Mechanical property degradation of a CuZr-based bulk metallic glass composite induced by sub-Tg annealing

The annealing effect at temperatures below glass transition temperature (Tg) on the mechanical behavior of a ductile Cu48Zr48Al4 bulk metallic glass composite (BMGC) containing a B2-CuZr phase was investigated. It was found that remarkable plasticity and fracture strength degradations of the CuZr-based BMGC occurred with increasing the annealing temperature in the condition without significantly changing the crystalline volume fraction in the BMGC, which were correlated with the annealing-induced microstructure variations. It is expected that the free volume in the glassy matrix of the BMGC still played an important role for its deformation behavior, in despite of the existence of the B2-CuZr phase. The sub-Tg annealing-induced free volume annihilation depressed the shear band generation in the glassy matrix, reduced the synchronous contribution of the “blocking effect” and “deformation-induced martensitic transformation effect” of the B2-CuZr phase to the multiplication of shear bands, resultantly caused the plasticity degradation. The annealing-induced martensitic transformation of the B2-CuZr phase at the temperature close to Tg would further expand the plasticity degradation due to the absence of the “deformation-induced martensitic transformation effect”. Furthermore, the plasticity degradation simultaneously resulted in the fracture strength reduction of the BMGC because its work-hardening-like behavior was conditioned by the plastic deformation ability. The present results indicate that the ductile CuZr-based BMGC reinforced by the B2-CuZr phase similarly suffers from sub-Tg annealing-induced embrittlement, as is the case for most monolithic BMGs.