Laser directed energy deposited Al0.8Nb0.5Ti2V2Zr0.5 lightweight refractory high entropy alloy: Regionalization of microstructure characteristics and strengthening mechanisms

Lightweight refractory high entropy alloys (LRHEAs) are emerging as promising candidates for lightweight and high-temperature applications, primarily due to their relatively low density and excellent specific strength at both room and elevated temperatures. However, due to large residual stress, relatively high melting point of constituent elements and the presence of the B2 or Laves phases with large volume fractions, LRHEAs fabricated by laser additive manufacturing often suffer from crack defeats, incomplete powder fusion and reduced room-temperature ductility. In this work, a crack-free LRHEA was successfully fabricated by laser directed energy deposition (LDED) using the pre-alloyed powder. The microstructure characteristics and compressive mechanical properties were systematically investigated. The alloy exhibited a fine equiaxed/columnar bimodal grain size distribution with BCC matrix and AlZrV C14_Laves phases near grain/subgrain boundaries. The high cooling rate during LDED inhibited elemental microsegregation to some extent, and the distribution of C14_Laves phases was regionalized due to the thermal history of the LDED process. The compressive yield strength reaches 1386 MPa at room temperature, 940 MPa at 873 K and 450 MPa at 1073 K. At 298 K, the LRHEA demonstrated a high specific yield strength (SYS) of 260 kPa m3/kg with an outstanding fracture strain of 20.5 % among several LAM-ed RHEAs, which can be attributed to synergetic strengthening from solid solution strengthening and second phase strengthening. The excellent SYS-ductility combination of the LRHEA highlights its great potential for advanced structural applications.