Band 22, Heft 3-4

This article discusses the experiments, computer simulations, and theoretical models addressing the conventional and specific mechanisms of plastic deformation in nanocrystalline metallic materials. Particular attention is devoted to the competition between lattice dislocation slip and specific deformation mechanisms mediated by grain boundaries as well as its sensitivity to grain size and other parameters of nanocrystalline metallic structures.

The plastic deformation properties of microscale and nanoscale specimens differ from those of their macroscopic counterparts as the discrete nature of the elementary processes governing plastic flow becomes directly visible. In such specimens, details of the initial defect microstructure may exert a strong influence on the recorded deformation behaviour, which accordingly exhibits significant scatter even amongst specimens that share an identical preparation history. The plasticity of microsamples appears as a sequence of spatially and temporally localised events and not as the smooth and continuous flow process envisaged by classical continuum elastoplasticity. These observations pose a significant challenge to constitutive modelling. In this feature article, we discuss the statistics of fluctuations in microscale and nanoscale plasticity and discuss the implications for computational modelling of plastic deformation processes on microscale and nanoscales. We propose a new type of constitutive models that combine a classical continuum description of the elastic problem with a stochastic description of the dynamics of plastic flow.

The analytical solution of the elastic-plastic response of a two-phase laminate microstructure subjected to periodic simple shear loading conditions is derived considering strain gradient and micromorphic plasticity models successively. One phase remains purely elastic, whereas the second one displays an isotropic elastic-plastic behavior. Although no classic hardening is introduced at the individual phase level, the laminate is shown to exhibit an overall linear hardening scaling with the inverse of the square of the cell size. The micromorphic model leads to a saturation of the hardening at small length scales in contrast to Aifantis strain gradient plasticity model displaying unlimited hardening. The models deliver qualitatively relevant size effects from the physical metallurgical point of view, but fundamental quantitative discrepancy is pointed out and discussed, thus requiring the development of more realistic nonlinear equations in strain gradient plasticity.

We derive an energy-based material model for thermomechanically coupled phase transformations in polycrystalline shape memory alloys. For the variational formulation of the model, we use the principle of the minimum of the dissipation potential for nonisothermal processes for which only a minimal number of constitutive assumptions has to be made. By introducing a condensed formulation for the representative orientation distribution function, the resulting material model is numerically highly efficient. For a first analysis, we present the results of material point calculations, where the evolution of temperature as well as its influence on the mechanical material response is investigated.

At low temperature, T →0, the yield stress of a perfect crystal is equal to its so-called theoretical strength. The yield stress of nonperfect crystals is controlled by the stress threshold of dislocation mobility. A noncrystalline solid has neither an ideal structure nor gliding dislocations. Its yield stress, that is, the stress at which the macroscopic inelastic deformation starts, depends on distribution of local, attributed to each atomic site, critical stresses at which the local inelastic deformation occurs. We describe exactly solvable model of planar layer strength and sliding with an arbitrary homogeneous distribution of local critical stresses. The rate of the thermally activated sliding is closely related to parameters of the low-temperature strength. The sliding activation volume scales with the applied external stress as where β <1. The proposed model accounts for mechanisms and the yield stress of the low-temperature deformation of polycluster metallic glasses, because intercluster boundaries of a polycluster metallic glass are natural sliding layers of the described type.

The drive to produce electrical energy by directly compressing piezoceramic material using mechanical stress stands behind the present test series. To be able to correctly choose the right material, PZT disks manufactured by three different manufacturers have been tested under static mechanical compressive and cyclic loads. It was shown that although the disks can withstand high mechanical stresses (up to 100 MPa) without any visible damage, their transduction is confined to much lower stresses (50–75 MPa), a range in which the electrical output is a function of the square of the applied stress. This range is further reduced, when the PZT is subjected to cyclic mechanical loading, yielding an applicable mechanical stress in the range of 30–40 MPa, from which electrical power can be produced without further deterioration. To compensate for the low electric power, due to relatively low mechanical stresses applied on the PZT disks, one can increase the volume of the material used by placing layers of piezoelectric material one on top of the other, each subjected to the same mechanical stress. This will yield the required electric power from a safe given mechanical stress without reduction in its output.