Effect of reinforcement fraction on phase constitution, microstructure and hardness of Al–FeCoCrNiMo composites

Aluminium matrix composites reinforced with high-entropy alloy (HEA) particles offer a promising route to enhanced mechanical performance through multicomponent solid-solution strengthening, yet composites processed by simple pressureless powder metallurgy (PM) remain comparatively under-reported. In this work, FeCoCrNiMo HEA powder was synthesised by high-energy ball milling (20 h) and incorporated into aluminium at 0, 1, 5 and 10 wt% by PM (warm compaction and pressureless vacuum sintering at 600 °C). Phase constitution, microstructure, elemental distribution and hardness were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and Brinell hardness testing. The milled HEA powder exhibited a predominant body-centred cubic (BCC) structure, with progressive diffraction-peak broadening indicating crystallite refinement from 20.49 nm to 5.28 nm after 20 h of milling. Composite XRD patterns retained the Al-matrix reflections without additional detectable phases within the detection limit of laboratory XRD. Increasing HEA content altered the microstructure and introduced more pronounced porous regions, with relative density falling from 97.8% to 90.1%. The Al–1 wt% HEA composite exhibited the highest hardness, 52.14 HB, against 36.99 HB for pure Al, a 41% increase, whereas 5 and 10 wt% additions were markedly less effective. These findings indicate a reinforcement-content–porosity–hardness relationship for PM-processed Al–FeCoCrNiMo composites.