Manifestations of point and extensive defects of bulk-metallic glasses

In spite of the fact that the one-parameter free-volume model is inadequate to provide a reasonable description of structural defects of bulk-metallic glasses (BMGs), this model is still in use due to its simplicity and conformity with naive ideas on the BMG structure. Meanwhile, the polycluster model of the metallic-glass structure takes into account the occurrence of locally preferred short-range and medium-range ordering, stable point defects (vacancies, interstitials, semi-vacancies, semi-interstitials, and impurities), and extensive defects (dislocations, cluster boundaries, and triple joints). Some of the defects, such as cluster boundaries and triple joints, are accessible for the direct observation by means of the field-emission microscopy (FEM). Others manifest themselves in macroscopic kinetic and mechanical phenomena. In this report, we briefly describe the point and extensive defects of metallic polyclusters and present some results of direct and indirect observations of their manifestations. The cluster boundaries, triple joints, and sub-clusters are identified by means of FEM. Vacancies and short-range ordering are revealed through the recovery annealing kinetics of BMGs irradiated with 2.5 MeV electrons. Since the vacancy production by irradiation is accompanied by the generation of interstitials (due to the Frenkel-pair formation), one can consider these results as an evidence of the interstitial formation in BMGs. However, the stability of interstitials in metallic glasses remains to be investigated. A pronounced manifestation of dislocations (first of all, cluster-boundary dislocations) is the Kaiser effect observed in a set of BMGs. A discussion of the observed phenomena is given.