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Focusing on nanocrystalline (nc) pure face-centered cubic metals, where systematic experimental data are available, this paper presents a brief overview of the recent progress made in improving mechanical properties of nc materials, and in quantitatively and mechanistically understanding the underlying mechanisms. The mechanical properties reviewed include strength, ductility, strain rate and temperature dependence, fatigue and tribological properties. The highlighted examples include recent experimental studies in obtaining both high strength and considerable ductility, the compromise between enhanced fatigue limit and reduced crack growth resistance, the stress-assisted dynamic grain growth during deformation, and the relation between rate sensitivity and possible deformation mechanisms. The recent advances in obtaining quantitative and mechanics-based models, developed in line with the related transmission electron microscopy and relevant molecular dynamics observations, are discussed with particular attention to mechanistic models of partial/perfect-dislocation or deformation-twin-mediated deformation processes interacting with grain boundaries, constitutive modeling and simulations of grain size distribution and dynamic grain growth, and physically motivated crystal plasticity modeling of pure Cu with nanoscale growth twins. Sustained research efforts have established a group of nanocrystalline and nanostructured metals that exhibit a combination of high strength and considerable ductility in tension. Accompanying the gradually deepening understanding of the deformation mechanisms and their relative importance, quantitative and mechanisms-based constitutive models that can realistically capture experimentally measured and grain-size-dependent stress-strain behavior, strain-rate sensitivity and even ductility limit are becoming available. Some outstanding issues and future opportunities are listed and discussed.

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Significant progress has been made during the past decade in incorporating micromechanics in continuum descriptions of inelastic deformation. This has led to the development of a rather comprehensive constitutive theory for rate-dependent and idealized rate-independent crystalline materials that deform plastically by crystalline slip. This theory is reviewed in some detail and examples are presented which illustrate how complex slip phenomena involving localized plastic flow and nonuniform crystallographic texture can be analyzed. The paper concludes by suggesting that it is now possible to develop accurate models for rate-dependent polycrystals undergoing arbitrarily large strains. Such models would have as principal aims the prediction of texture development and the rigorous assessment of such anisotropy on constitutive behavior. An example of how this would be of immediate value in analyzing strain-hardening behavior of metal polycrystals at large strains is provided.

This paper is concerned with an analysis of strain localization in ductile crystals deforming by single slip. The plastic flow is modelled as rate-insensitive and localization, viewed as a bifurcation from a homogeneous deformation mode to one which is concentrated in a narrow "shear band", is found to be possible only when the plastic hardening modulus for the slip system has fallen to a certain critical value, h , where h is sensitive to the cr ' cr precise form of the constitutive law governing incremental shear. We develop the general form of this constitutive law, incorporating within it the possibility of deviations from the Schmid rule of a critical resolved shear stress, and we show that h may in fact be positive when there are deviations from the Schmid rule. It is suggested that micromechanical processes such as "cross-slip" in crystals provide specific cases for which stresses other than the Schmid stress may influence plastic response and, further, there is an experimental association of localization with the onset of large amounts of cross-slip. Thus we give the specific form of h for a constitutive model that corresponds to the non-Schmid effects in cross-slip, and we develop a dislocation model of the process from which we estimate the magnitude of the parameters involved. The work supports the notion that localization can occur with positive strainhardening, h > 0 , and the often invoked notions of the attainment of an ideally plastic or strain softening state for localization may be unnecessary.