The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for metallic glasses (Acta Mater. 54 (2006) 4293), based on small-scale molecular dynamics simulations up to 20,000 atoms and thermomechanical analysis. The model predicts the existence of a critical lengthscale10 nm, above which melting could occur in shear-alienated glass. Large-scale molecular dynamics simulations with up to 5 million atoms have directly verified this prediction. When the applied stress exceeds the glue traction (computed separately before in a shear cohesive zone, or an amorphous-amorphous ‘‘generalized stacking fault energy’ ’ calculation), we indeed observe maturation of the shear band embryo into bona fide shear crack, accompanied by melting. In contrast, when the applied stress is below the glue traction, the shear band embryo does not propagate, becomes diffuse, and eventually dies. Thus this all-important quantity, the glue traction which is a property of shear-alienated glass, controls the macroscopic yield point of well-aged glass. We further suggest that the disruption of chemical short-range order (‘‘chemical softening’’) governs the glue traction microscopically. Catastrophic thermal softening occurs only after chemical alienation and softening in our simulation, after the shear band embryo has already run a critical length. [doi:10.2320/matertrans.MJ200769

Although aluminum has a smaller modulus in {111}〈112̄〉 shear than that of copper, we find by first-principles calculation that its ideal shear strength is larger because of a more extended deformation range before softening. This fundamental behavior, along with an abnormally high intrinsic stacking fault energy and a different orientation dependence on pressure hardening, are traced to the directional nature of its bonding. By a comparative analysis of ion relaxations and valence charge redistributions in aluminum and copper, we arrive at contrasting descriptions of bonding characteristics in these two metals that can explain their relative strength and deformation behavior.

Using density functional theory we analyze the stress-strain responses of 22 simple metals and ceramics to determine the maximum shear strain a homogeneous crystal can withstand, a property for which we suggest the name shearability. A shearability gap is found between metals and covalent ceramics. Shearability of metals further correlates with the degree of valence charge localization and directional bonding. Depending on the deformation constraints, ionic solids may possess even larger shearability than covalent solids. The Frenkel model of ideal shear strength works well for both metals and ceramics when shearability is used in the scaling.

Energy landscapes of $\left(2\overline{1}\overline{1}\right)⟨111⟩$ deformation twinning in bcc Mo and $\left(111\right)⟨11\overline{2}⟩$ deformation twinning in fcc Al and Cu are determined using density functional theory for sliding of layers numbering up to 7. In bcc Mo, the minimum thickness of a metastable twin is two layers, while twin embryos of three and four layers are unstable. Starting from five layers, the Mo twin can grow in a layer-by-layer fashion. The twin boundary formation and migration energies are found to be 607 and $40\mathrm{mJ}∕\mathrm{m}$, respectively, implying that partial dislocations on twin boundaries will have wide cores and high mobilities. The stress to homogeneously nucleate a partial loop on the boundary of a thick twin is determined to be only $1.4\mathrm{GPa}$, indicating that once a deformation twin in Mo reaches a critical thickness, which we estimate to be six layers, it can grow rather easily. Based on simple defect mechanics considerations, we estimate the condition for runaway defect growth requires twin embryo thickness to be tens of layers. Comparing the twinning energy landscape for Mo with those of Al and Cu, we find the former to have a longer ranged interlayer mechanical coupling, which is due to angular bonding and weaker electron screening in the intervening layers. Between Al and Cu, interactions in the former are relatively longer ranged.