Nanohardness of grains and grain boundaries in reactive sintered (Ti-Zr-Nb-Hf-Ta)B2 + (Ti-Zr-Nb-Hf-Ta)C composite

Abstract

Nanohardness of grains and grain boundaries in reactive spark plasma sintered dual-phase (Ti_0.82Zr_0.04Nb_0.08Hf_0.03Ta_0.03)B₂ + (Ti_0.49Zr_0.12Nb_0.13Hf_0.11Ta_0.15)C high-entropy composite was investigated. The composite with a high relative density of 99.8 %, consists predominantly of carbide (∼56.4 vol%) and boride (∼39.7 vol%) phases with average grain sizes of 2.2 μm and 3.4 μm of the carbide and boride phases, respectively. Most grain/phase boundaries show a continuous sharp ∼ 1.5 nm wide segregation of Fe, Co, and Ni impurities. Nanohardness was measured by nanoindentation on polished/non-deformed and worn/deformed surfaces. At non-deformed surface, the average hardness/Young’s modulus of boride and carbide grains are 41.3 ± 2.9 GPa/596 ± 25 GPa and 38.3 ± 2.3 GPa/553 ± 24 GPa, respectively. At the vicinity of grain boundaries, the hardness/Young modulus is 37.3 ± 2.5 GPa/600 ± 32 GPa. At deformed surfaces, the average hardness of boride grains is higher and the average hardness of carbide grains is slightly higher compared to the values at non-deformed surfaces, with values of 42.4 ± 3.9 GPa, and 38.4 ± 2.4 GPa, respectively.