Effect of AlCrFeCuNi High-Entropy Alloy Reinforcement with a Nanocrystalline Internal Structure on the Microstructure, Electrical Conductivity and Tribological Behavior of SPS-Processed Cu–B4C Composites

This study investigates the effect of mechanically alloyed AlCrFeCuNi high-entropy alloy (HEA) reinforcement with a nanocrystalline internal structure on the microstructural, mechanical, electrical, and tribological properties of Cu–1 wt.% B4C composites containing 0–30 wt.% HEA. The AlCrFeCuNi reinforcement used in this work had previously been produced by 25 h of mechanical alloying and characterized in detail, exhibiting a dual FCC–BCC structure and an average crystallite size of 10.2 nm while retaining micrometer-scale particle dimensions. The Cu–B4C–HEA powder mixtures were subsequently milled and consolidated by spark plasma sintering at 800 °C under 35 MPa. Increasing the HEA content progressively refined the powder mixture and increased the hardness from 78.8 HB for 0HEA to 138.94 HB for 20HEA, while the latter retained a relative density of 96.1% and an electrical conductivity of 67% IACS. The 20HEA composite exhibited the lowest average friction coefficient (0.36) and specific wear rate (1.01 × 10−3 mm3/N·m), corresponding to an approximately 88% reduction in wear rate relative to 0HEA. Microstructural and worn-surface analyses showed that this behavior was associated with the combined effects of hard HEA reinforcement, preserved Cu-matrix continuity, and the formation of an oxygen-rich mechanically mixed layer. Increasing the HEA content to 30 wt.% promoted reinforcement clustering and residual porosity, which reduced densification and partially deteriorated the tribological performance.