Enhancement of plasticity in Co–Nb–B ternary bulk metallic glasses with ultrahigh strength

A family of ultrahigh strength Co-based bulk metallic glasses (BMGs) with critical diameters up to 2 mm was fabricated in ternary Co65 − xNb8B27 + x (at.%, x = 1–5) alloy system. The highest glass forming ability for the alloy composition with x = 3 is in agreement with its largest supercooled liquid region, lowest liquidus temperature and eutectic behavior, which are closely related to the presence of backbone structure formed by transition metal (Nb) and metalloid elements (B). The present glassy alloy system also possesses excellent mechanical properties including both yield strength of 4610–5340 MPa and plasticity up to 5% simultaneously, which is rarely reported for other BMGs known so far. It is found that changes in the proportion of metalloid B contents have significant influence on the atom packing fraction and therefore the enhanced yield strength. Meanwhile, the enhancement of plasticity is ascribed to the formation of nanoscale-range order clusters in the CoNbB BMGs.