Deformation of metallic glasses: Recent developments in theory, simulations, and experiments

Radiation damage in nanostructured materials

The deformation and flow of disordered solids, such as metallic glasses and concentrated emulsions, involves swift localized rearrangements of particles that induce a long-range deformation field. To describe these heterogeneous processes, elastoplastic models handle the material as a collection of 'mesoscopic' blocks alternating between an elastic behavior and plastic relaxation, when they are too loaded. Plastic relaxation events redistribute stresses in the system in a very anisotropic way. We review not only the physical insight provided by these models into practical issues such as strain localization, creep and steady-state rheology, but also the fundamental questions that they address with respect to criticality at the yielding point and the statistics of avalanches of plastic events. Furthermore, we discuss connections with concurrent mean-field approaches and with related problems such as the plasticity of crystals and the depinning of an elastic line.

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Deformation and flow of amorphous solids: Insights from elastoplastic models

Elastostatically induced structural disordering in amorphous alloys

Nearly 90 per cent of service failures of metallic components and structures are caused by fatigue at cyclic stress amplitudes much lower than the tensile strength of the materials involved. Metals typically suffer from large amounts of cumulative, irreversible damage to microstructure during cyclic deformation, leading to cyclic responses that are unstable (hardening or softening) and history-dependent. Existing rules for fatigue life prediction, such as the linear cumulative damage rule, cannot account for the effect of loading history, and engineering components are often loaded by complex cyclic stresses with variable amplitudes, mean values and frequencies, such as aircraft wings in turbulent air. It is therefore usually extremely challenging to predict cyclic behaviour and fatigue life under a realistic load spectrum. Here, through both atomistic simulations and variable-strain-amplitude cyclic loading experiments at stress amplitudes lower than the tensile strength of the metal, we report a history-independent and stable cyclic response in bulk copper samples that contain highly oriented nanoscale twins. We demonstrate that this unusual cyclic behaviour is governed by a type of correlated 'necklace' dislocation consisting of multiple short component dislocations in adjacent twins, connected like the links of a necklace. Such dislocations are formed in the highly oriented nanotwinned structure under cyclic loading and help to maintain the stability of twin boundaries and the reversible damage, provided that the nanotwins are tilted within about 15 degrees of the loading axis. This cyclic deformation mechanism is distinct from the conventional strain localizing mechanisms associated with irreversible microstructural damage in single-crystal, coarse-grained, ultrafine-grained and nanograined metals.

History-independent cyclic response of nanotwinned metals

Indentation size effect and shear transformation zone size in a bulk metallic glass in two different structural states

Indentation size effect in bulk metallic glass

Plasticity in amorphous alloys is associated with strain softening, induced by the creation of additional free volume during deformation. In this paper, the role of free volume, which was a priori in the material, on work softening was investigated. For this, an as-cast Zr-based bulk metallic glass (BMG) was systematically annealed below its glass transition temperature, so as to reduce the free volume content. The bonded-interface indentation technique is used to generate extensively deformed and well defined plastic zones. Nanoindentation was utilized to estimate the hardness of the deformed as well as undeformed regions. The results show that the structural relaxation annealing enhances the hardness and that both the subsurface shear band number density and the plastic zone size decrease with annealing time. The serrations in the nanoindentation load-displacement curves become smoother with structural relaxation. Regardless of the annealing condition, the nanohardness of the deformed regions is ∼12–15% lower, implying that the prior free volume only changes the yield stress (or hardness) but not the relative flow stress (or the extent of strain softening). Statistical distributions of the nanohardness obtained from deformed and undeformed regions have no overlap, suggesting that shear band number density has no influence on the plastic characteristics of the deformed region.

Role of free volume in strain softening of as-cast and annealed bulk metallic glass

Nanoindentation analysis of time-dependent deformation in as-cast and annealed Cu-Zr bulk metallic glass

The elastic deformation behaviors of bulk amorphous alloys during elastostatic compression were examined. Depending on the alloy deformation characteristics, the elasticity of amorphous alloys consists of strain components characterized by ideal elasticity, anelasticity, and viscoelasticity. The strain component associated with viscoelasticity is irreversible and causes the generation of excess free volume, which in turn alters the mechanical properties. This article discusses how the properties of amorphous alloys can be altered by the application of elastostatic compression.

Byung Gil Yoo

Papers

Indentation size effect and shear transformation zone size in a bulk metallic glass in two different structural states

Indentation size effect in bulk metallic glass

Role of free volume in strain softening of as-cast and annealed bulk metallic glass

Nanoindentation analysis of time-dependent deformation in as-cast and annealed Cu-Zr bulk metallic glass

Irreversible structural change induced by elastostatic stress imposed on an amorphous alloy and its influence on the mechanical properties