Shear-coupled migration of grain boundaries: the key missing link in the mechanical behavior of small-grained metals?

Romain Gautier,Marc Legros,Armin Rajabzadeh,Melvyn Larranaga,Nicolas Combe,Frédéric Mompiou

doi:10.5802/crphys.52

Abstract

Grain size reduction is a very efficient way to block dislocation movements and therefore create very strong metals and alloys. Not only grain boundaries are known obstacles for dislocations, but when reaching nanometer dimensions, crystallites usually become dislocation free, which imposes an additional constraint to develop plasticity. A recent effort to understand grain boundaries-based deformation mechanisms has therefore emerged. These mechanisms can be manifold, involving conservative and diffusive processes that are very poorly understood. A first approach consisting in downscaling mechanisms that are documented at large scale such as Coble creep, proved very limited. On the other hand, stress-assisted grain growth or shear-coupled grain boundary migration, that were recently observed in small-grained materials at room or low temperature may provide a crucial step to fully understand dislocation-less plasticity in nanocrystals. As this is a completely new field with many more degrees of freedom, a continuous research effort has to be carried out to link the mechanical properties of nanocrystals to these mechanisms specifically linked to grain boundaries.

Highlights

IntroductionAs the main vector of plastic deformation in crystals, the mobility of dislocations directly influences the mechanical properties
A mechanism called shear-coupled grain boundary migration (SCGBM) seems the most capable to alleviate for plasticity in nanocrystalline and small-grained metals
Current models and many simulations suggest that the coupling factor depends directly on the misorientation carried by a given grain boundaries (GB), and as such reaches values on the order of tens of percents

Summary

Introduction

As the main vector of plastic deformation in crystals, the mobility of dislocations directly influences the mechanical properties This is why hardening a metal mainly consists in blocking or hindering dislocation movements. Similar confinement-driven hardening is observed in Severe Plastic Deformation processes, that lead to the formation of dislocation cells, low- high-angle grain boundaries (HAGB) upon increasing strain [4,5,6]. Such top-down methods usually manage to reduce the grain size to a few hundreds of nm, but going further down requires bottom-up methods such as physical or chemical deposition, crystallite condensation and agglomeration [7, 8]. The microstructure of pure metals becomes thermodynamically unstable when they contain too many grain boundaries, and grain growth or recrystallization is observed at room temperature [9,10,11]

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Conclusion