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Showing papers on "Grain boundary strengthening published in 2011"


Journal ArticleDOI
20 Jan 2011-Nature
TL;DR: This work determines the location and identity of every atom at a grain boundary and finds that different grains stitch together predominantly through pentagon–heptagon pairs, and reveals an unexpectedly small and intricate patchwork of grains connected by tilt boundaries.
Abstract: The properties of polycrystalline materials are often dominated by the size of their grains and by the atomic structure of their grain boundaries. These effects should be especially pronounced in two-dimensional materials, where even a line defect can divide and disrupt a crystal. These issues take on practical significance in graphene, which is a hexagonal, two-dimensional crystal of carbon atoms. Single-atom-thick graphene sheets can now be produced by chemical vapour deposition on scales of up to metres, making their polycrystallinity almost unavoidable. Theoretically, graphene grain boundaries are predicted to have distinct electronic, magnetic, chemical and mechanical properties that strongly depend on their atomic arrangement. Yet because of the five-order-of-magnitude size difference between grains and the atoms at grain boundaries, few experiments have fully explored the graphene grain structure. Here we use a combination of old and new transmission electron microscopy techniques to bridge these length scales. Using atomic-resolution imaging, we determine the location and identity of every atom at a grain boundary and find that different grains stitch together predominantly through pentagon-heptagon pairs. Rather than individually imaging the several billion atoms in each grain, we use diffraction-filtered imaging to rapidly map the location, orientation and shape of several hundred grains and boundaries, where only a handful have been previously reported. The resulting images reveal an unexpectedly small and intricate patchwork of grains connected by tilt boundaries. By correlating grain imaging with scanning probe and transport measurements, we show that these grain boundaries severely weaken the mechanical strength of graphene membranes but do not as drastically alter their electrical properties. These techniques open a new window for studies on the structure, properties and control of grains and grain boundaries in graphene and other two-dimensional materials.

1,824 citations


Journal ArticleDOI
TL;DR: In this paper, the authors provide an overview of metal-based material classes whose properties as a function of external size have been investigated and provide a critical discussion on the combined effects of intrinsic and extrinsic sizes on the material deformation behavior.

1,515 citations


Journal ArticleDOI
TL;DR: The Interdependence Theory as mentioned in this paper links grain formation and nucleant selection to improve the ability to reveal the mechanisms of grain refinement, predict as-cast grain size and account for observations that only a small proportion of added inoculant particles nucleate grains.

437 citations


Journal ArticleDOI
TL;DR: In this paper, friction stir processing was applied to a magnesium alloy to generate various grain sizes with the same intense basal texture, and subsequent tensile deformation along two orthogonal directions by easy activation or inhibition of basal slip followed the Hall-Petch relationship between yield stress and grain size in both directions.

320 citations


Journal ArticleDOI
TL;DR: In this article, a multiscale, theoretical study of twin nucleation from grain boundaries in polycrystalline hexagonal close packed (hcp) metals is presented, and a key element in the model is a probability theory for the nucleation of deformation twins based on the idea that twins originate from a statistical distribution of defects in the grain boundaries.
Abstract: A multi-scale, theoretical study of twin nucleation from grain boundaries in polycrystalline hexagonal close packed (hcp) metals is presented. A key element in the model is a probability theory for the nucleation of deformation twins based on the idea that twins originate from a statistical distribution of defects in the grain boundaries and are activated by local stresses at the grain boundaries. In this work, this theory is integrated into a crystal plasticity constitutive model in order to study the influence of these statistical effects on the microstructural evolution of the polycrystal, such as texture and twin volume fraction. Recently, a statistical analysis of exceptionally large data sets of {1012} deformation twins was conducted for high-purity Mg ( Beyerlein et al., 2010a ). To demonstrate the significantly enhanced accuracy of the present model over those employing more conventional, deterministic approaches to twin activation, the model is applied to the case of {1012} twinning in Mg to quantitatively interpret the many statistical features reported for these twins (e.g., variant selection, thickness, numbers per grain) and their relationship to crystallographic grain orientation, grain size, and grain boundary misorientation angle. Notably the model explains the weak relationship observed between crystal orientation and twin variant selection and the strong correlation found between grain size and the number of twins formed per grain. The predictions suggest that stress fluctuations generated at grain boundaries are responsible for experimentally observed dispersions in twin variant selection.

