Nanoscale structural heterogeneities in metastable refractory high-entropy alloy

Most refractory high-entropy alloys (RHEAs) with the dominant body-centered-cubic (bcc) phase are lack of room temperature tensile ductility, which strictly limits their large-scale applications in industry and aerospace. Recently, it has been found that the strategy of metastability engineering can be utilized to ductilize the TiZrHf-based RHEAs. However, up to now, a nanoscale understanding of the potential structural heterogeneities in this system remains exclusive. Herein, through performing the transmission electronic microscope (TEM) characterization on a prototype TaZrHfTa_0.5 RHEA with exceptional tensile properties, we have found that this alloy contains a high density of nanoscale structural heterogeneities that are randomly distributed throughout the bcc matrix, including the hexagonal ω phase forming by the collapse of {1 1 1}_bcc atomic pairs and the orthorhombic α″ phase forming by coupled shuffle-shear mechanism. It may provide a microstructural basis for manipulating the macroscopic properties of metastable RHEAs.