Cryogenic thermal cycling induced simultaneous improvement of strength and ductility in a Zr-based bulk metallic glass composite

Preparing bulk metallic glass composites (BMGCs) is an effective way to improve the plasticity of bulk metallic glasses (BMGs). However, the enhancement of plasticity is usually at the cost of deterioration in strength. In this study, a series of BMGCs with the nominal composition of Zr48Cu48Al4 was prepared. Finely dispersed B2 CuZr crystals are embedded in a glassy matrix and subsequently, these composites were cryogenically cycled in liquid nitrogen followed by heating to various temperatures below the glass transition temperature. It was found that the samples after cryogenic thermal cycling (CTC) treated at an intermediate upper temperature shown a simultaneous improvement of yielding strength from 1680 MPa to 1832 MPa and plasticity from 5.6% to 9.1%. The enhancement of yielding strength is ascribed to the existence of the B19 ' phase resulted from the partial martensitic transformation during the CTC treatment, while the improvement of plasticity is attributed to the higher free volume content and the larger "blocking effect" from the B19' after the samples were treated by CTC. The combination of CTC treatment and precipitation of crystals provides a new strategy for improving the mechanical properties of the BMGs.