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Wear during dry sliding friction depends on the features of deformation accumulation. The regularities of this process are traced on polycrystalline Hadfield steel with structure certified by Electron Backscatter Diffraction (EBSD). The root mean square values of the vibration acceleration and the friction coefficient change a little during friction. The grain deformation and the volume of the material involved in deformation are increased by friction. The area with deformed grains increases during the first three test cycles. The deformation propagation from the friction surface is slower than in single crystals. The activity of grain deformation is determined by a high value of the Schmid factor for the normal load and the friction force. The values of the shear trace parameters of grains are similar to those of the single crystals. The grain orientation difference and the hardening effect of grain boundaries delay the deformation accumulation and destruction, increasing the wear resistance of the polycrystalline steel compared to the single crystals. [ABSTRACT FROM AUTHOR]

The article is devoted to the study of the mechanism of nanopore formation in the junctions of polycrystal grains under the plastic deformation of a polycrystal due to the conservative sliding of lattice dislocations. A mechanism for the formation of a stress concentrator at the junction of the polycrystal grain boundaries is proposed. The possibility of relaxation of the stress state due to the formation of a junction nanopore is considered in the paper. [ABSTRACT FROM AUTHOR]

This work presents two methods to improve the understanding of the mechanical behavior of nanocrystalline thin films. Firstly, a simple two-phase model is proposed to explain the inverse Hall–Petch effect. The model suggests that the nanocrystalline material consists of the grain-boundary and the grain interior with different mechanical properties. Secondly, a computational method is developed to create more realistic grain boundaries in simulations of polycrystals. Traditionally created grain boundaries by Voronoi tessellation are too dense and do not accurately reflect the reality and the experimental results. The strength of the nanocrystalline aluminum thin film is simulated using molecular dynamics. The grain size dependence of the elastic modulus, ultimate tensile stress, and engineering yield strength is demonstrated. The experimental values of modulus and strength are approximately 6 times smaller probably due to the presence of porosity in the real sample. An approach is developed to introduce porosity in the simulated samples at the grain boundaries and in the grain interior to reduce this discrepancy. The modeled value of modulus for a grain size of 40 nm without porosity is 67 GPa, whereas, with 50% grain-boundary porosity and 20% intra-granular porosity it is 30 GPa. For the ultimate tensile strength, we get 3 GPa and 1.4 GPa for samples without porosity and with porosity, respectively. The simulated values of modulus with porosity are still 4 times higher and the values of strength are about 2 times higher than the experimental ones. The amount of porosity in our method can be adjusted to fit the experimental values; however, high values of porosity cause the mechanical instability of the simulated samples. [Display omitted] [ABSTRACT FROM AUTHOR]

Schwarz crystals with uniformly distributed twins enclosed by general grain boundaries (GBs) resembling triply minimal-surfaces were observed in high stacking fault energy metal Pt which is believed not readily to twin. By refining the grains of pure Pt from 140 μm to several nanometers via plastic deformation technique, deformation twins were found to emerge numerously and the structural morphologies of Schwarz crystals can be identified at grain sizes below 4 nm. Both experiments and atomistic simulations suggest that Pt Schwarz crystals are thermally stable at temperatures approaching the thermal equilibrium melting point. The measured room temperature microhardness as a function of grain size, on the other hand, may increase monotonically up to 12.9 GPa, revealing a sustained Hall-Petch relationship. The observed outstanding mechanical stabilities can be attributed to that, regardless of the critical twin size and the stacking fault energy of the material, the plastic deformation at the grain size limit is solely limited by twinning and both the nucleation and bowing-out of twinning partial dislocations from GB sources are notoriously difficult according to simulations and theory. [Display omitted] [ABSTRACT FROM AUTHOR]

• Numerical sensitivity of the level-set finite element method is shown to depend on physical parameters of the problem. • A heterogeneity scale for a grain boundary energy function applied to an untextured microstructure is developed. • Grain boundary energy functions are shown to have critical effects on the evolution of microstructures. In a previous article (Fausty et al., 2018) a new level-set finite element formulation for pure grain growth with heterogeneous grain boundary energies (i.e. one energy per grain interface) was developed and validated for simple configurations. In this work, the authors apply this new tool to the simulation of two dimensional grain growth of polycrystals using different disorientation dependent grain boundary energy functions. The results of these full-field calculations are assessed using the time dependent evolution of the following criteria: grain size, grain number, total interface energy, grain boundary disorientation distribution, grain boundary energy distribution and number of neighboring grains distribution. Of particular interest is the relationship between the grain boundary energy function and the evolution of the grain boundary network in the sense of both its morphology and its constitution. Some notable results are that the disorientation distribution evolution is inversely correlated to the grain boundary energy function itself and that the kinetics of grain growth are heavily effected by the heterogeneity of the system. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were employed to investigate the aggregation of monocrystal and polycrystal nanoparticles. The lattice structure, displacement vector, potential energy, shrinkage ratio, relative gyration radius and mean square displacement of the two systems are compared. The results indicate that the aggregation of polycrystal nanoparticles is more drastic than that of monocrystal nanoparticles. Besides, the polycrystal nanoparticles are found contacted and melted at lower-temperature than that of monocrystal nanoparticles. The reason for all these phenomena is that there is additional surface energy in the grain boundary of polycrystal nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract The effective properties, elastic and conductive, of a polycrystalline material with intergranular cracks are considered. The specifics of intergranular cracking are that crack locations are restricted to intergranular boundaries. This necessitates re-examination of the usual homogenization schemes (where crack locations are unrestricted). The model developed here addresses this issue. [ABSTRACT FROM AUTHOR]

