β-type Ti-based bulk metallic glass composites with tailored structural metastability.

Bulk metallic glass composites (BMGCs) containing an in-situ formed metastable β phase (β-type BMGCs) being capable of transforming to martensites, are a promising class of alloys. Nonetheless, the influence of the structural metastability of β phases on the deformation mechanisms of BMGCs remains largely unclear. This, however, is crucial for understanding and optimizing plastic deformability. In this study, three Ti-based β-type BMGCs with nominal compositions of (Ti 0.505 Zr 0.353 Fe 0.028 Be 0.114 ) 100− x Cu x ( x = 1, 5, 8) have been developed, in each of which the β-Ti phase has a different metastability. For x = 8, the β phase precipitates exclusively and does not transform martensitically during deformation. At the lowest Cu content ( x = 1), however, the β phase contains a high density of fine ω phase upon quenching, which hinders the deformation-induced martensitic transformation. β phases in both BMGCs deform plastically by dislocation glide. Only when x = 5 does the β phase undergo the martensitic transformation to α″-Ti due to an appropriate degree of metastability. The reversible phase transformation between β and α″ followed by twinning in α″ on continuing deformation leads to a “double yielding” behavior and results in strong work-hardening capability as well as superelasticity during compression (cyclic loading) tests. [ABSTRACT FROM AUTHOR]