Synergistic effects of microstructural inhomogeneity and phase-transformation-induced plasticity for ductility improvements in metallic glass composites

Recent experiments show some successes in ductility improvements in bulk metallic glass (BMG) composite with metastable crystalline second-phase particles that are capable of phase transformation induced plasticity (TRIP), e.g., austenite to martensite upon straining. The underlying mechanisms are yet to be quantified for a few reasons. First, ductility enhanced by TRIP mechanism in crystalline materials is often credited to the delayed necking due to the enhanced work hardening rate, while BMG composites with such metastable second phases never fail by necking. Second, load partitioning amongst austenite and martensite phases and BMG matrix can be probed by in situ diffraction techniques, but its consequence on preventing or delaying the transition from shear bands to failure has not been revealed. In this work, synergistic effects of microstructural inhomogeneity and TRIP mechanism are quantitatively investigated from micromechanical finite element simulations, using the free-volume theory that is capable of explicitly modeling individual shear bands in the BMG matrix and a strain-driven TRIP model that can be calibrated from available neutron diffraction measurements. For metastable second phases to improve the BMG composite ductility, our parametric studies have found that the strength of the BMG matrix has to be in between the strengths of soft austenite and hard martensite phases, thus leading to the effectively confinement of shear bands near the second phase, the reduction of maximum shear band strain, and therefore the improvement of tensile ductility. Our micromechanical simulations also allow us to identify the roles played by geometric factors such percolation and core-shell or inverse core-shell distributions.