Abstract The dislocation-induced shielding effect on the crack-tip depends on not only the number of dislocations but also the distance of dislocations from the crack-tip. The former is closely related to the nucleation of dislocation but the latter to the grain size and the obstruction of grain boundaries (GBs) to dislocations. In this paper, a discrete dislocation dynamic framework was further developed with a crack-tip dislocation emission scheme especially introduced, and then a series of discrete dislocation dynamic simulations were performed to study the shielding effect of dislocations on the crack-tip in poly-crystals. The different roles of the dislocation nucleation from the Frank-Read sources (DNFR) and dislocation emission from the crack-tip (DECT) in the shielding effect were studied, which indicates the DECT mechanism plays more important role compared with the DNFR one for the cases considered here. The shielding effect of dislocation is dominated by the grain size as well as the energy barrier for dislocation transmission across GB. With the increase of grain size, the shielding effect of dislocations on the crack-tip increases significantly, showing strong grain size effect. Moreover, with the increase of the GB energy barrier for dislocation transmission, the shielding effect of dislocations on the crack-tip first increases and then reaches its peak and finally decreases, showing an interesting mountain-shaped dependence on the GB energy barrier. This mountain-shaped dependence is gradually flattened with the increases of grain size. These interesting results enrich our understanding of the dislocations-induced shielding effect on the crack-tip in the poly-crystals. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Severe microcracks of polycrystal LiNi x Co y Mn z O 2 cathodes during cycling have led to increasing attention to the development of single-crystal cathodes. The single-crystal cathode particle is considered to induce a straight Li-ion diffusion through two-dimensional pathways in bulk compared with the pathways of grain boundary and cracks inside polycrystal particles. However, the effect of different paths on diffusion kinetics and the resulting electrochemical properties still needs to be better understood. In this work, the electrochemical performance of polycrystal and single-crystal cathodes caused by different diffusion paths was compared in detail. The results showed that grain boundaries and cracks of polycrystalline cathodes facilitate Li-ion diffusion. In contrast, relatively slow Li-ion diffusion occurs in the single-crystal cathode particles due to long diffusion paths. Moreover, the single-crystal cathode particles experience severe structural inhomogeneity due to the slow diffusion kinetics and inhomogeneous Li concentrations, resulting in structural distortion and localized strain increasing during cycling. [ABSTRACT FROM AUTHOR]

• A polycrystal DDD framework that accounts for key short- and long-range dislocation interactions. • DDD framework accounts for transmission of dislocations through the grain boundaries. • Model predicts lower Hall-Petch strengthening when slip transmissions across the grain boundaries are allowed. • Deformation-induced GND density near the grain boundaries increases with decrease in grain size. A two-dimensional discrete dislocation dynamics (DDD) framework is extended to model plastic deformation in polycrystalline materials having realistic grain morphology in finite geometries. The formulation includes constitutive rules to represent intra-grain dislocation mechanisms as well as dislocation/slip transmission across grain boundaries. Rules for intra-granular interactions mimic key three-dimensional dislocation mechanisms, such as line tension and dynamic junction and source formation, in addition to other short-range dislocation interactions. Various effects of idealized three-dimensional dislocation mechanisms on the mechanical response in tension are explored. The scaling of flow strength with grain size is investigated with and without slip transmission at grain boundaries. To aid in the analysis, spatial mappings of relevant stress components and geometrically necessary dislocation density are produced, which bring out competing dislocation-based strengthening mechanisms leading to the observed grain size effect in polycrystals. [ABSTRACT FROM AUTHOR]

