Competing plastic deformation mechanisms in nanophase metals

Swygenhoven, H. Van; Spaczer, M.; Caro, A.; Farkas, Diana

doi:10.1103/physrevb.60.22

Cited by 327 publications

(154 citation statements)

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“…For nanocrystalline metals without nano-twin substructures, molecular dynamics simulations [6][7][8][9] have shown a strength softening mechanism as grain size is reduced to about 10 nm in Cu, which has been attributed to a transition from dislocation-mediated plastic deformation to grain-boundary-associated mechanisms such as grain-boundary sliding, grain-boundary diffusion and grain rotation 5,[14][15][16][17][18] . In nano-twinned ultrafine-grained Cu, the observed strength softening cannot be attributed to grain-boundary-associated mechanisms for the following reasons: (1) twin planes are coherent in nature, and shearing along them is as difficult as most other atomic planes; and (2) grain sizes and grain-boundary properties for samples with different twin thicknesses are similar 10,11 .…”

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confidence: 99%

Dislocation nucleation governed softening and maximum strength in nano-twinned metals

Wei

Lü

et al. 2010

Nature

1,022

525

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In conventional metals, there is plenty of space for dislocations-line defects whose motion results in permanent material deformationto multiply, so that the metal strengths are controlled by dislocation interactions with grain boundaries 1,2 and other obstacles 3,4 . For nanostructured materials, in contrast, dislocation multiplication is severely confined by the nanometre-scale geometries so that continued plasticity can be expected to be source-controlled. Nanograined polycrystalline materials were found to be strong but brittle 5-9 , because both nucleation and motion of dislocations are effectively suppressed by the nanoscale crystallites. Here we report a dislocation-nucleation-controlled mechanism in nano-twinned metals 10,11 in which there are plenty of dislocation nucleation sites but dislocation motion is not confined. We show that dislocation nucleation governs the strength of such materials, resulting in their softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, occurring at a critical twin-boundary spacing for which strength is maximized. At this point, the classical Hall-Petch type of strengthening due to dislocation pile-up and cutting through twin planes switches to a dislocation-nucleationcontrolled softening mechanism with twin-boundary migration resulting from nucleation and motion of partial dislocations parallel to the twin planes. Most previous studies 12,13 did not consider a sufficient range of twin thickness and therefore missed this strength-softening regime. The simulations indicate that the critical twin-boundary spacing for the onset of softening in nano-twinned copper and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twinboundary spacing, and the higher the maximum strength of the material.Ultrafine-grained Cu with nanoscale thin twins embedded in individual grains has recently been synthesized, achieving a strength increase by a factor of 7 to 10 relative to conventional coarse-grained polycrystalline Cu, as well as considerable ductility and high electrical conductivity 10,11 . More interestingly, the strength of such nanotwinned Cu first increases as the twin-boundary spacing l decreases, reaching a maximal strength at l 5 15 nm, then decreases as l is further reduced 11 . The trend of increasing strength in nano-twinned ultrafine-grained Cu with decreasing l can be relatively well explained by the Hall-Petch effect because the twin planes can serve as barriers to dislocations gliding on inclined slip planes. However, the strength softening with a further decrease of l from 15 nm to 4 nm is intriguing.For nanocrystalline metals without nano-twin substructures, molecular dynamics simulations 6-9 have shown a strength softening mechanism as grain size is reduced to about 10 nm in Cu, which has been attributed to a transition from dislocation-mediated plastic deformat...

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confidence: 99%

Dislocation nucleation governed softening and maximum strength in nano-twinned metals

Wei

Lü

et al. 2010

Nature

1,022

525

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“…In these materials, different deformation mechanisms are active (as different from coarse grained metals where plastic accommodation is controlled by the activity of dislocation sources and conventional dislocation glide). As discussed above, these mechanisms include diffusion controlled deformation (for very small grains), sliding and rotation of grains, as well as the different conditions of emission, glide and absorption of dislocation nucleation or partial dislocations (sometimes accompanied by twinning) [45,46]. Many of these mechanisms can be active in parallel, interact and interplay and transfer from one to another under some conditions.…”

Section: Dislocation Mechanisms Of Deformation and Polycrystal Plastimentioning

confidence: 99%

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

Mishnaevsky

Левашов

2015

Computational Materials Science

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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...

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“…In pure tension, growth and coalescence of nanovoids smaller than a few nm in diameter can participate collectively in shear band formation and localized plastic deformation processes that result in significant material softening in metals [1][2][3][4]. It is also now well-accepted that the mechanical behavior of GB networks governs the plastic deformation of nanocrystalline and nanotwinned metals due to GB-induced mechanisms, such as GB migration [5][6][7][8][9][10], dislocation nucleation [11][12][13][14], and interface sliding [10,11,[14][15][16][17]. However, recent investigations focusing on the atomic-scale structure of GBs by nanodiffraction [18,19] have revealed experimentally that columnar grains in nanotwinned Cu have GBs with curves and defects that have not been observed in previous studies.…”

Section: Introductionmentioning

confidence: 99%

Quasicontinuum study of the shear behavior of defective tilt grain boundaries in Cu

2014

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Atomistic simulations using the quasicontinuum method are used to study the role of vacancy defects and angström-scale voids on the mechanical behavior of five tilt bicrystals containing grain boundaries (GBs) that have been predicted to exhibit characteristic deformation processes of nanocrystalline and nanotwinned metals : GB-mediated dislocation emission, interface sliding, and shear-coupled GB migration. We demonstrate that such nanoscale defects have a profound impact on interfacial shear strength and underlying deformation mechanisms in copper GBs due to void-induced local stresses. In asymmetric high and low angle GBs, we find that voids become preferential sites for dislocation nucleation when the void size exceeds 4 Å. In symmetric Σ9(221) GBs prone to sliding, voids are shown to shield the local shear stress, which considerably reduces the extent of atom shuffling at the interface. In symmetric Σ5(210) and Σ27(115) GBs, we find that the effect of voids on shear-coupled GB migration depends on the GB tilt direction considered, as well as on the size and number of voids. Remarkably, large voids can completely abate the GB migration process in Σ27(115) GBs. For all GB types, the interfacial shear strength is shown to decrease linearly as the volume fraction of voids at the interface increases ; however, this study also suggests that this decrease is much more pronounced in GBs deforming by sliding than by dislocation nucleation or migration, owing to larger void-induced stresses.

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Competing plastic deformation mechanisms in nanophase metals

Cited by 327 publications

References 15 publications

Dislocation nucleation governed softening and maximum strength in nano-twinned metals

Dislocation nucleation governed softening and maximum strength in nano-twinned metals

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

Quasicontinuum study of the shear behavior of defective tilt grain boundaries in Cu

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