Effect of hot isostatic pressing on the microstructure and mechanical properties of additive manufactured Al_xCoCrFeNi high entropy alloys

Three high entropy alloys (HEAs), based on the Al _x CoCrFeNi alloy system have been prepared by direct laser fabrication (DLF) with aluminium molar fractions (x) of 0.3, 0.6 and 0.85. These three alloys had FCC, duplex FCC + BCC, and BCC crystal structures, respectively. The effect of hot isostatic pressing (HIP) on alloy density, microstructure and mechanical properties of these DLF bulk high entropy alloys was studied for the first time. HIP was found to decrease the number of large pores (> 5 µm) in the as-deposited alloys, which equated to a marginal increase in density. HIP also induced microstructural coarsening, chemical homogenisation and resulted in a general improvement in the mechanical properties of FCC HEA (x = 0.3). HIP improved the compressive properties of the dual phase HEA (x = 0.6), however, degraded the tensile properties as a result of the coarsening of hard BCC grain boundary precipitates. The mechanical properties were compromised in the high aluminium (x = 0.85) HEA due to the formation of σ-phase at the phase and grain boundaries, which induced a brittle fracture in tension and compression. A cooling curve transformation diagram (CCT) for the σ-phase was determined by a dilatometric method and the critical cooling rate to inhibit σ-phase formation was found to be 1 K/s.