Size-dependent Deformation Behavior In Nanosized Amorphous Metals Suggesting Transition from Collective to Individual Atomic Transport

The underlying atomistic mechanism of deformation is a central problem in mechanics and materials science. Plastic deformation in crystalline materials typically utilizes dislocations which slide in slip planes. For amorphous metals, the plastic deformation mechanism is generally recognized as viscous flow. At low temperature and high strain rates, deformation is highly localized in narrow shear bands. At high temperature and low strain rates, deformation is essentially homogenous.<br/><br/>We study the size-dependency of the deformation mechanism in amorphous metals. Thermomechanical nanomolding (TMNM) allows us to deform bulk metallic glasses under well-defined size confinement and temperatures. By applying compression deformation across a temperature range from Tg – 50 K to Tg + 10 K, and a size range from 10 nm to 250 nm, we found that the underlying deformation mechanism of amorphous metals changes at small scales, typically below 100 nm, from collective atomic transport through viscous flow to an individual atomic transport based on atomic diffusion. Scaling experiments further reveal that the critical length scale of the transition between diffusion- and viscous-based flow is temperature dependent. This critical length scale increases and then decreases with temperature, exhibiting a maximum at the glass transition. While deformation based on viscous flow does not discriminate among alloy constituents, diffusion does and the constituent element with the highest diffusivity deforms fastest. Hence, composition changes during deformation.<br/><br/>In this presentation, I will discuss the size-dependency of deformation mechanism under nano-scale confinements, including a dislocation-diffusion transition in crystalline and a viscous flow-diffusion in amorphous metals. I will discuss the theoretical framework behind the mechanism transitions with size based on a maximization of energy release rate. Discussion on nano-mechanics and physics of amorphous materials will also be covered, including but not limited to the bulk and surface glass transition, breakdown of the Stokes-Einstein relation, and glass structures.