Designing lightweight dual-phase refractory VNbTiSi-based eutectic high-entropy alloys for use at elevated temperatures

The emergence of high-entropy alloys (HEAs) provides a vast compositional space for designing high temperature materials for next generations of advanced propulsion systems. In this study, novel lightweight (about 6.0 g/cm3 in density), refractory (melting point higher than 1630 °C) VNbTiSi-based dual-phase eutectic high entropy alloys (EHEAs) that consist of body-centered cubic (BCC) solid solution and tetragonal-structured silicide phase are designed. The as-cast compressive strength at room temperature (RT) can be as high as 2.4 GPa and the RT fracture toughness achieves 24.58 MPa∙m1/2, higher than many of the as-cast and even directionally solidified Nb–Si based alloys. The tensile yield strength and elongation at elevated temperature (1100 °C) can be above 300 MPa and 10%, respectively. The anomalous eutectic structure with rod-like silicide phase dispersed in the continuous BCC network is revealed, and two kinds of orientation relationships between the two phases are identified: {100}BCC//{110}Silicide (or <100>BCC//<110>Silicide), and <111>BCC//[011]Silicide or <111>BCC//[101]Silicide with no paralleled planes observed. The BCC phase provides ductility both at room and elevated temperatures, where plastic deformation is dominated by the multiplanar glide of long straight screw dislocations, whereas the silicide phase undergoes no plastic deformation and acts as the strengthening phase at elevated temperatures. This study evidences the feasibility of phase-constituent simplification in a complex composition space based on the high entropy strategy and the possibility of optimizing materials on demand.