Dynamic glass transition in Zr-Al-Ni-Cu bulk metallic glass

The elastic and viscoelastic behavior of a bulk metallic glass Zr55Al10Ni5CU30 (in mol%) has been studied by isothermal forced-vibration measurements of the dynamic shear modulus. At temperatures of the calorimetric glass transition, on decreasing the frequency from 10(+1) to 10(-3) Hz at a constant temperature, the storage modulus exhibits a striking drop, of an order of magnitude, which is similar to the change observed under continuous heating. The lowering of the storage modulus and the accompanying maximum in the loss modulus indicate a dynamic glass transition; the material behaves as a solid at high frequencies but as a liquid at low frequencies. The range of frequencies where the storage and loss moduli exhibit the characteristic behavior is much wider than expected from a simple Maxwell model of viscoelasticity. A parallel combination of Maxwell units is found to reproduce the observed trend, and is used to evaluate the characteristic rate constant. Its activation energy and the pre-exponential factor turn out to be anomalously high, indicating that the dynamic glass transition is controlled by collective movements of atoms. (c) 2006 Elsevier B.V. All rights reserved.