By amalgamating the X-ray diffraction technique with the grain boundary tracking technique, a novel method, diffraction-amalgamated grain boundary tracking (DAGT), has been developed. DAGT is a non-destructive in-situ analysis technique for characterising bulk materials, which can be applied up to near the point of fracture. It provides information about local crystal orientations and detailed grain morphologies in three dimensions, together with high-density strain mapping inside grains. As it obtains the grain morphologies by utilising X-ray imaging instead of X-ray diffraction, which latter is typically vulnerable to plastic deformation, DAGT is a fairly robust technique for analysing plastically deforming materials. Texture evolution and localised deformation behaviours have here been successfully characterised in Al–Cu alloys, during tensile deformation of 27% in applied strain. The characteristic rotation behaviours of grains were identified, and attributed to the effects of interaction with adjacent grains on the basis of the 3D local orientation and plastic strain distributions. It has also been revealed that 3D strain distribution in grains is highly heterogeneous, which is not explained by known mechanisms such as simple incompatibility with adjacent grains or strain percolation through soft grains. It has been clarified that groups consisting of a few adjacent grains may deform coordinately, especially in shear and lateral deformation, and the characteristic deformation pattern is thereby formed on a mesoscopic scale. [ABSTRACT FROM AUTHOR]

This work presents a phase field model (PFM) for understanding crack propagation in a brittle polycrystalline material during quasi-static loading. A parametric study is performed by varying the elastic anisotropy and the fracture toughness anisotropy. The fracture toughness anisotropy is defined as a function of misorientation between neighboring grains and elastic anisotropy as a function of single crystal elasticity coefficients. The grain boundary fracture toughness is degraded compared to the grains within the PFM. This approach allows modeling intergranular and transgranular crack growth physically informed by the grain structure. A series of numerical simulations of single crystals, bicrystals, and polycrystals are performed to decouple the effects of elastic anisotropy and fracture toughness anisotropy on crack propagation. The model agrees well with similar studies found in the literature. In addition, the proposed PFM formulation is applied to simulate fracture in dual phase brittle polycrystals and the effect of the arrangement of phases on the overall fracture toughness of a polycrystal. • Analyses of the role of elastic anisotropy and fracture toughness anisotropy on crack path. • Modeling the effect of grain boundary fracture toughness on intra-granular vs. inter-granular cracks. • A combined analysis using single crystals, bicrystals and polycrystals. • Effect of material phases on crack path and overall fracture-toughness of polycrystals. [ABSTRACT FROM AUTHOR]

This study focuses on calculation of the effective diffusion coefficient of a polycrystalline material accounting for the grain size and shapes. Polycrystal is modelled as a composite consisting of a matrix with high diffusivity (grain boundaries and triple junctions) and inhomogeneities with low diffusivity (bulk grains including crystal defects like dislocations). The segregation at the grain boundaries is accounted for. Typical micromechanical models are re-written for diffusivity assuming that the grains have the shape of ellipsoids of revolution (spheroids). The results are compared with (1) numerical results for hydrogen diffusion in an imaginary polycrystalline material and (2) experimental results for diffusion of hydrogen in nickel polycrystal available in the literature. The approach can be used for extraction of information on diffusivity along the grain boundaries. [ABSTRACT FROM PUBLISHER]

Numerical simulations on α-iron and polycrystalline iron are conducted using discrete element method (DEM) combined with undiffused two-phase transition model, thermodynamic consistent free energy function, and phase transition kinetics of relaxation equation. The phase boundary and Hugoniot relation are obtained through numerical simulation. The simulation result of polycrystalline iron shows that the shock-front irregularities rise when the propagation distance of shock wave increases; and the shock-front is more irregular in the coarse grain model than in the fine grain model; the phase transitions in the polycrystalline are heterogeneous, and the transitions are observed first along grain boundaries. The curve of local average pressure and transformed mass fraction is obtained. [ABSTRACT FROM AUTHOR]

Abstract: In this work, the effect of external loading on the martensitic transformation is analyzed using an elasto-plastic phase field model. The phase field microelasticity theory, incorporating a non-linear strain tensor and the effect of grain boundaries, is used to study the impact of applied stresses on an Fe–0.3%C polycrystalline alloy, both in two and three dimensions. The evolution of plasticity is computed using a time-dependent equation that solves for the minimization of the shear strain energy. Crystallographic orientation of the grains in the polycrystal is chosen randomly and it is verified that the said assumption does not have a significant effect on the final volume fraction of martensite. Two-dimensional (2-D) and three-dimensional (3-D) simulations are performed at a temperature significantly higher than the martensitic start temperature of the alloy with uniaxial tensile, compressive and shear loading, along with hydrostatic stresses. It is found that the 3-D simulations are necessary to investigate the effect of external loading on the martensitic transformation using the phase field method since the 2-D numerical simulations produce results that are physically incorrect, while the results obtained from the 3-D simulations are in good agreement with the empirical observations found in the literature. Finally, it is concluded that the given model can be used to predict the volume fraction of martensite in a material with any kind of external loading. [Copyright &y& Elsevier]

