Finite-width adiabatic shear banding and dislocation patterning in mesoscale polycrystalline aggregates

TL;DR

This study combines mesoscale dislocation mechanics modeling and experiments to investigate adiabatic shear banding and dislocation patterning in polycrystalline materials, revealing size effects and microstructural evolution during deformation.

Contribution

It introduces a mesoscale model capturing finite-width shear bands and dislocation patterns without heat conduction, validated by large-scale 3D simulations and experiments.

Findings

Finite-width shear bands observed in simulations and experiments.

Dislocation densities form patterned structures at grain boundaries.

Material strength and stress evolve during large deformations, showing non-softening steady states.

Abstract

Dynamic shear banding under adiabatic conditions in a mesoscale polycrystalline aggregate is studied using a model of mesoscale dislocation mechanics and experiments. The model involves a length scale related to hardening induced by excess/polar/geometrically necessary dislocation (GND) density, and utilizes a simple classical crystal plasticity model with isotropic Voce law hardening. Simulations of statistically representative volume elements of a polycrystal determined from experimental samples are conducted. Studies in 2-d (section) and 3-d capture the experimentally observed finite-width shear bands and the formation of low-angle subgrain boundaries even in the absence of heat conduction in the model, as well as size-dependent strengthening for grain sizes from 1 to 20 $\mu$m. The 2-d and large-scale 3-d simulations, the latter involving 1 million finite elements, provide access to the progressive evolution of material strength, stress state, and temperature in the course of large deformations. GND distributions accumulate at grain boundaries and form patterned structures within grain interiors, offering insight into the microstructural changes that precede failure in adiabatic shear bands. Mesh-converged, delocalized and localized plastic flow to very large deformations without softening is observed for a significant range of parameters, reflecting a competition between GND hardening and thermal softening in setting the non-softening steady state in the absence of other ductile damage mechanisms in the model.