Dislocation structure analysis in the strain gradient of torsion loading: a comparison between modelling and experiment

Markus Stricker,Daniel Weygand,Mario Walter,Sabine M Weygand,Patric Gruber,Michael Ziemann

doi:10.1088/1361-651x/ac4d77

Abstract

Complex stress states due to torsion lead to dislocation structures characteristic for the chosen torsion axis. The formation mechanism of these structures and the link to the overall plastic deformation are unclear. Experiments allow the analysis of cross sections only ex situ or are limited in spacial resolution which prohibits the identification of the substructures which form within the volume. Discrete dislocation dynamics simulations give full access to the dislocation structure and their evolution in time. By combining both approaches and comparing similar measures the dislocation structure formation in torsion loading of micro wires is explained. For the ⟨100⟩ torsion axis, slip traces spanning the entire sample in both simulation and experiment are observed. They are caused by collective motion of dislocations on adjacent slip planes. Thus these slip traces are not atomically sharp. Torsion loading around a ⟨111⟩ axis favors plasticity on the primary slip planes perpendicular to the torsion axis and dislocation storage through cross-slip and subsequent collinear junction formation. Resulting hexagonal dislocation networks patches are small angle grain boundaries. Both, experiments and discrete dislocation simulations show that dislocations cross the neutral fiber. This feature is discussed in light of the limits of continuum descriptions of plasticity.

Highlights

Mechanical properties of small scale metallic structures have been investigated in detail focusing on the size effect [1,2,3,4]
From the surface traces observed in the discrete dislocation dynamics (DDD) simulations of the 100 torsion axis specimen (cf figures 1(a) and 2(a)), it is clearly visible that dislocation glide is active on all primary slip planes
Even though the employed methods are different in nature with different shortcomings, we show that it is possible to fuse the data from both methods to understand dislocation substructure formation and explain deformation modes

Summary

Introduction

Mechanical properties of small scale metallic structures have been investigated in detail focusing on the size effect [1,2,3,4]. The dislocation structure forming in twisted nano- and microwires and its connection to the mechanical properties as well as deformation mechanisms is not yet clear: torsion experiments on thin metallic wires [9] triggered the development of a complex strain gradient model in order to describe the size dependency of the mechanical response of polycrystalline wires under torsion. Continuum dislocation dynamics (CDD) investigations show dislocation density distributions within the wire consistent with DDD simulations [12, 16, 31] under torsion. Due to the small diameters and high loading rate of the wires in MD simulations, dislocations are nucleated at free surfaces.

Methods

Results

Discussion

Conclusion