Atomic bond deficiency as a structural defect in amorphous metals: Relevance to glass transitions

Atomic bond-deficiency (BD) is considered to be a deviation from the coordination number of the ideal atomic structures caused by chemical and topological short-range ordering (CSRO/TSRO). It is presented as the characteristic structural defect in metallic glasses and deeply undercooled melts. BD defects are treated as thermally inherent and are based on the notion of restricted, rather than free, atomic movement. They are formed under the SRO constraint, and exhibit significantly smaller energy and volume costs when compared with crystal vacancies. Statistical thermodynamics are employed for the analysis of binary systems where the formation of the critical BD defects is related to the bond strengths and atomic sizes. Application to Zr–Cu, Zr–Ni and Zr–Be indicates that the experimentally observed glass transitions occur at temperatures where a critical concentration of BD defects is present. The dependence of BD defect concentration on alloy composition agrees with the experimental glass transition variations.