Robert Maass

The future of metallic glasses as an advanced structural and functional material will to a great extent depend on the understanding and control of their mesoscopic flow defects called shear bands. These defects are sweet‐and‐sour; sweet because they mediate macroscopic plasticity at room temperature, and sour because they quickly promote failure. In the past decade, fundamental research generated great progress in characterizing the role that shear bands play during plastic deformation of disordered systems, including metallic glasses. Similar to those in many other materials, shear bands in metallic glasses are only active for a very short time, which directed research focus towards topological, structural, chemical, and thermal properties of formed, but inactive shear bands. In this paper, recent progress in directly characterizing the shear‐band dynamics in situ during straining experiments is presented. Various shear‐banding stages are outlined, including formation, propagation, and arrest, as well as shear‐band creep and aging. The results are discussed in a more general context of disordered materials, concluding with a summarizing overview of time‐scales involved in shear banding, and describing future research directions that may lead to controlled shear‐band plasticity in metallic glasses.

Two dimensional mapping of structural properties near a single shear band in a Zr-based bulk metallic glass reveals the presence of hardness and modulus reductions at a micrometer length scale. The isolated shear band had carried all the macroscopic plastic strain and the material near the shear-band exhibits structural variations both along and normal to the shear plane. Analyzing the nanoindentation data indicates that long range internal stresses are the primary cause of the spatially varying structure. The results demonstrate that a nano-scale defect in a metallic glass may have a micrometer range signature.

Directly tracing the spatiotemporal dynamics of intermittent plasticity at the micro- and nanoscale reveals that the obtained slip dynamics are independent of applied stress over a range of up to ∼400 MPa, as well as being independent of plastic strain. Whilst this insensitivity to applied stress is unexpected for dislocation plasticity, the stress integrated statistical properties of both the slip size magnitude and the slip velocity follow known theoretical predictions for dislocation plasticity. Based on these findings, a link between the crystallographic slip velocities and an underlying dislocation avalanche velocity is proposed. Supporting dislocation dynamics simulations exhibit a similar regime during microplastic flow, where the mean dislocation velocity is insensitive to the applied stress. Combining both experimental and modeling observations, the results are discussed in a framework that firmly places the plasticity of nano- and micropillars in the microplastic regime of bulk crystals.

Homogeneous ductile flow of metallic glasses is observed at the micrometer scale. It is shown that this unusual deformation mode of an otherwise brittle material depends on both specimen size and applied loading rate. The results are explained by intrinsic length-scale effects of nanometer-sized defects, and provide a rationale for the long term debated brittle-to-ductile transition of amorphous metals.

At temperatures well below their glass transition, the deformation properties of bulk metallic glasses are characterised by a sharp transition from elasticity to plasticity, a reproducible yield stress, and an approximately linear decrease of this stress with increasing temperature. In the present work it shown that when the well known properties of the under-cooled liquid regime, in terms of the underlying potential energy landscape, are assumed to be also valid at low temperature, a simple thermal activation model is able to reproduce the observed onset of macro-scopic yield. At these temperatures, the thermal accessibility of the complex potential energy landscape is drastically reduced, and the statistics of extreme value and the phenomenon of kinetic freezing become important, affecting the spatial heterogeneity of the irreversible structural transitions mediating the elastic-to-plastic transition. As the temperature increases and approaches the glass transition temperature, the theory is able to smoothly transit to the high temperature deformation regime where plasticity is known to be well described by thermally activated viscoplastic models.