The Effect of Atomic-Scale Open-Volume on Flow and Fracture Processes in a Zr-Ti-Ni-Cu-Be Bulk Metallic Glass
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The Effect of Atomic-Scale Open-Volume on Flow and Fracture Processes in a Zr-Ti-NiCu-Be Bulk Metallic Glass Daewoong Suh1 and Reinhold H. Dauskardt Department of Materials Science and Engineering Stanford University, Stanford, CA 94305 1 Currently at Materials and Engineering Sciences Center Sandia National Laboratories, Livermore, CA 94551 ABSTRACT Effects of atomic-scale open-volume regions in metallic glass structure on the flow and fracture behavior of a Zr-Ti-Ni-Cu-Be bulk metallic glass were examined. Studies of relaxation time scales showed that atomic arrangement processes for viscous flow were significantly retarded with annealing. Plane strain fracture toughness was significantly decreased and fatigue crack crack-growth rates were dramatically increased, indicating degradation of resistance to crack extension as a result of annealing. Fracture morphology completely changed from vein patterns to cleavage-like features with little evidence of plasticity with annealing. The positron lifetime and Doppler broadening experiments revealed decreased open-volume regions as a result of annealing. The loss of stress relief ability by retarded crack tip viscous flow as a result of the anneal-out of open-volume regions is believed to contribute to observed annealing embrittlement. INTRODUCTION Atomic transport processes such as diffusion and flow in materials are generally described in terms of defects. In crystalline materials, vacancies and dislocations are wellknown diffusion and flow defects. Although the nature of defects in amorphous materials is somewhat diffuse, they can be described as local fluctuations in the short-range order often present in amorphous materials [1]. The concept which has been successfully applied to atomic transport processes in molecular and metallic glasses is the local density fluctuation or free volume [2-4]. According to the free volume theory, if the local free volume of an atom becomes larger than some critical value, the atom can escape from its nearest neighbor cage and perform a diffusive or shear flow jump, i.e., the free volume becomes a diffusion or flow defect. Furthermore, fracture surfaces of metallic glasses exhibit vein patterns indicating local viscous flow during fracture [5-8]. The physical process behind this softening is often modeled in terms of crack tip stress-aided creation of free volume which, in turn, results in reduced viscosity at the crack tip [5,9,10]. In the free volume theory, the free volume is generally defined as an excess volume relative to some reference state. Although the fractional free volume has been successfully used as a key parameter in free volume formulations, the atomistic characterization of the free volume has not been studied in detail. In this study, the focus is on atomic-scale open-volume regions as an atomistic entity for the free volume. The objective of the present paper is therefore to examine effects of open-volume regions on the flow and fracture processes in metallic glasses. Specifically, the relaxation time scale
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