First-Order, Structural Transformations in Metallic Glasses

Magnetic evidence is presented for a first-order, structural transformation in the local order of a cobalt-base metallic glass. The transformation is centered at $T_0=180\mathrm{}°$ C and shows a scan-rate-dependent thermal hysteresis of $\Delta T=100\mathrm{}°$ C. A critical volume for nucleation of the transformation is estimated to contain approximately 200 atoms. The transformation appears in samples which show no microcrystallites greater than 30 Å and vanishes in samples with appreciable crystallinity. The observations are discussed in terms of model clusters of icosahedral, trigonal, and octahedral symmetry.