Mechanical relaxation time scales in a Zr-Ti-Ni-Cu-Be bulk metallic glass
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laxation time scales in a commercial-grade Zr41.25Ti13.75Ni10Cu12.5Be22.5 (at.%) bulk metallic glass were examined using transient and dynamic mechanical experiments. The viscoelastic and sub-Tg 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 Tg 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-Tg relaxation suggesting localized atomic adjustments which may involve different structural units or mechanisms. The glass transition was manifested as a decoupling of the sub-Tg and viscoelastic relaxation. The resulting transition temperature determined at a selected time scale was in agreement with the value obtained from calorimetric studies.
In response to mechanical, electrical, or temperature perturbations, the glassy state of materials may exhibit various types of viscoelastic or anelastic relaxations. These are typically manifested by a transient response of physical or thermodynamic properties such as enthalpy, volume, strain, or stress following an instantaneous change of stimuli in the time domain. Alternatively, energy dissipation associated with relaxations may be studied using sinusoidally applied perturbations in the frequency domain. Such dynamic experiments reveal changes in the complex part of properties, for example, the loss modulus. Selection of transient or dynamic techniques depends on the magnitude of relaxation time scales. Knowledge of relaxation time scales is essential for the understanding of not only such fundamental topics as glass transition and transport phenomena but also a range of technologically important properties including damping, yield strength, ductile-to-brittle transition temperature, fracture toughness, and fatigue crack growth rate in a variety of glassy materials.1–5 In addition, relaxation behavior was recently found to be associated with the development of tempering stresses in the current metallic glasses.6 Although significant research has been conducted on the relaxation behavior of inorganic and polymeric glasses, relatively few studies have been conducted on metallic glasses. This is in part due to their poor thermal stability at elevated temperatures where relaxation predominates and the dimensional limitation of thin specimens needed to ensure high cooling rates necessary for 1254
http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 6, Jun 2002 Downloaded: 14 Mar 2015
glass formation. In recent years, however, a series of multicomponent bulk glass forming alloy systems with a high resistance to crystallization have been developed.7 Their stable supercooled liquid region and excellent thermal stability, for the first time, allow the examination of mechanical relaxation behavior using conventional experimental te
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