A multiscale modeling methodology involving discrete dislocation dynamics (DDD) and crystal plasticity finite element method (CPFEM) was used to study the grain size effect in FCC polycrystalline plasticity. The developed model is based on the dislocation density storage-recovery framework and is expanded to the scale of slip systems. DDD simulations were used to establish a constitutive law incorporating the main dislocation mechanisms that are involved in the strain hardening process observed in monotonically deformed FCC polycrystals. This was achieved by calculating the key features controlling the accumulation of the forest dislocation density within the grains and the polarized dislocation density at the grain boundaries during plastic deformation. The model was then integrated with a CPFEM model at the polycrystalline aggregate scale to compute short- and long-range internal stresses within the grains. These simulations quantitatively reproduced the deformation curves of the FCC polycrystals as a function of grain size. Because of its predictive ability to reproduce the Hall–Petch effect in a physically justified approach, the proposed framework has significant potential for further applications. • Based on DDD simulation results, a governing relationship between surface GND density and long-range internal stress is established. • The computation of long-range internal stress at the scale of slip system is free of fitting parameters. • With a simple geometry of polycrystalline aggregate, the computed Hall–Petch effect agrees with the experimental data of pure polycrystalline copper. • Two history-related and grain size-dependent features are responsible for the Hall–Petch effect: initial dislocation density and surface GND density accumulated at grain boundaries. [ABSTRACT FROM AUTHOR]

Simulation results from finite element models using two types of 3D polycrystal geometric representations, one with a voxel representation and stair-stepped grain boundaries and the other with smooth grain boundaries, are compared. Both models start with a periodic grain structure representation, which is in the form of a regular, rectangular 3D array of points, where each point is assigned an orientation. The voxel representation is obtained by simply sampling the array of grid points on a coarser regular grid with a prescribed resolution and forming a voxel centered at each grid point, which is assigned the grain orientation from the sampled grid point. The voxel representation may be meshed directly by decomposing each voxel into finite elements. In the second case, a method is presented that extracts geometric topology information for a grain structure with smooth, flat grain boundaries from the discrete grain structure representation. From the geometric topology information, a finite element mesh is created. The two representations are then subjected to large strain deformations, and the simulation results and efficiencies are compared. The macroscopic behavior, overall texture evolution, and statistical distribution of stress and slip are found to be nearly identical for both models. However, noticeable differences are observed in the misorientation distribution within grains and the smoothness of the stress field. The voxel representation is found to be more efficient because of the uniform finite element mesh. [ABSTRACT FROM AUTHOR]

Abstract: In a previous work [Acta Materialia 2012;60:4787], we developed a new method that utilizes discrete, voxel-based data for microstructure quantification. We successfully calculated some relatively simple microstructural quantities and relations. In this paper, we apply and extend this method to compute more complex microstructural quantities and, in particular, map out the connection between grain growth rate and various topological properties. We present detailed results for several local and average topological and geometric properties of the microstructures during grain coarsening, including the curvature of grain boundaries and triple junction lines, grain cell shape, and their relations with growth dynamics. We also examine several well-known topological relations, i.e. Euler relations, the Lewis rule and the Aboav–Weaire law. These quantities and relations are the centerpiece of the grain growth models and theories developed so far. We also compare our results with some existing results in three dimensions. The quantitative description of the dynamic behaviors of the microstructural attributes adds a valuable data set to grain growth that can be used for benchmarking for phase field modeling and comparison with other approaches. [Copyright &y& Elsevier]

Abstract: While digital microstructures can be produced using various numerical models, quantitative characterization of these microstructures is often limited to a few attributes, such as grain diameter, the number of faces per grain and their distributions. A large number of topological and geometric properties that are useful for both experimental and theoretical analysis of microstructure–property relations have yet to be explored. To address this problem, we developed a series of numerical methods to compute geometric and topological properties of the microstructures. To test these methods, we performed a quantitative analysis of the digital microstructures obtained from a phase field simulation during isotropic grain growth. The new characterization methods allow us to not only identify each individual microstructure entity, including grain cell, grain boundary interface, triple junction line and vertex point, but also calculate the interface area, the triple junction length, and the curvature of grain boundary interfaces and triple junction lines. The detailed time evolution of these microstructural attributes, particularly the distributions of interface area and the length of triple junction lines, is extracted during the grain-coarsening process. The quantitative information gives us a detailed picture of grain growth in the phase field model that has not been seen before. Both the quantitative microstructures and the dynamic behavior are expected to serve as a benchmark for comparison to the microstructures obtained from other methods, including experiments. [Copyright &y& Elsevier]

