Berlin 2008 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 31: Mechanical Properties III

MM 31.1: Talk

Wednesday, February 27, 2008, 17:30–17:45, H 0111

Inhomogeneous flow characteristics of bulk metallic glasses — •Alban Dubach, Florian Dalla Torre, and Jörg Löffler — Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerland

In contrast to crystalline metals, which exhibit dislocation mediated deformation, bulk metallic glasses (BMGs) bear a high resistance to plastic flow at low temperatures (T) but viscous like properties close to their glass transition temperature. Both mechanical responses underlie a single mechanism: nucleation and propagation of shear transformation zones (STZs). As opposed to high T deformation at low T is temporarily and spatially restricted within narrow shear bands and therefore difficult to assess experimentally and less well understood.

In this study the inhomogeneous flow kinetics of Zr-based BMGs are investigated at sub-ambient T by strain rate change tests and the temporal and spatial characteristics of shearing are compared with the appearance of shear bands and fracture surfaces. Using the time durations of individual shear events an "apparent viscosity" at the moment of shearing can be estimated. Our results show that serrated flow, typically observed at room temperature, disappears below a critical T or above a critical strain rate, in accordance with a change of the strain rate sensitivity. A constitutive model for inhomogeneous flow in BMGs is presented which describes the deformation behaviour according to a thermally activated (cooperative) motion of STZs, including a time varying state variable representing the local state of relaxation in the shear band.