Kinetics of structural relaxation in bulk metallic glasses by mechanical spectroscopy: Determination of the stretching parameter β _kWW

Structural relaxation in Cu ₄₆Zr ₄₅Al ₇Dy ₂, Ti ₄₀Zr ₂₅Ni ₈Cu ₉Be ₁₈ and Zr _41.2Ti _13.8Cu _12.5Ni ₁₀Be _22.5 (Vit 1) bulk metallic glasses has been studied by mechanical spectroscopy during annealing below the glass transition temperature T _g. Structural relaxation in bulk metallic glasses leads to an increase of the storage modulus G′ and a decrease of the loss factor tan δ, which are observed after an incubation time. Kinetics of this phenomenon can be well described by a stretched exponential, similar to the Kohlrausch-Williams-Watts (KWW) equation used to fit differential scanning calorimetry (DSC) experiments. In the various investigated bulk metallic glasses the stretching exponent β _KWW does not depend on annealing temperature implying therefore that this exponent β _KWW is nearly independent of the microstructure. Decrease in the loss factor is due to a decrease in the atomic mobility. Various models have been proposed in the literature to describe this evolution: decrease in the free volume, decrease in the liquid-like regions or decrease in the defect concentration (flow defects or quasi-point defects). Present results are discussed using this concept of defect. In any case it corresponds to an increase of the short range order during annealing below the glass transition temperature T _g.