Influence of the starting powder and ball milling on microstructure and mechanical properties of Al0.3CoCrFeNi high entropy alloy, obtained by spark plasma sintering

This work investigates the effect of ball milling on the microstructure and mechanical properties of Al 0.3 CoCrFeNi high entropy alloy (HEA), starting from two different powders: pre-alloyed powders obtained by gas atomization and a pure elemental mixture. The powders were densified using spark plasma sintering (SPS). For the atomized powders, ball milling leads to the formation of a heterogeneous core–shell structure that remains after sintering, with grain sizes ranging from 1 to 10 μm in the cores and less than 1 μm in the shells. Slight air contamination during the milling stage promoted the formation of nanometric AlN at grain boundaries within the shells, which inhibited grain coarsening during SPS. Additionally, Al₂O₃ precipitates formed at prior particle boundaries (PPBs). Grain size after sintering is refined from 13 μm to 3.3 μm (without and with 28 h milling of atomized powder), resulting in an increase in hardness (from 182 to 280 HV₂), yield strength (from 370 to 615 MPa), and ultimate tensile strength (from 653 to 770 MPa), accompanied by a loss in ductility (from 54 % to 18 %). However, below 28 h of milling, the strengthening is not as high as would be expected from the Hall-Petch effect. PPBs have been identified as crack initiation sites, leading to lower-than-expected tensile properties. This negative effect of PPBs can be decreased by increasing powder particle size through prolonged milling, thereby reducing PPBs density. In contrast, mechanical activation of elemental powder mixture leads to the formation of an ultrafine-grained microstructure with dispersed nanometric B2 and Al₂O₃ phases. This elaboration route allows a higher mechanical strength (1175 MPa) and hardness (512 HV 2 ), but significantly lower ductility (2 %). The random distribution of minor secondary phases does not lead to PPB-related failure. This study demonstrated the strong influence of starting powders on the milling-induced microstructure, phase formation, and the resulting strengthening mechanisms governing the mechanical behavior of Al 0.3 CoCrFeNi HEAs.