291 citations


Journal ArticleDOI
TL;DR: In this article, the role of dislocation slip in superelastic deformation of thin Ni-Ti wires containing various nanograined microstructures was investigated by tensile cyclic loading with in situ evaluation of electric resistivity.

282 citations


Journal ArticleDOI
TL;DR: In this article, the authors evaluate how variation in grain size and processing impact the corrosion resistance of high purity aluminium and find that a Hall-Petch type relationship may exist for corrosion rate and grain size.

280 citations


Journal ArticleDOI
TL;DR: In this paper, solid solution strengthening in nanocrystalline alloys is studied using sputtered Ni-W as a model system, and the authors show that solute addition increases strength to very high levels, almost in proportion to the solute content.

197 citations


Journal ArticleDOI
TL;DR: In this paper, the radiotracer technique was applied by using the 63Ni isotope and high-precision mechanical grinding, and ultra-fast diffusion rates, which exceed those along general high-angle grain boundaries in annealed coarse-grained Ni, were observed.

197 citations


Journal ArticleDOI
TL;DR: In this paper, a mechanism-based plasticity model of nanotwinned metals was developed to investigate the effect of twin spacing on strength, ductility and work hardening rate of such materials.

196 citations


Journal ArticleDOI
TL;DR: In this article, a tensile test was conducted with annealed pure copper foils with different thicknesses and grain sizes to study the size effects on fracture behavior, and it was found that flow stress, fracture stress and strain, and the number of micro-voids on the fracture surface decrease with the decreasing ratio of specimen size to grain size.

Journal ArticleDOI
TL;DR: In this paper, the authors show that in surface-dominated structures with sub-micron dimensions, 60 nm grained Ni-W alloys exhibit lower tensile strength with decreasing pillar diameter, forming shear bands and undergoing mechanical twinning.

Journal ArticleDOI
TL;DR: In this paper, a two-scale method to treat predictively the interactions of large numbers of dislocations with grain boundaries has been developed, implemented, and tested, and a meso-scale simulation (MSS) redistributes the mobile part of the dislocation density within grains consistent with the plastic strain, computes the associated inter-dislocation back stress, and enforces local slip transmission criteria at grain boundaries.

Journal ArticleDOI
TL;DR: In the Gran Paradiso metagranodiorite (Western Alps) small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture.

Journal ArticleDOI
TL;DR: In this paper, the use of reactive wetting and stir mixing to disperse nano-sized ceramic particles in molten metal was studied using Al2O3 nanoparticle reinforced Al-Cu-Mg composites that were synthesized using gravity casting in a permanent mold and a single sample synthesized by squeeze casting.
Abstract: The use of reactive wetting and stir mixing to disperse nano-sized ceramic particles in molten metal was studied using Al2O3 nanoparticle reinforced Al–Cu–Mg composites that were synthesized using gravity casting in a permanent mold and a single sample synthesized by squeeze casting. These experiments have shown that reactive wetting of nanoparticles of Al2O3 in Al–Cu–Mg alloys, combined with stir mixing, can result in significant improvements in the hardness of gravity cast specimens provided that a reaction between the reinforcement and the matrix was either on-going or recently completed at the time of solidification. The results indicate that microstructure and strengthening mechanisms depend on the degree of clustering after reaction completion. It is hypothesized that a small amount of clustering resulted in particle engulfment and Orowan strengthening and increased clustering resulted in particle pushing and grain refinement strengthening described by the Hall–Petch relation. TEM analysis of a squeeze cast sample showed that individual particles and small flocculi were present within the grains rather than at grain boundaries, suggesting that the higher solidification rate of squeeze casting should follow particle pushing models and result in engulfment of larger particles than gravity casting. A comparison of yield stress or hardness variation with the inverse of the square root of grain size or dendrite arm spacing clearly shows the dominant strengthening mechanism as a result of the different process parameters. This method is also used to compare the results of this study to those of other studies.

