Ultra‐Fast Atomic Transport in Severely Deformed Materials—A Pathway to Applications?

Abstract

Severe plastic deformation of pure Cu and Cu‐rich alloys was found to create a hierarchical combination of fast and ultra‐fast diffusion paths ranging from non‐equilibrium grain boundaries to non‐equilibrium triple junctions, vacancy clusters, nano‐ and micro‐pores, and finally to general high‐angle grain boundaries. Under certain conditions, a percolating network of porosity can be introduced in the ultra‐fine grained materials by a proper mechanical and thermal treatment. This network may offer promising opportunities for creating materials with tailor‐made properties, including combinations of improved mechanical performance with a possibility of self repair using “vascular structures” for atom transport. Applications in such areas as drug eluting bioimplants and lead or polymer eluting materials for reduction of friction based on impregnation of porosity networks with these agents are also envisaged.

Plasticity and Grain Boundary Diffusion at Small Grain Sizes

“…As a matter of fact, it can even be desirable in that it provides novel opportunities in areas such as self-repairing structures, improved biomaterial compatibility, or fast gas permeation for energy storage applications. [11] Yet, the present study has uncovered a fundamentally significant feature of SPD-the occurrence of a connected network of ultra-fast diffusion paths (microvoids, microcracks, and/or open channels). This important finding sheds a new light on the low tensile ductility of SPD processed materials, which is often the price to be paid for their high strength.…”

“…[10] While some applications might benefit from the presence of percolating ultra-fast transport pathways (some of which are described in ref. [11] ), for other applications internal porosity or percolating pathways with drastically increased atomic transport would compromise the final performance. On the other hand, materials after similar processing treatment have shown largely enhanced properties and, as also summarized below, experimental results indicate a strong dependence of the formation of percolated open porosity on details of the processing route.…”

Grain boundary excess volume and defect annealing of copper after high-pressure torsion

“…It has already been shown that athermally produced excess vacancies are present in high concentrations that are otherwise found only close to the melting temperature [5,6] . A complex defect annealing kinetics is suggested from volume and grain boundary diffusion studies in SPD-processed Cu and Ni [7,8] . During annealing, abundant and highly mobile vacancies may, for example, agglomerate or form a percolating porosity network in combination with triple junctions of grain boundaries.…”

Ultrafine-Grained Magnesium Alloys for Hydrogen Storage Obtained by Severe Plastic Deformation

“…The role of these defects in facilitating the nucleation of the hydride phase and in optimizing its growth morphology is to be considered as well. It should also be mentioned that SPD processing may generate channels for ultra-fast diffusion, which is more rapid than what could be expected from an increased area of a grain boundary network in a grain-refined material (Divinski et al, 2010(Divinski et al, , 2011. Texture effects may also be at play (Crivello et al, 2016).…”