Elucidating the factors governing strength–plasticity synergy in B2-containing bulk metallic glass composites

The mechanical properties of in situ bulk metallic glass (BMG) composites depend on multiple characteristics of the crystalline second phase, whose individual contributions are difficult to distinguish. Here, the coupled roles of four factors were examined in TiZrCuNiSiSn BMG composites: (i) second-phase fraction, (ii) phase transformation behavior, (iii) second-phase distribution, and (iv) amorphous-matrix enclosure. Compression tests and in situ synchrotron X-ray diffraction provided insights into how these factors jointly influence deformation. First, the B2-phase fraction was associated with the plastic-deformation mode, with increasing fractions reducing the stress drops associated with collective shear-band motion. Second, the B2 transformation behavior was closely associated with plasticity: a lower transformation stress enhanced plasticity, whereas its effect on yield strength depended on the degree of constraint imposed by the surrounding amorphous matrix. Third, in both the isolated and interconnected B2 microstructures, martensitic transformation first became detectable at approximately 1.7 GPa, indicating that the transformation was delayed until a similarly high stress level. Fourth, the sharp strength decrease at high B2 fractions was interpreted in terms of a local loss of amorphous matrix enclosure, which may reduce constraint on the surface-exposed B2 regions and increase the influence of their transformation on macroscopic yielding. These findings suggest that incorporating a low-transformation-stress B2 phase while maintaining sufficient constraint through amorphous-matrix enclosure is an effective strategy for achieving both high strength and large plasticity.