Journal ArticleDOI
01 Jun 2011-Carbon
TL;DR: In this paper, the authors performed a series of hybrid molecular dynamics simulations to study the structures, energies, and structural transformations of symmetric tilt grain boundaries of graphene and found that the grain boundary comprises an array of edge dislocations, with the dislocation density increasing upon increasing the grain boundaries misorientation angle.


Journal ArticleDOI
TL;DR: In this article, the authors employ non-equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium, under the assumption that the whole grain size distribution remains self-similar.
Abstract: P>We employ basic non-equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium, under the assumption that the whole grain size distribution remains self-similar. We show that the grain size reduction is controlled by the rate of mechanical dissipation in agreement with recent findings. Our formalism is self consistent with mass and energy conservation laws and allows a mixed rheology. As an example, we consider the case where the grain size distribution is lognormal, as is often experimentally observed. This distribution can be used to compute both the kinetics of diffusion between grains and of dynamic recrystallization. The experimentally deduced kinetics of grain size coarsening indicates that large grains grow faster than what is assumed in classical normal grain growth theory. We discuss the implications of this model for a mineral that can be deformed under both dislocation creep and grain size sensitive diffusion creep using experimental data of olivine. Our predictions of the piezometric equilibrium in the dislocation-creep regime are in very good agreement with the observations for this major mantle-forming mineral. We show that grain size reduction occurs even when the average grain size is in diffusion creep, because the largest grains of the grain size distribution can still undergo recrystallization. The resulting rheology that we predict for olivine is time-dependent and more non-linear than in dislocation creep. As the deformation rate remains an increasing function of the deviatoric stress, this rheology is not localizing.

Journal ArticleDOI
TL;DR: In this paper, a finite element model embedded with the dislocation material models captures the essential features of the deformation field and grain refinement mechanism during cutting, and the model predicts the grains in the machined chips are refined from an initial size of 50-100μm to about 100-200nm for Al 6061 T6 and OFHC Cu at a low cutting speed of about 0.02m/s with negative rake angle tools.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the yield stress of oxide dispersion-strengthened (ODS) PM2000 steel and 14YWT steel as a function of temperature for grain sizes from 0.2μm to ∞.

Journal ArticleDOI
TL;DR: In this article, a biased Monte Carlo method is used to approximate representative non-equilibrium bicrystalline grain boundaries based on an embedded atom method potential, leveraging the concept of excess free volume.

Journal ArticleDOI
TL;DR: In this article, the yield strength of dilute nc-Cu alloys was investigated using molecular dynamics simulations, and it was shown that adding lower grain boundary energy significantly increased the yield of the alloy.

Journal ArticleDOI
TL;DR: In this article, an analytical model is developed to account for the effect of solute elements on grain size in an isothermal melt, and the analytical model shows that the final grain size can be related to the maximum undercooling, average growth velocity and solid fraction at the moment of recalescence.

Journal ArticleDOI
TL;DR: In this article, simultaneous shear coupling and grain rotation were observed experimentally during grain boundary migration in high-purity Al bicrystals subjected to an external mechanical stress at elevated temperatures.

Journal ArticleDOI
TL;DR: In this paper, grain boundary diffusivity on the cathode side has an apparent activation energy about 1 ǫ-eV lower than that of normal grain boundary diffusion.
Abstract: Grain growth in 8 mol% Y2O3-stabilized zirconia ceramics (8YSZ) under an electric current has been investigated. Enhanced grain growth on the cathode side starts at 1150°C, well below the conventional sintering temperature, while grain growth is dormant on the anode side until 1400°C. In fully dense samples, the grain size undergoes an abrupt transition, differing by a factor of more than 10 on the two sides. Porous samples also experience faster densification on the cathode side, but grain growth is postponed until full density is first reached. Estimated grain boundary diffusivity on the cathode side has an apparent activation energy about 1 eV lower than that of normal grain boundary diffusion. These results are attributed to supersaturated oxygen vacancies accumulated on the cathode side, causing cation reduction that lowers their migration barrier.

