Mechanical relaxation time scales in a Zr-Ti-Ni-Cu-Be bulk metallic glass

摘要

The relaxation time scales in a commercial-grade Zr_(41.25)Ti_(13.75)Ni_(10)Cu_(12.5)Be_(22.5) (at. percent) bulk metallic glass were examined using transient and dynamic mechanical experiments. The viscoelastic and sub-T_g relaxations were well described by the Kohlrausch-Williams-Watts relaxation function. A large activation energy (4.0 eV) and small nonexponentiality parameter (approximately 0.5) were observed for viscoelastic relaxation above T_g consistent with the cooperative nature of atomic movements leading ultimately to viscous flow. Conversely, a small activation energy (0.1 eV) and large nonexponentiality parameter (approximately 0.9) were observed for the sub-T_g relaxation suggesting localized atomic adjustments which may involve different structural units or mechanisms. The glass transition was manifested as a decoupling of the sub-T_g and viscoelastic relaxation. The resulting transition temperature determined at a selected time scale was in agreement with the value obtained from calorimetric studies.