that these properties are due to group IV elements such as Ti, Zr, and Hf, which are expected to be developed into an element-strategy alloy design.

This work was supported by JSPS KAKENHI Grant Number JP18H05453, Modeling and simulation on mechanical properties of high entropy alloys.