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 T_g is supposed to be sufficient for thermoplastic forming of Co–Ta–B BMGs.