Formation of CuZr-based bulk metallic glass composites containing nanometer-scale B2-CuZr phase through sub-T_g annealing

Annealing-induced crystallization behavior of a typical CuZr-based bulk metallic glass (BMG) with a nominal composition of Cu₄₈Zr ₄₈Al₄ was investigated at a temperature below the glass transition temperature (T_g). A metastable B2-CuZr phase was found to primarily precipitate during isothermally annealing the monolithic BMG at the sub-T_g temperature. Afterwards, a eutectoid reaction from the primary metastable B2 phase to the low-temperature equilibrium phases, Cu ₁₀Zr₇ and CuZr₂, occurred with increasing the annealing time. There is a rather large time window at the sub-T_g temperature for the formation of the CuZr-based BMG composites (BMGCs) containing the single B2-CuZr phase. In the annealing-induced BMGCs, the size of the B2 phase is in nanometer-scale. Vickers hardness and compression experimental results display that no ductilizing but only strengthening occurred for the nanometer-scale B2-CuZr phase reinforced BMGCs. Furthermore, it is shown that enhanced plasticity could be obtained for the BMGCs after a free-volume recovery treatment at a temperature above the T_g. The enhanced plasticity is further proved by three-point bending tests.