Abstracts:Several experiments have shown that, with a small amount of graphene volume concentration, the maximum strength of graphene-metal nanocomposites could increase notably while its failure strain decrease drastically, but at present no theory seems to exist to explain these opposing trends. In this paper we present a unified theory of plasticity and progressive damage that ultimately leads to the failure of composite. The theory is written in a two-scale framework, with the small scale constituting the ductile matrix and the microvoids generated during progressive damage, and the large scale combining the damaged metal matrix with 3-D randomly oriented graphene. To calculate the overall stress-strain relations the method of field fluctuation and interface effect are both considered, and to assess the evolution of microvoids during progressive damage a new damage potential is suggested. The final outcome is a simple and analytical model for the strength and ductility of the nanocomposite. We highlight the developed theory with a direct application to reduced graphene oxide/copper (rGO/Cu) nanocomposites, and demonstrate how, in line with experiments, the tensile strength can increase by 40% and the failure strain can drop from 0.39 to 0.14 as graphene volume concentration increases from 0 to 2.5 vol%. The rapid increase of damage effect at high graphene volume concentration was found to be responsible for the sharp drop of ultimate strain.

Abstracts:In this work, we have systematically investigated the effect of cyclic deformation on the strength and rate sensitivity of Cu/Ru multilayers with different individual layer thickness (h) by nanoindentation tests. It was found that cyclic deformation remarkably enhances the hardness of Cu/Ru multilayers comparing with the specimens by monotonic loading. The rate sensitivity (m) of multilayer exhibits an anomalous size dependence after nanoscale cyclic deformation. When h > 10 nm, the m linearly increases with increasing cycle number of loading-unloading (s). However, the m sharply decreases with increasing s when h < 10 nm, presenting an inverse cyclic deformation effect on m. An obvious Bauschinger effect is observed during cyclic loading, where the evolution of effective stress is consistent with the m. Cyclic deformation induced dislocation accumulation and arrays at the heterogeneous interface are the intrinsic plastic mechanism for the enhanced rate sensitivity. The formation of amorphous layers at the critical h is mainly responsible for the inverse size m.

Abstracts:We have developed a thermodynamically–consistent finite-deformation-based constitutive theory to describe strain induced grain boundary migration due to the heterogeneity of stored deformation energy in a plastically deformed polycrystalline cubic metal. Considering a representative volume element, a mesoscale continuum theory is developed based on the coupling between dislocation density-based crystal plasticity and phase field methods. Using the Taylor model-based homogenization method, a multiscale coupled finite-element and phase-field staggered time integration procedure is developed and implemented into the Abaqus/Standard finite element package via a user-defined material subroutine. The developed constitutive model is then used to perform numerical simulations of strain induced grain boundary migration in polycrystalline tantalum. The simulation results are shown to qualitatively and quantitatively agree with experimental results.

Abstracts:We combine pre-compressive test, reverse tensile test, re-compressive test, in-situ electron back-scattered diffraction, and high-resolution transmission electron microscopy to systematically investigate the effect of annealing on the reciprocal motion of twin boundary (TB) in pure Mg and Mg alloys AZ31 and AZ91. We find that the twin boundary mobility (TBM) can be enhanced by decreasing the dislocation density and increasing the number of coherent TBs after annealing for a short time. On the other hand, after prolonged annealing in Mg alloys, TBM decreases since TBs are stabilized by segregated solute atoms and precipitates. As a result, the TBM significantly depends on both the alloying element content and the annealing time. We demonstrate, for the first time, that friction stress and back stress can be applied to clarify the variation of TBM during annealing in Mg alloys. Our findings show that the TBM can be regulated by annealing, opening up a novel avenue for developing Mg alloys with high damping capacity or enhanced mechanical properties.