Plasticity and Grain Boundary Diffusion at Small Grain Sizes

Abstract

Severe plastic deformation (SPD) processes have attracted considerable attention due to their potential for fabricating large quantities of material with an overall small grain size. [1,2] In the last decade, various SPD processes have been proposed for ultragrain refinement, such as high pressure torsion (HPT), [1] equal channel angular pressing (ECAP) [3] accumulative roll bonding (ARB) [4] or-for achieving the smallest grain sizes for pure metals through SPD-repeated cold rolling (RCR) [5,6] as attractive routes for fabricating bulk nanoand submicron-grained materials. With these approaches, high densities of lattice defects are introduced into the material, which, according to the general view, can then rearrange to attain a minimum energy configuration by forming a submicron cell/sub-grain structure that can evolve to a fine grained microstructure with large fractions of high angle grain boundaries (HAGBs) upon continued straining. It has been observed that materials with fine grain sizes that had been synthesized by such a severe deformation route, exhibit spectacular properties and property combinations, such as, e.g., a very high yield strength and high ductility at the same time, enhanced hydrogen storage capacity and enhanced hydrogen permeation velocity or combinations of high mechanical strength and high electrical conductivity, see, e.g., the recent overview in ref. [7] Along with the modification of the grain structure towards finer grains, the high number of lattice defects that are created led to the postulation of modifications of the grain boundary structure to explain the unusual (mechanical) properties that were observed. In the simplest description, high numberBulk nanostructured-or ultrafine-grained materials are often fabricated by severe plastic deformation to break down the grain size by dislocation accumulation. Underlying the often spectacular property enhancement that forms the basis for a wide range of potential applications is a modification of the volume fraction of the grain boundaries. Yet, along with the property enhancements, several important questions arise concerning the accommodation of external stresses if dislocation-based processes are not longer dominant at small grain sizes. One question concerns so-called ''non-equilibrium'' grain boundaries that have been postulated to form during severe deformation and that might be of importance not only for the property enhancement known already today, but also for spectacular applications in the context of, e.g., gas permeation or fast matter transport for self-repairing structures. This contribution addresses the underlying issues by combining quantitative microstructure analysis at high resolution with grain boundary diffusion measurements.758

Phase separation of a supersaturated nanocrystalline Cu–Co alloy and its influence on thermal stability

“…In addition, recent studies showed that diffusion or segregation processes in ufg materials are considerably influenced by the highly-distorted non-equilibrium grain boundaries [65][66][67][68]. The vacancy-type free volumes are supposed to be mainly located in the grain boundary regions, thus enhancing grain boundary diffusion.…”

Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

“…For example, recent studies demonstrated that transport properties of UFG materials (diffusion, segregation, etc.) are markedly affected by a so-called "nonequilibrium" grain boundary state [8][9][10]. At this place it is important to highlight that a broad spectrum of diffusivities of short-circuit paths is observed in UFG materials -contributions of high-angle grain boundaries with both "normal" and significantly enhanced diffusion rates can be differentiated in SPD-processed materials [11,12].…”

“…2(a) was analyzed by the method of geometric phase analysis (GPA) that allows calculating relative magnitudes of the in-plane components of the strain tensor and of the tensor of rigid body rotation with respect to a reference lattice, based on the intensity distribution in high resolution electron micrographs [28,29]. Details concerning the analysis and concerning the conditions under which the HRTEM micrograph was taken can be found in [9]. One result of this analysis is displayed in Fig.…”

Revealing interfacial diffusion kinetics in ultra-fine-laminated Ni with low-angle grain boundaries

“…And the geometric phase analysis was applied to determine the atomic level strains in high-resolution TEM (HRTEM) images, following the experimental process in Ref. [15]. Foils were mechanically thinned and electropolished in an electrolyte of 10 vol.% perchloric acid and 90 vol.% ethanol at −20°C.…”