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In this work, a viscoplastic fast Fourier transform (FFT)-based code is combined with a continuum dislocation dynamics (CDD) framework to analyze the mechanical behavior of polycrystalline MgAZ31 material under unidirectional tensile test. A crystal plasticity formulation including the size effects through a stress/strain gradient theory, dislocation density flux among neighboring grains and grain boundary back stress field is implemented into the CDD and coupled with VPFFT for this purpose. Then, an electron backscatter diffraction-based orientation image microscopy of a sample microstructure is applied as an input to the code. The model predicts, among other things, distributions of stress, strain, mobile dislocation density, geometrically necessary dislocation and stress–strain behavior. The numerical findings are compared with experimental results, and the micromechanical behavior of the polycrystal is discussed regarding dislocation density evaluation in different stages of strain hardening.

This paper studies the effects of high-pressure torsion (HPT) at ambient temperature on microstructural evolution and mechanical properties enhancement in pure copper. The aim is to introduce gradient microstructure, with various statistical distributions of grain size and grain orientations to examine their effect on strength and ductility. To this end, extruded cylindrical pure copper subjected to HPT for 1, 2, and 3-turns resulted in grain refinement down to the grain size of 500 nm. Combination of microhardness test and EBSD scans through the radial direction confirm the creation of a heterogeneous structure through the thickness and radial directions. The results demonstrate that increasing the shear strain leads to (1) ultra-fine grain (UFG) generation at deformed coarse-grain boundaries, (2) an increase in the fraction of recrystallized grains and high angle grain boundaries, and (3) a homogenous structure in the last step. A unique mixture has been obtained due to the particular shape of the anvils. The mixture included a chain of UFGs and coarse grains contain dislocations and subgrains. The highest level of gradient structure through the thickness was observed after 1-turn, which leads to the best combination of strength and ductility.

Although metallic materials with gradient microstructure exhibit notable performance in harsh environmental conditions, they can also exhibit unusual mechanical behaviour. This is attributed to bot...

Traditionally, modeling the effect of grain size on the mechanical behavior of crystalline materials is based on assuming an equivalent homogenous microstructure with strength being dependent on the average grain size, for example the well-known Hall-Petch relation. However, assuming an equivalent homogenized microstructure for a highly heterogeneous microstructure can lead to inaccurate prediction of strength and ductility, especially when the gradients in the spatial heterogeneity are severe. In this work, we employ a multiscale dislocation-based model combined with a strain/stress-gradient theory to investigate the effect of spatial heterogeneity of the microstructure on strength and ductility. We concentrate on understanding the effect of various grain size spatial distributions on the mechanical properties of interstitial free (IF)-steel. The results show that by controlling some parameters in the spatial distribution of the microstructure with regions composed of micro-grains and nano-grains one can achieve improved strength and ductility. Based on these results, it is suggested that the mechanical properties of gradient materials can be described by phenomenological relations that include two structural parameters, grain size and grain-size gradient, in contrast to Hall-Petch relation for homogenous materials where only grains size appears in the equation.

Additive manufacturing (AM) methods such as Aerosol Jet (AJ) printing allow the fabrication of structures via sintering of micro and/or nanoparticles, leading to microstructures that consist of various combinations of pore and grain sizes. It has been reported that AJ printed and sintered silver micropillars show an unusual behavior of high stiffness and high strain-to-failure for structures with high porosity and vice versa (Saleh et al. 2018 [1]). This behavior, however, is accompanied by the stiffer structures having smaller grain sizes and softer structures having larger grain sizes. To explain the physics of this behavior where a trade-off between hardening caused by size effects (grain refinement and gradients) and softening caused by porosity is expected to play a critical role, a multi-scale modeling approach is proposed in this paper. The model formulation consists of a continuum dislocation dynamics (CDD) framework, coupled with continuum plasticity and finite element analysis. The dislocation dynamics formulation is introduced into a user material subroutine and coupled with a finite element commercial solver, in this case, LS-DYNA, to solve the model in three-dimensional scale with the same size as the AM micropillars. The results from the model capture the general trends observed in compression tests of AM micropillars. In particular, it is shown that the grain size and dislocation density have a disproportionately higher influence over the mechanical deformation of metallic structures when compared to the porosity. These results show that the behavior of AM structures in the plastic regime is dominated by grain size effects rather than porosity. Some limitations of the model and possible future refinements are discussed. The paper provides an important analytical framework to model the mechanical behavior of AM structures with internal porosity in the plastic regime.

This chapter, outlines a multiscale dislocation-based plasticity framework coupling discrete dislocation dynamics (DDD) with continuum dislocation-based plasticity. In this framework, and guided by DDD, a continuum dislocation dynamics (CDD) plasticity model involving a set of spatio-temporal evolution equations for dislocation densities representing mobile and immobile species is developed. The evolution laws consist of a set of components each corresponding to a physical mechanism that can be explicitly evaluated and quantified from DDD analyses. In this framework, stochastic events such as cross-slip of screw dislocations and uncertainties associated with initial microstructural conditions are explicitly incorporated in the continuum theory based on probability distribution functions defined by activation energy and activation volumes. The result is a multiscale dislocation-based plasticity model which can predict not only the macroscopic material mechanical behavior but also the corresponding microscale deformation and the evolution of dislocation patterns, size and gradient-dependent deformation phenomena, and related material instabilities at various length and time scales.

In this study, a dislocation density-based model is introduced to analyze slip transmission across grain boundaries in polycrystalline materials. The method applies a combination of the misorientation of neighboring grains and resolved shear stress on relative slip planes. This model is implemented into a continuum dislocation dynamics framework and extended to consider the physical interaction between mobile dislocations and grain boundaries. The model takes full account of the geometry of the grain boundary, the normal and direction of incoming and outgoing slip systems, and the extended stress field of the boundary and dislocation pileups at the boundary. The model predicts that slip transmission is easier across grain boundaries when the misorientation angle between the grains is small. The modeling results are verified with experimental nanoindentation results for polycrystalline copper samples.