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We report the results of a molecular dynamics study of the effect of texture on the yield and peak stresses in columnar-grained nanotwinned Ag and Cu. The simulations suggest that in pure nanotwinned face-centered cubic metals, the strength is determined primarily by the cooperation or competition between two major factors: the magnitude of the Schmid factors for the available slip systems and the effectiveness of grain boundaries (and their triple-junctions) in generating dislocations. These factors and their relative impact depend on the geometry of the specimen relative to the applied stress, which is typically reflected in the texture of the material in experimental studies. The detailed mechanisms of plastic deformation are discussed for seven specific geometries that represent a range of different textures. [ABSTRACT FROM AUTHOR]

We present the results of large-scale molecular dynamics simulations of grain growth in polycrystalline nickel with nanoscale grains. The simulations show that grain growth is accompanied by coherent twin boundary (CTB) generation. As the grains grow, twins collide; such collisions result in twin junctions. We catalog all possible twin junctions and show examples of each from the simulations. These include junctions of 2–4 CTBs with grain boundaries and five-fold twin junctions (penta-twins). We elucidate the mechanisms by which all of these junctions form and their relative frequencies. Penta-twins, which are rare in coarse microstructures, occur frequently in nanocrystalline metals. Their absence in macro-scale samples can be traced to the wedge-disclination character (and, consequently, an elastic energy that diverges with sample size). In the nanocrystalline case, the presence of penta-twins can be traced to this twin collision formation mechanism, which is responsible for their wedge-disclination dipole character (relatively small elastic energy). We demonstrate how all CTB junctions, especially penta-twins, retard grain growth. [ABSTRACT FROM AUTHOR]

Nanoscale twins form in many metallic materials, especially those with low stacking fault energy. Their presence can significantly enhance the strength of a material. However, nanotwins are unstable and can be annihilated, e.g. by thermal annealing. We present the results of molecular dynamics (MD) simulations, which demonstrate that additions of solutes can significantly stabilize nanotwinned structures. The MD simulations reveal that the mechanism of the solute drag on the twin boundary may be associated not with the solute segregation on the incoherent twin boundary (ITB), but rather with changing of the ITB shape or position to accommodate as many solutes as possible. [ABSTRACT FROM PUBLISHER]

We review the application of Brandon’s criterion to identifying so-called “special” grain boundaries. The underlying principles of the Brandon criterion, and others that have followed it, are explained, and the choices of particular parameters within the criteria are considered in the light of experimental information in the literature. It is suggested that varying choices of the parameters may be appropriate for differing applications. An experimental study of the stability of CSL-related triple junctions is used to evaluate the applicability of Brandon-like criteria to these microstructural features. [ABSTRACT FROM AUTHOR]

Solute additions are commonly used to stabilize nanocrystalline materials against grain growth, and can simultaneously enhance the strength of the material by impeding dislocation emission from the grain boundaries. In this study we demonstrate using molecular dynamics (MD) simulations that the effect of solutes on dislocation nucleation depends on the distribution of solutes at the grain boundary, and can vary dramatically depending on the solute type. Solutes with a smaller positive size mismatch to the host can be more effective in suppressing dislocation emission from grain boundaries than others that have larger mismatch. In particular, although Ag solutes have a smaller misfit with Cu than Zr solutes, the effect of Ag on the dislocation nucleation from grain boundaries in Cu can be larger than the corresponding effect of Zr. These findings are relevant to the search for optimal solute additions, which can strengthen a nanocrytalline material by suppressing the nucleation of dislocation slip from grain boundaries, while stabilizing it against grain growth. [ABSTRACT FROM AUTHOR]

When grain sizes are reduced to the nanoscale, grain boundaries (GB) become the dominant sources of the dislocations that enable plastic deformation. We present the first molecular dynamics (MD) study of the effect of substitutional solutes on the dislocation nucleation process from GBs during uniaxial tensile deformation. A simple bi-crystal geometry is utilized in which the nucleation and propagation of dislocations away from a GB is the only active mechanism of plastic deformation. Solutes with atomic radii both larger and smaller than the solvent atomic radius were considered. Although the segregation sites are different for the two cases, both produce increases in the stress required to nucleate a dislocation. MD simulations at room temperature revealed that this increase in the nucleation stress is associated with changes of the GB structure at the emission site caused by dislocation emission, leading to increases in the heats of segregation of the solute atoms, which cannot diffuse to lower-energy sites on the timescale of the nucleation event. These results contribute directly to understanding the strength of nanocrystalline materials, and suggest suitable directions for nanocrystalline alloy design leading toward structural applications. [ABSTRACT FROM AUTHOR]

Highlights • MD simulation shows that addition of Zr can lead to disorder in the grain boundary (GB) structure in Cu. • The full disorder of the GB structure leads to the suppression of dislocation emission. • A partial disorder may also occur and this aids dislocation nucleation rather than suppressing it. Abstract It was recently reported that segregation of Zr to grain boundaries (GB) in nanocrystalline Cu can lead to the development of disorder in the intergranular structure (Khalajhedayati et al., 2016; Khalajhedayati and Rupert, 2015). In this study we employ atomistic computer simulations to investigate how this disorder affects dislocation nucleation from the GBs under applied stress. It was found that a fully disordered grain boundary structure suppresses dislocation emission and significantly increases the yield stress. Depending on the solute concentration and heat-treatment, however, partial disorder may also occur and this aids dislocation nucleation rather than suppressing it, reducing or eliminating the strengthening effect. [ABSTRACT FROM AUTHOR]