Abstract: A new concept of a representative atomistic volume for a grain(RAVG), defined as a crystal having random shape, random crystallographic orientation, and a specific number of atoms, is introduced and investigated numerically. The grain is embedded into a 3D polycrystal with a fixed number of similar size grains of body-centered cubic lattice(iron). The computer generated polycrystals comprised of such grains are subjected to tensile tests using a molecular statics approach and the Morse pairwise potential. Two sizes of polycrystals were considered: the 8-grain (two-by-two-by-two) and the 27-grain one (three-by-three-by-three). For each type and a specific grain size 15 structurally random polycrystal samples were generated to assess the Young’s modulus and its scatter. It is shown that the grain size having 12 lattice constant units (around 3.4nm with 3270 atoms per grain) is sufficient to describe its meso-scale properties and thus can be accepted as a representative atomistic volume for a grain embedded in a polycrystal. [Copyright &y& Elsevier]

Abstract: In polycrystalline elastic simulations, grain boundaries can be considered as volume interphases or as elastic interfaces assuming a displacement jump across the interface. Such an interface description does not account for the in-plane deformation of the interface and Poisson effects cannot be reproduced. The purpose of this Note is to provide an enriched description of the elastic interface which takes into account such effects. When considering a multilayer material, the interphase description and the enriched interface description yield identical homogenized behaviour while quite important discrepancies can be observed with the classical interface description. [ABSTRACT FROM AUTHOR]

Abstract: The lattice and grain boundary diffusion coefficients of As in 260nm-thick Ni2Si films were measured. The Ni2Si layers were prepared via the reaction between a Si layer deposited by low pressure chemical vapor deposition and a Ni layer deposited by magnetron sputtering on a Si substrate covered with a SiO2 film. As was implanted in the silicide. Its concentration profiles were measured using secondary ion mass spectroscopy before and after annealing (550–700°C). 2D finite element diffusion simulations taking into account lattice diffusion and grain boundary (GB) diffusion were performed based on the microstructure of the samples. They were found to fit accurately the measured profiles and allowed to measure the diffusion coefficients for each temperature. Lattice diffusion is characterized by a pre-exponential factor D 0 v ∼1.5×10−1 cm2 s−1 and an activation energy Qv ∼2.72eV. In the case of GB diffusion P 0 = sδD 0 gb =9.0×10−3 cm3 s−1 and the activation energy was found to be higher than for lattice diffusion with Qgb ∼3.07eV. Existing data concerning diffusion in silicides and other materials is used to discuss these results. The diffusion of As in Ni2Si could be reduced due to impurity segregation in GBs. [Copyright &y& Elsevier]

Abstract: Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting new possibilities for mapping 3D grain shapes and crystallographic orientations in different classes of polycrystalline materials. X-ray diffraction contrast tomography (DCT) is a monochromatic beam imaging technique combining the principles of X-ray micro-tomography and three-dimensional X-ray diffraction microscopy (3DXRD). DCT provides simultaneous access to 3D grain shape, crystallographic orientation and attenuation coefficient distribution at the micrometer length scale. The microtexture of the material can be quantified in more detail by post-processing of the volume data provided by DCT. In particular one can determine the local crystallographic habit plane of the grain boundary by analysing the surface normal of the grain boundary with respect to the crystal orientation. The resulting five parameter description of the character of individual grain boundaries could previously be produced only by destructive characterization techniques. Statistical analysis of this kind of data can be expected to provide new insight into various physico-chemical processes, driven by the grain boundary energy (corrosion, coarsening). [Copyright &y& Elsevier]

Abstract: Classical molecular dynamics simulations have been carried out to study the primary damage due to -decay self-irradiations in single-, bi-, and poly-crystal UO2 matrices. In all the cases no amorphization has been found, only the creation of few point defects is observed. However, in all grain boundary systems numerous point defects are created along the interfaces. Furthermore, cascade morphologies depend strongly on the grain boundary structure. For symmetrical tilt grain boundaries with small misorientation angles (lower than 20°) the structure at the grain boundaries is composed of edge dislocations, whereas for higher misorientation angles is formed by Schottky defects. The grain boundary structure in the poly-crystal is found to be highly disordered. For the last two systems, cascades seem stopped by the interfaces unlike those with edge dislocation grain boundaries. These types of interface act like sink which traps moving atoms. [Copyright &y& Elsevier]

