Mechanical properties of reactive sintered high-entropy boride/carbide composite

• The dual-phase high-entropy boride/carbide composite exhibits a dense, homogeneous microstructure with fine grains and minimal oxide content. • Mechanical testing reveals the highest hardness in boride grains, intermediate hardness in carbide grains, and lower hardness at grain boundaries • Toughening occurs through crack deflection, crack branching, and crack bridging along grain boundaries and transgranular fracture paths. • Adding SiC whiskers is a promising strategy to enhance toughness and wear resistance in future composite development. High-entropy equimolar (Ti-Zr-Nb-Ta-Hf)B2 - (Ti-Zr-Nb-Ta-Hf)C dual–phase boride/carbide composite was prepared via a two-step spark plasma sintering (SPS). The aim of this research is to quantify the nano, micro, and macro-scale hardness and the indentation fracture resistance of this dual-phase composite, and to relate these properties to its microstructure and observed crack-toughening mechanisms. The composite exhibits a high relative density (∼99%) and a homogeneous microstructure, with average grain sizes of the boride and carbide phases measuring 4.4 μm and 4.1 μm, respectively, with a negligible oxide content (HfO₂/ZrO₂ < 1 wt%). The nanohardness and indentation modulus of the boride and carbide grains are 41.4 GPa and 628 GPa, as well as 39.3 GPa and 577 GPa, respectively. The nanohardness and indentation modulus at the boride/carbide grains/phase boundary are 35.7 GPa and 592 GPa, respectively. The hardness, HV1, and the indentation fracture resistance of the composite are 22.4 GPa and 4.9 MPa·m 0.5 , respectively. Toughening mechanisms in the form of crack deflection, crack branching and crack bridging, are often found at the grain boundaries and during the transgranular fracture of larger boride and carbide grains, probably connected with the high-entropy effects.