A Ni–CoWB composite developed by devitrification of Ni–Co–W–B bulk metallic glass

Highlights: • The first composite reinforced by the ultra-hard CoWB phase is developed. • The fully crystallized composites contain about 60 vol% FCC-Ni and 40 vol% CoWB. • Composites having microhardness values higher than 1400 HV are obtained. • The microhardness of CoWB phase is determined to be between 4500 and 5000 HV. • Increasing boron content is expected to improve both microhardness and toughness. The effects of annealing on microstructure and mechanical properties of Ni_36.3Co₂₅W_23.7B₁₅ bulk metallic glass are investigated. Composite samples were produced by annealing amorphous samples of the alloy above the crystallization temperature. Microstructural examinations revealed that nickel solid solution and CoWB phase precipitate upon annealing. Fully crystallized composite samples contain about 60 vol% nickel solid solution and 40 vol% CoWB phase. Microhardness of the amorphous alloy is determined to be 1168 HV. Composites having microhardness values higher than 1400 HV are obtained. Improvement in microhardness is caused by the formation of CoWB phase, whose microhardness is determined to be between 4500 and 5000 HV. This is the first composite reinforced by CoWB phase. Indentation fracture toughness of the composite having peak hardness ranges between $2.44$ and $4.67MPa\sqrt{m}$. It is found that further annealing results in slightly lower microhardness but higher indentation fracture toughness, which is between $3.55$ and $6.45MPa\sqrt{m}$. The effects of microstructure on microhardness and indentation fracture toughness of the composites are discussed.