Many physical properties of rocks are sensitive to grain size and hence to the structure of grain boundaries. Depending on their properties, such as deformation and transport behaviour, boundaries may be divided into two broad types, namely special and general grain boundaries. Electron backscattered diffraction (EBSD) is used to investigate the misorientation distributions of grain boundaries and, more recently, to determine the population of grain boundary planes. Studies on metals and ceramics suggest that the grain boundary plane, rather than the misorientation, is the key parameter when defining special and general grain boundaries. In this study, the distribution of grain boundary plane orientations has been successfully determined using EBSD for a slightly deformed, synthetic NaCl material containing 22 ppm water. Boundaries showed a preference for {100} planes, which occurred with twice the frequency of a random distribution. The grain boundary plane distributions found in NaCl were largely in agreement with studies on MgO. Grain boundaries, with a coincident site lattice (CSL) misorientation, also showed a preference for {100} planes, rather than the planes of high coincident density associated with the CSL. Three main types of boundary were identified, namely {100} twist boundaries, boundaries with {100}{hkl} planes and general {hkl}{hkl} boundaries. As the properties of these three types of boundary differ, then the transport and creep properties in wet NaCl will depend on the fraction of the different boundary types found in the grain boundary population. [ABSTRACT FROM AUTHOR]

We present a variant of diffusion creep in polycrystalline bodies, which is based on solving a diffusion-viscous flow model problem. A polycrystal is treated as a composite material and the relevant boundary-value problem is solved through approximation of the self-consistent method. Simple expressions have been derived for viscosity coefficients of the polycrystal and grain boundaries. The creep equation is analyzed for a uniaxial compression case. The conclusions inferred from this equation are qualitatively consistent with the available experimental findings. [ABSTRACT FROM AUTHOR]

The deformation behavior and strengthening mechanism for Mo-3Nb single crystal with 〈111〉 orientation and polycrystal have been investigated and disclosed comprehensively in a wide temperature range by quasi-static compression with 7% plastic strain. The slip traces of single crystal and polycrystal at room temperature show different features, which long continuous slip traces appear on the surface of deformed Mo-3Nb single crystals while some branch-off slip lines can only be observed in large grains for polycrystals. The conjugate slip planes of single crystals are activated at the elevated temperature, while the slip lines of polycrystals are difficult to be observed because of the grain boundary sliding effect. The compressive stress of single crystals and polycrystals shows different results at different deformation temperatures, suggesting that grain boundaries in polycrystals play an important role in the strengthening mechanism. TEM analysis shows that dislocation entanglement is the main strengthening mechanism both in single crystal and polycrystal during the room temperature deformation. With the increase of temperature, the strengthening mechanism caused by dislocation entanglement becomes ineffective, while grain boundaries continue to hinder the dislocation movement and the strengthening effect greatly weakened due to the dynamic recovery. [Display omitted] • <111> oriented Mo-3Nb single crystal and polycrystal have been firstly designed • Different slip features are observed between single crystal and polycrystal • The grain boundary strengthening effect weakens when the deformation temperature reaches to 1500 °C [ABSTRACT FROM AUTHOR]

Abstract: Nanoindentation has been extensively used to measure the mechanical behavior of materials at the micro- and nanoscale. However, the material response of nano/microindentation and its intrinsic mechanisms are very complicated, especially when considering heterogeneous polycrystalline materials. In this contribution, nanoindentation in polycrystals, performed with a circular indenter, is studied by numerical modeling based on the two-dimensional discrete dislocation plasticity of Van der Giessen and Needleman [Van der Giessen E, Needleman A. Model Simul Mater Sci Eng 1995;3:689]. The dependence of indentation hardness is investigated with respect to four typical characteristic dimensions: indenter radius, grain size, indentation depth, and the distance between the grain boundary and the indenter. Results show that these characteristic dimensions have considerable influence on the nanoindentation hardness. Further investigation shows that their influence takes effect mainly in two ways, i.e. via strain hardening and the indentation size effect. Although both effects are size dependent, their underlying mechanisms are clearly different. For the present polycrystal case, the strain hardening effect is mainly associated with the constraints of grain boundaries and dislocation obstacles to the dislocation glide, while the indentation size effect is related to the average strain gradient beneath the circular indenter. [Copyright &y& Elsevier]

Abstract: Grain boundary segregation of phosphorus and silicon was investigated in polycrystals and bicrystals of an Fe–2.6Si–0.055P (wt.%) alloy after annealing at 600 or 800°C by Auger electron spectroscopy. Lower segregation effects were revealed in bicrystals if compared with polycrystalline samples annealed at the same conditions. Intergranular fracture was found to be enhanced when the grain boundary concentration of phosphorus was increased. [Copyright &y& Elsevier]

