Size-effects in deformation behavior of metallic glasses
Meduri, Chandra Sekhar
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Metallic glasses are metal alloys with disordered atomic structure. Due to their amorphous structure, they exhibit a unique set of properties that are ideal for wide range of applications including electrical transformers, sporting goods, fuel cells, precision gears for micromotors etc. The near-theoretical strength (1-3 GPa), exceptionally high elastic limit (2-3%), and excellent formability (down to nanoscale) of these materials are desirable in structural applications, micro and nanodevices in particular. On the downside, the amorphous structure also results in zero tensile ductility at room temperature. The plastic strain localizes in narrow shear bands (~20-40 nm in thickness) at low temperatures and high stresses. The nucleation and propagation of shear bands depends on multiple parameters such as, the elastic constants, the sample size and processing, and the testing conditions (temperature, strain-rate, and loading geometry). Studying the effects of these variables and linking them to a unifying flow model is critical for fundamental understanding and improving the intrinsic ductility of metallic glasses. While many of these aspects are well documented, the sample size dependence has been poorly understood, and even hotly debated. This study investigates the sample size and testing temperature effects on shear banding process through tensile and bend tests. Under tensile loading, sample size and temperature effects on fracture morphologies and deformation modes were explored in Pt-based metallic glass. Thermoplastic drawing procedure was utilized to fabricate ASTM grade tensile specimens with diameters ranging from 500 µm to 150 nm. Constant strain-rate (10-2 s-1) tensile tests at different temperatures ranging from cryogenic to glass transition temperature were conducted using a customized setup. Analysis of fracture morphologies from these high-throughput tests show a gradual transition from catastrophic shear bands to slow shear bands, to shear bands plus some distributed plastic flow, and eventually necking to a point flow was observed as sample size was reduced. In addition, it was observed that a decrease in sample size has a similar effect as a decrease in testing temperature on the deformation behavior both in shear localization, and homogeneous/necking regime. In the shear localization regime, an increasing contribution of thermal softening (through shear offset) and decreasing contribution of defect development (through coalescence of nanovoids and formation of microcracks) to the final fracture was observed as sample size and/or temperature decreases. In the homogeneous regime, the shear band stability and thus the ductility increased with decreasing sample size and/or testing temperature. In addition, bend tests were conducted on Zr- and Pt-based metallic glasses in the temperature range (0.1Tg-0.8Tg). The results show an increase in bending strain, shear band density, and critical shear offset with decreasing temperature. The observed size-temperature equivalence in both bending and tension was discussed based on fundamental plastic flow units, Shear Transformation Zones (STZs). In an attempt towards a unified flow theory to describe the fracture of metallic glasses, the results obtained in this study were analyzed using the existing plastic flow models.