Fabrication of ultra-fine grained Hf-based materials with superior hardness and temperature-independent electrical conductivity by a combination of high-energy ball milling and spark plasma sintering

Ultra-fine grained (UFG) hexagonal-close-packed (HCP) hafnium (Hf) and Hf-5 wt%Y2O3 (HYO) were prepared by a combination of high-energy ball milling and spark plasma sintering (SPS), and both possess high relative density (95–99%), high hardness, and low electrical conductivity. The HYO sample shows superior hardness of 12.11 GPa, which is about 6–7 times of that of coarse-grained Hf, and electrical conductivity of 2 × 105 S/m, which keeps constant and is almost independent of temperature. It is very possible for HYO with these good properties to become an excellent plasma torch cathode. The analysis of the microstructure under transmission electron microscope (TEM) and scanning electron microscope (SEM) shows that the superior hardness of the samples originates from the grain boundary (GB) strengthening and the pinning effects of the Y2O3 particles, while their almost temperature-independent electrical conductivity originates from the combining effects of the dispersion of large electrical resistivity particles Y2O3 and the dense high-angle grain boundaries (GBs). •Ultra-fine grained (UFG) hexagonal-close-packed (HCP) Hf and Hf-5 wt%Y2O3 (HYO) were prepared for the first time.•HYO shows super hardness of 12.11 Gpa, which is about 6–7 times of that of traditional Hf.•The conductivity of HYO is 2 × 105 S/m, which keeps constant and almost independent of temperature.