Cohesive zone model was used to simulate two-dimensional plane strain crack propagation at the grain level model including grain boundary zones. Simulated results show that the original crack-tip may not be separated firstly in an elastic-viscoplastic polycrystals. The grain interior’s material properties (e.g. strain rate sensitivity) characterize the competitions between plastic and cohesive energy dissipation mechanisms. The higher the strain rate sensitivity is, the larger amount of the external work is transformed into plastic dissipation energy than into cohesive energy, which delays the cohesive zone rupturing. With the strain rate sensitivity decreased, the material property tends to approach the elastic-plastic responses. In this case, the plastic dissipation energy decreases and the cohesive dissipation energy increases which accelerates the cohesive zones debonding. Increasing the cohesive strength or the critical separation displacement will reduce the stress triaxiality at grain interiors and grain boundaries. Enhancing the cohesive zones ductility can improve the matrix materials resistance to void damage. [ABSTRACT FROM AUTHOR]

Abstract: Polycrystalline Ca3Co1.8 M 0.2O6 (M=Mn, Fe, Co, Ni, Cu) and Ca2.7Na0.3Co2O6 were synthesized by solid-state reaction to evaluate the effect of substitution on the thermoelectric properties of Ca3Co2O6. Substitution by Mn, Cu and Na appears to increase carrier density, given that electrical resistivity () and the Seebeck coefficient (S) were simultaneously reduced. Conversely, Fe substitution seems to reduce carrier density, resulting in a simultaneous increase in S and . Cu and Na substitution resulted in a significant decrease in due to enhancement of grain size and grain boundary connectivity, which could have a strong impact on . Not only the intrinsic substitution effect on the electronic state but also this modification of the microstructure plays an important role in improvement of the thermoelectric power factor, particularly in the case of the Na-substituted sample. [Copyright &y& Elsevier]

The kinetics and topology of grain growth in three dimensions are simulated using a phase-field model of an ideal polycrystal with uniform grain-boundary mobilities and energies. Through a dynamic grain-orientation-reassignment routine, the computational algorithm avoids grain growth via coalescence, thus eliminating the dependence of the simulation results on the number of order parameters implemented in the phase-field description of the polycrystalline microstructure. Consequently, far fewer order-parameter values must be computed than in previous formulations of the phase-field model, which permits handling simulation cells large enough to contain a statistically significant number of grains. The kinetic and topological properties of the microstructure induced by coarsening closely resemble those obtained by other methods for simulating coalescence-free grain growth in 3D. [Copyright &y& Elsevier]

In this work we develop a new phase field method for modeling fracture in polycrystalline materials. Many studies addressed this problem by combining cohesive models at the grain boundaries and damage models for the bulk. In this paper, a unified formulation is proposed in the phase field framework. Anisotropic failure is considered within the grain. Each of the preferential failure direction is associated with a damage variable. An additional damage variable is dedicated to the grain boundaries which are considered of finite thickness. Furthermore the value of the fracture energy at the grain boundaries is set depending on the local misorientation. The obtained model allows modeling competition and interactions between intergranular and transgranular fracture. The proposed model is illustrated through several numerical examples including crack initiation and propagation in polycrystalline aggregates. • Modeling both intergranular and transgranular fractures in polycrystalline materials. • Implementation of a damage variable dedicated to grain boundaries. • Implementation of effects of misorientation on grain boundaries fracture energy. • Numerical correction of the phase field method directionality formalism. [ABSTRACT FROM AUTHOR]

[Display omitted] A framework for generating tetrahedral meshes of polycrystalline microstructures with optimal element-size distribution, as determined by the underlying defects (e.g., cracks and voids), is proposed. The framework utilizes a conformal surface mesh of each grain boundary and a point-cloud file to describe, respectively, the polycrystal and its underlying defects. The input meshes are first preprocessed to remove undesired mesh artifacts. To create a suitable element-size distribution informed by the defect structure, elements near the defects are locally refined via edge splitting, while elements far from the defects are coarsened by edge collapsing. The quality of the gradated mesh is improved via edge swapping. The presented framework is robust and flexible, and its ability to selectively refine and coarsen a given mesh is demonstrated by three proof-of-concept models: (1) a polycrystalline open-cell foam with intra-ligament voids, (2) an experimentally measured microstructure with an observed fatigue crack, and (3) an additively manufactured polycrystalline microstructure with an evolving crack. The framework is shown to generate gradated meshes that are converged with respect to both global (e.g., macroscopic stress–strain curve) and local (e.g., crack-front kink angles) metrics, with a significant reduction in both element count and computation time and a minimal reduction in element quality as compared to a uniform mesh. The gradated meshes generated using this framework also provide similar simulation results as produced by their highly refined and uniform mesh counterparts, with the advantage of being more computationally efficient. [ABSTRACT FROM AUTHOR]

