Due to the absence of long-range order, bulk metallic glasses (BMGs) exhibit unique properties, such as high strength, high hardness, large elastic limit, good soft magnetic properties, and high corrosion resistance. However, there is a fatal weakness that prevents wide application of BMGs: that is almost zero plasticity under tension and limited plasticity under compression. Therefore, improving the plasticity of metallic glass is currently the subject of extensive study. Recent research has shown that impressive improvements in plasticity can be achieved for some Zr-, Pd-, Pt-, Cu-Zr-. Ti-based bulk metallic glasses (BMGs) under compression, in which the enhanced plasticity was attributed to the presence of small nanocrystals, stress-induced nanocrystallization, and a high Poisson ratio. However, the mechanism is still not fully understood. The excellent plasticity of a Ti-based BMG sample with 1mm diameter and a Cu-based BMG under compression were attributed to an increase in free volume introduced by high cooling rate and minor alloying, respectively. Moreover, many earlier investigations have shown that annealing imparts severe brittleness to the BMGs. This is attributed to various factors including reduction in free volume, precipitation of crystalline phases and phase separation. Of these mechanisms, the most prominent is the reduction in free volume that occurs during structural relaxation of metallic glasses, whereby free volume gets redistributed and annihilated, resulting in the loss of ductility. There have also been several reports that the ductility of thermally embrittled amorphous ribbon samples could be recovered by thermal treatment or neutron-irradiation. Furthermore, Nagel et al. demonstrated that the annihilated free volume can be restored by heat treatment above the glass transition temperature (T{sub}g). Thus, detailed understanding of the effect of the free volume on the mechanical properties of BMGs, in particular ductility is still important from the applications point of view. In this paper, Cu{sub}46Zr{sub}47Al{sub}7 BMG alloys with different free volume states were obtained through thermal treatments such as annealing and quenching. We demonstrate that the ductility of BMGs is closely related to the free volume and the ductility of thermally embrittled BMGs can be restored by thermal treatments.
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