Ultra-high strength Co–Ta–B bulk metallic glasses: Glass formation, thermal stability and crystallization

Co61Ta6B33, Co59Ta8B33, Co57Ta10B33 and Co53Ta10B37 bulk metallic glasses (BMGs) exhibit a good combination of the ultra-high fracture strength in the range of about 5.4–6.2 GPa, Vickers hardness of 1459–1653 HV0.2 and plastic strain of 0.3–3.2%, depending on composition. Glass forming-ability and physical properties of Co–Ta–B BMGs are discussed in relationship to the Ta and B content and atomic structure. Crystallization upon isochronal and isothermal annealing is studied using differential scanning calorimetry and X-ray diffraction. A novel crystalline phase with ~Ta2Co15B8 or ~TaCo7B4 formula is shown to form upon glass annealing. The incubation time for isothermal crystallization in the supercooled liquid state near Tg is supposed to be sufficient for thermoplastic forming of Co–Ta–B BMGs.