Triple yielding and deformation mechanisms in metastable Cu47.5Zr47.5Al5 composites

A triple yielding phenomenon resulting in high fracture strength (up to 2360 ± 10 MPa) and large ductility (up to 13.5 ± 0.5%) during compressive deformation is first discovered and systematically investigated in metastable Cu47.5Zr47.5Al5 composites. Based on electron microscopy and X-ray diffraction studies, the major deformation mechanisms at the different stages of compression are revealed to be: (1) the constraint effect of the amorphous phase; (2) the pre-existence of a small number of microtwins and a small amount of martensitic phases; (3) the initiation and development of martensitic transformation; (4) the interplay between CuZr crystals and shear bands; and (5) the activation of partial detwinning, together with a high density of dislocations. The yield strengths of the composites are modeled as a function of the volume fraction of the constituent phases and a transition from the rule-of-mixtures to a load-bearing model is observed. The underlying ductile nature of the composites is further correlated with the unique electronic structure of the B2 CuZr intermetallics. The present results provide an understanding of the deformation mechanism of metastable CuZr-based composites and give guidance on how to improve the ductility/toughness of bulk metallic glasses.