Micro-to-nano-scale microstructural evolution and mechanical characteristics of (CrFeNiCu)100-xAlxhigh entropy alloys with x=0, 3, 5, 7, 10, 12 and 15 at.%

High entropy alloys (HEAs), as advanced materials, have gained significant attention over the past decade owing to their exceptional properties. As-cast (CrFeNiCu)100-xAlx HEAs (x = 0, 3, 5, 7, 10, 12, and 15 at.%) were designed by adding Al as a minor element to the quaternary CrFeNiCu HEA to lead microstructural evolution and enhancement of mechanical properties. Systematic microstructural analyses found that addition of Al content more than 7 at.% induced significant micro-to-nano-scale microstructural evolution. A minor addition of Al content lower than 5 at.% was ineffective to modulate the microstructure of the HEAs composed of CrFeNi-rich FCC1 and Cu-FCC2 phases. In contrast, the HEAs containing Al content higher than 7 at.% exhibited crucial microstructural evolution from dual-phase FCC1/FCC2 structure to multi-phase FCC1/FCC2/BCC structure. Furthermore, the primary BCC dendrite revealed a phase separation into nanoscale A2/B2 phase, and L12 nanoprecipitates formed in the FCC phase. The Al content with large negative mixing enthalpy with constituent elements induced the micro-to-nano-scale chemical evolution and heterogeneity, which resulted in micro-tonano-scale microstructural evolution. The volume fraction of the BCC phase strongly depended on Al content and significantly improved the yield strength from 291 MPa to 1366 MPa and Vickers hardness from 134 HV to 475 HV. Although the increase in volume fraction of BCC phase in the HEAs decreased plasticity from a higher than 45 %-16.02 %, this result could be considered reasonable plasticity for engineering materials.