Elastic-plastic memory and kinematic-type hardening

A mechanism of strain hardening and Bauschinger effect: shear-history-dependent microstructure of elasto-plastic suspensions

Dispersing solid hard particles in an elasto-plastic material leads to important shear-history dependence of the behavior, namely strain hardening and Bauschinger effect. Strain hardening is observed as the progressive strengthening of a material during its plastic deformation and is usually associated with ductility, a property often sought after in composite materials to postpone fractures and failure. In addition, anisotropic mechanical properties are developed, the material resistance being larger in the direction of the imposed flow, which is referred to as the Bauschinger effect. We show that this is related here to shear-history-dependent particle-pair distribution functions. Roughness and interparticle contacts likely play a major role, as replacing hard particles by non-deformable bubbles modifies the suspension microstructure and suppresses strain hardening. Beyond suspensions, our study provides new insight in the understanding and control of strain hardening and Bauschinger effect in composite materials.

On a mixed energetic–dissipative constitutive law for non-proportional loading, with focus on small-scale plasticity

We analyse the mixed energetic–dissipative potential (MP) recently proposed by our group to predict, within higher-order strain gradient plasticity (SGP), reliable size-dependent responses under general loading histories. Such an MP follows former proposals by Chaboche, Ohno and co-workers for nonlinear kinematic hardening in the context of size-independent metal plasticity. The MP is given by M quadratic addends that each transitions, at a different threshold value, into a linear dissipative contribution. Hence, the MP involves 2 M positive material parameters, given by the M threshold values and the M moduli weighing each quadratic recoverable term. We analytically demonstrate that, under proportional loading, the MP limit for M  → ∞ converges to a less-than-quadratic potential with well-defined properties. This result is of crucial importance for identifying the material parameters of any model adopting the MP. Moreover, our analysis provides a formula for the characterization of the energetic and dissipative parts of any possible MP limit, showing that, regarding the capability to describe the effect of diminishing size within SGP, the MP can be selected such that its contribution to the strengthening (i.e. an increase in yield point) is mostly dissipative, whereas its contribution to the increase in strain hardening is mostly recoverable.

Origin of the Bauschinger Effect in Amorphous Solids

We study the structural origin of the Bauschinger effect by accessing numerically the local plastic thresholds in the steady state flow of a two-dimensional model glass under athermal quasistatic deformation. More specifically, we compute the local residual strength, Δτ^{c}, for arbitrary loading orientations and find that plastic deformation generically induces material polarization, i.e., a forward-backward asymmetry in the Δτ^{c} distribution. In steady plastic flow, local packings are on average closer to forward (rather than backward) instabilities, due to the stress-induced bias of barriers. However, presumably due to mechanical noise, a significant fraction of zones lie close to reverse (backward) yielding, as the distribution of Δτ^{c} for reverse shearing extends quasilinearly down to zero local residual strength. By constructing an elementary model of the early plastic response, we then show that unloading causes reverse plasticity of a growing amplitude, i.e., reverse softening, while it shifts away forward-yielding barriers. This result in an inversion of polarization in the low-Δτ^{c} region and, consequently, in the Bauschinger effect. This scenario is quite generic, which explains the pervasiveness of the effect.

A Review on Heterogeneous Nanostructures: A Strategy for Superior Mechanical Properties in Metals

Generally, strength and ductility are mutually exclusive in homogeneous metals. Nanostructured metals can have much higher strength when compared to their coarse-grained counterparts, while simple microstructure refinement to nanoscale generally results in poor strain hardening and limited ductility. In recent years, heterogeneous nanostructures in metals have been proven to be a new strategy to achieve unprecedented mechanical properties that are not accessible to their homogeneous counterparts. Here, we review recent advances in overcoming this strength–ductility trade-off by the designs of several heterogeneous nanostructures in metals: heterogeneous grain/lamellar/phase structures, gradient structure, nanotwinned structure and structure with nanoprecipitates. These structural heterogeneities can induce stress/strain partitioning between domains with dramatically different strengths, strain gradients and geometrically necessary dislocations near domain interfaces, and back-stress strengthening/hardening for high strength and large ductility. This review also provides the guideline for optimizing the mechanical properties in heterogeneous nanostructures by highlighting future challenges and opportunities.

Nucleationless three-dimensional island formation in low-misfit heteroepitaxy

The formation of faceted three-dimensional islands during growth of low-misfit Si1-xGex alloys on Si(100) has been investigated by low-energy electron microscopy. The formation of the islands in these alloy systems does not involve three-dimensional nucleation, but rather proceeds via a precursor array of shallow, stepped mounds on the surface that result from the inherent morphological instability of the strained alloy film.