Microstructure evolution of compressed micropillars investigated by in situ HR-EBSD analysis and dislocation density simulations

Kolja Zoller,Péter Dusán Ispánovity,Szilvia Kalácska,Katrin Schulz

doi:10.5802/crphys.55

Abstract

With decreasing system sizes, the mechanical properties and dominant deformation mechanisms of metals change. For larger scales, bulk behavior is observed that is characterized by a preservation and significant increase of dislocation content during deformation whereas at the submicron scale very localized dislocation activity as well as dislocation starvation is observed. In the transition regime it is not clear how the dislocation content is built up. This dislocation storage regime and its underlying physical mechanisms are still an open field of research. In this paper, the microstructure evolution of single crystalline copper micropillars with a $\langle1\,1\,0\rangle$ crystal orientation and varying sizes between $1$ to $10\,\mu\mathrm{m}$ is analysed under compression loading. Experimental in situ HR-EBSD measurements as well as 3d continuum dislocation dynamics simulations are presented. The experimental results provide insights into the material deformation and evolution of dislocation structures during continuous loading. This is complemented by the simulation of the dislocation density evolution considering dislocation dynamics, interactions, and reactions of the individual slip systems providing direct access to these quantities. Results are presented that show, how the plastic deformation of the material takes place and how the different slip systems are involved. A central finding is, that an increasing amount of GND density is stored in the system during loading that is located dominantly on the slip systems that are not mainly responsible for the production of plastic slip. This might be a characteristic feature of the considered size regime that has direct impact on further dislocation network formation and the corresponding contribution to plastic hardening.

Highlights

Plastic deformation of bulk metals is usually a smooth process due to the large dislocation content of the crystal
The experimental results provide insights into the material deformation and evolution of dislocation structures during continuous loading. This is complemented by the simulation of the dislocation density evolution considering dislocation dynamics, interactions, and reactions of the individual slip systems providing direct access to these quantities
The analysis comprised experimental in situ HR-electron backscatter diffraction (EBSD) measurements as well as 3d continuum dislocation dynamics simulations

Summary

Introduction

Plastic deformation of bulk metals is usually a smooth process due to the large dislocation content of the crystal. As it was shown by compression experiments of single crystalline micron and sub-micron sized pillars (called microor nanopillars, respectively), deformation gets localized in distinct slip bands giving rise to an inhomogeneous slip surface [1]. The strength exhibits inverse dependence on the specimen size in this regime, a phenomenon called size effect [3] The reason for these specific properties of submicron-scale deformation is that the number of dislocations in the volume is rather limited. In the case of nanopillar compression experiments, most of the dislocations can escape the sample before being able to multiply or interact with other dislocations This process has been termed dislocation starvation [4, 5] and results in a limited amount of possible dislocation sources where accumulation of plastic strain is possible. These processes have been successfully modelled with discrete dislocation dynamics (DDD) simulations that track the motion of individual dislocation lines [7, 8] and the size effects were explained in terms of weakest link arguments [9,10,11]

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