A real-time TEM study of the deformation mechanisms in β-Ti reinforced bulk metallic glass composites

The deformation mechanisms in a β-Ti reinforced Zr-based bulk metallic glass composite (BMGC) are studied by extracting submicron sized tensile coupons of the crystalline and amorphous phase in their monolithic and bilaminate composite forms and tensile testing them inside a TEM. Results show that the monolithic BCC crystalline phase and amorphous phase have high yield strains, owing to small length scale effects, but undergo negligible post-yield elongation before failure. However, the ductility of the bilaminate composite is significantly higher (~12.4%), provided the thickness of its amorphous portion is < 100 nm. Real time videos, which are supplemented by molecular dynamics simulations, show that the negligible post yield elongation of the monolithic BCC crystalline phase is caused by planar slip on one of the {110} planes. However, the bilaminate composite form exhibits strain hardening, despite the occurrence of planar slip, as dislocations pile up at the impervious amorphous/crystalline interface, which in turn, activates slip on other {110} planes. Strain hardening in the crystalline phase ceases when a shear band nucleated in the amorphous phase penetrates the dendrite along one of the slip planes. These mechanisms are extended to explain the flow behavior of β-Ti reinforced BMGCs and strategies to improve the ductility are discussed.