We measured the thermal conductivity κ (T) of polycrystalline diamond with natural ( nat C) and isotopically enriched ( 12 C content up to 99.96 at.%) compositions over a broad temperature T range, from 5 to 410 K. The high quality polycrystalline diamond wafers were produced by microwave plasma chemical vapor deposition in CH 4 -H 2 mixtures. The thermal conductivity of 12 C diamond along the wafer, as precisely determined using a steady-state longitudinal heat flow method, exceeds much that of the nat C sample at T > 60 K. The enriched sample demonstrates the value of κ (298 K) = 25.1 ± 0.5 W cm − 1 K − 1 that is higher than the ever reported conductivity of natural and synthetic single crystalline diamonds with natural isotopic composition. A phenomenological theoretical model based on the full version of Callaway theory of thermal conductivity is developed which provides a good approximation of the experimental data. The role of different resistive scattering processes, including due to minor isotope 13 C atoms, defects, and grain boundaries, is estimated from the data analysis. The model predicts about a 37% increase of thermal conductivity for impurity and dislocation free polycrystalline chemical vapor deposition (CVD)-diamond with the 12 C-enriched isotopic composition at room temperature. [ABSTRACT FROM AUTHOR]

This work provides an experimental and computational analysis of low cycle fatigue of a tantalum polycrystalline aggregate. The experimental results include strain field and lattice rotation field measurements at the free surface of a tension–compression test sample after 100, 1000, 2000, and 3000 cycles at ± 0.2 % overall strain. They reveal the development of strong heterogeneites of strain, plastic slip activity, and surface roughness during cycling. Intergranular and transgranular cracks are observed after 5000 cycles. The Crystal Plasticity Finite Element simulation recording more than 1000 cycles confirms the large strain dispersion at the free surface and shows evidence of strong local ratcheting phenomena occurring in particular at some grain boundaries. The amount of ratcheting plastic strain at each cycle is used as the main ingredient of a new local fatigue crack initiation criterion. [ABSTRACT FROM AUTHOR]

This study presents prediction of crack propagation in polycrystalline materials under cyclic loads using Peridynamic (PD) theory. The PD fatigue model utilizes the strain-load cycle (ε-N) data and introduces the "remaining life" of each bond calculated from its cyclic strain, which changes over time. The model also captures the traditional Paris law, which is widely used for computing the fatigue crack growth. Moreover, a new crack tip detection algorithm is introduced to adaptively detect the crack tip which is necessary for the PD fatigue model. Finally, several numerical cases are considered to investigate the effect of the strength of grain boundaries and the size of grains on the fatigue behaviour of polycrystalline materials. • Peridynamic fatigue analysis is performed for polycrystalline materials. • A new crack tip detection algorithm is proposed. • Effect of the strength of grain boundaries on fatigue behaviour is investigated. • Effect of size of grains on fatigue behaviour is studied. [ABSTRACT FROM AUTHOR]

A new efficient framework coupling the two dimensional (2D) discrete dislocation dynamics (DDD) and the extended finite element method (XFEM) is developed for modeling the dynamic evolution of multiple dislocations in crystalline materials with abundant interfaces/surfaces. By a specially introduced stress calculation scheme with cut-off radius and virtual elements, the present algorithm can capture interaction between neighboring dislocations accurately although no time-consuming dislocation-core enrichment is employed. Since it is not sensitive to the FE mesh size and large XFEM time step can be used, its efficiency is evidently improved compared with other DDD frameworks. Due to the merits of XFEM itself, the present DDD-XFEM scheme can treat intractable surface/interface problems conveniently, with no additional image stress calculation as in the DDD-FEM superposition framework and special plastic-strain distribution near the surfaces/interfaces as in the discrete-continuous method (DCM) needed. For these, it can be used to model plastic behaviors of crystals with abundant interfaces/surfaces (i.e. grain/phase boundaries and crack/void surfaces). To show the ability of present DDD-XFEM scheme, it is used to simulate the mechanical responses of polycrystalline aluminum with assumed rigid and penetrable grain boundaries. A three-stage model for dislocation penetrating through grain boundary (DPTGB) is suggested. The results show that the present DDD-XFEM scheme can capture the formation of dislocation pile-ups and thus the Hall-Petch effect successfully. The DPTGB can decrease the flow stress and hardening rate evidently; moreover, it can weaken the grain size effect and may enhance the cyclic stress relaxation of polycrystals greatly. Image 1 • A more efficient and accurate XFEM-DDD framework for simulating the micro-plasticity of materials is developed. • The newly developed XFEM-DDD framework can well deal with various complex discontinuities in materials. • The present XFEM-DDD framework can simulate efficiently the problems on much longer/larger temporal/spatial scale. • A three-stage model for dislocation penetrating through GB (DPTGB) is suggested. • The grain size effect on strength and the cyclic stress relaxation are captured efficiently by the present scheme. [ABSTRACT FROM AUTHOR]