Journal ArticleDOI
TL;DR: In this article, a focused ion beam (FIB) was used to nucleate cracks crystallographically on single slip planes identical to natural stage I cracks and the crack path through a grain boundary was shown in 3D by FIB tomography.

Journal ArticleDOI
An-Chou Yeh1, Kang-Wei Lu1, Chen Ming Kuo1, Bor Hui-Yun, Wei Chao-Nan 
TL;DR: In this article, the effect of serrated grain boundaries on the high temperature creep behavior of IN718 superalloy was investigated, and it was shown that the morphology of grain boundaries can be a function of heat treatment conditions.
Abstract: This article presents an investigation of the effect of serrated grain boundaries on the high temperature creep behaviour of IN718 superalloy. XRD, DSC, SEM and TEM have been conducted to study the microstructures; experimental results have shown that the morphology of grain boundaries can be a function of heat treatment conditions. In addition, creep tests have been performed on samples with and without serrated grain boundaries under 650 °C/625 MPa. In conclusions, the formation of δ phase during the heat treatment can influence the morphology of grain boundaries, and an improvement of 400 h creep rupture life has been attributed to the serrated grain boundaries.

Journal ArticleDOI
TL;DR: The significant segregation of alloying elements at the boundaries of ultrafine-grained alloys implies that less solutes will be available in the matrix for precipitation with a decrease in the average grain size.

Journal ArticleDOI
TL;DR: In this paper, a model based on lattice dislocations piling up mechanism is proposed to illustrate the breakdown of the inverse Hall-Petch effect and calculate the critical grain size.
Abstract: For the purpose of studying the inverse Hall–Petch effect in nanocrystalline hard coatings, nanocrystalline ZrN coatings have been fabricated using magnetron sputtering with grain sizes ranging from 45 nm to 10 nm by varying negative biases from 0 V to 150 V. The transition from the classical Hall–Petch effect to an inverse Hall–Petch effect in nanocrystalline ZrN coatings is observed at a grain size between 19.0 nm and 14.2 nm. The reality of the inverse Hall–Petch effect in the present study is validated by exclusion of other possible effects on hardness of nanocrystalline ZrN coatings, such as porosity, multiphase, chemical composition, texture, and residual stress. Furthermore, a concise model based on lattice dislocations piling up mechanism is proposed to illustrate the breakdown of the Hall–Petch effect and calculate the critical grain size. The predictions of the model fit well with experimental data in some nitride and carbide nanocrystalline coatings. Both experimental and theoretical results indicate that the inverse Hall–Petch effect is an essential property of nanocrystalline hard coatings as similar to nanocrystalline metals and alloys.

Journal ArticleDOI
TL;DR: In this article, the authors describe methods of minimizing or eliminating grain boundary α formation by using metastable transition precipitates to nucleate α more rapidly, and the effects on fracture behavior also are described.
Abstract: Beta-Ti alloys contain sufficient concentrations of β stabilizing alloy additions to permit retention of the metastable β phase after cooling to room temperature. Decomposition of the metastable β phase results in the formation of several possible phases, at least two of which are metastable. Concurrently, equilibrium α phase often forms first by heterogeneous nucleation at the α grain boundaries with an accompanying precipitate free zone observed adjacent to the grain boundary α. The grain boundary regions are softer than the precipitation hardened matrix. As a consequence, fracture follows the prior β grain boundaries, especially in high-strength conditions. This fracture mode results in low tensile ductility and/or fracture toughness. This article will describe methods of minimizing or eliminating grain boundary α formation by using metastable transition precipitates to nucleate α more rapidly. The effects on fracture behavior also will be described.