Micromechanical modelling of nanocrystalline and ultrafine grained metals: A short overview

Abstract

Abstract:An overview of micromechanical models of strength and deformation behaviour of nanostructured and ultrafine grained metallic materials is presented. Composite models of nanomaterials, polycrystal plasticity based models, grain boundary sliding, the effect of nonequilibrium grain boundaries and nanoscale properties are discussed and compared. The examples of incorporation of peculiar nanocrystalline effects (like large content of amorphous or semi-amorphous grain boundary phase, partial dislocation GB emission/glide/GB absorption based deformation mechanism, diffusion deformation, etc.) into continuum mechanical approach are given. The possibilities of using micromechanical models to explore the ways of the improving the properties of nanocrystalline materials by modifying their structures (e.g., dispersion strengthening, creating non-equilibrium grain boundaries, varying the grain size distributions and gradients) are discussed.Keywords: Micromechanics; Finite element modelling; Nanomaterials; Nanocrystalline materials IntroductionDuring recent decades, growing interest of scientific community has been attracted to the nanostructured materials. The expectations on nanostructuring as a way to enhance material performances and to improve competing materials properties are very high, and some of them have been delivered, indeed.The extraordinary properties of nanocrystalline and ultrafine grained metallic materials (i.e., materials with the grain sizes of the order of several to several hundred nanometers) include superductility at room temperature, high hardness and high strength 2 to hardness value (which might be 2…7 times higher than in coarse grained materials), lower elastic modulus, negative Hall-Petch slope, enhanced strain rate sensitivity and difference between tensile and compression response [1][2][3][4][5]. The yield strength of nanocrystalline materials can be up to 5…10 times higher than of coarse-grained materials [6]. Other peculiar effect of nanocrystalline materials are the deviation fromHall-Petch relation at ultrafine and nanoscale grain sizes (below 100 nm), which goes into negative Hall Petch slope at about 10 nm, as well as asymmetry of tensile and compressive behaviour and enhanced diffusion properties [7].In order to predict the service properties of the materials and to explore the potential and reserves of their improvement, computational models linking the macroscale (service) In this paper, we present an overview of micromechanical models of nanocrystalline and ultrafine grained materials, their mechanical behaviour, deformation and strength.Composite models, crystal plasticity based models, grain boundary sliding, the effect of non-equilibrium grain boundaries, etc. are reviewed. The main constraints and challenges in the considered models are discussed. Composite models of nanocrystalline metallic materialsThe main structural feature of nanocrystalline and ultrafine grained materials, apart from the small grain sizes, is the high relative volume of grain boundary surface pha...

Grain-size dependent mechanical behavior of nanocrystalline metals

“…Nevertheless, the effect of grain size has been investigated by introducing an artificial length scale. A short review of composite methods is presented by Mishnaevsky and Levashov [205]. Importantly, FEM predicts correctly the internal stresses due to compatibility requirements.…”

Dislocation and back stress dominated viscoplasticity in freestanding sub-micron Pd films

“…Other advanced models including non equilibrium grain boundaries and GB defects have been reviewed by Mishnaevsky and Levashov [12]. Due to the large amount of internal interfaces combined to large local stress and small volume available for dislocation accumulation, thermally activated mechanisms substitute the regular forest dislocation type plasticity in nanocrystalline metals.…”

Strain-delocalizing effect of a metal substrate on nanocrystalline Ni film

“…Nanocrystalline (NC) metals, of which the grain size range less than 100 nm, have attracted a great number of researchers in the past few decades due to their remarkable physical and mechanical properties compared with conventional coarse-grained (CG) polycrystalline materials, especially the ultra-high yield and fracture strength [1][2][3][4]. However, most of the superior mechanical properties is accompanied with the decreased ductility, which performs as limited uniform elongation due to the high propensity of NC materials to the deformation localization [5,6].…”