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Showing papers on "Grain boundary strengthening published in 2014"


Journal ArticleDOI
TL;DR: In this paper, the influence of changes in crystallographic texture on the Hall-Petch (H-P) relationship for an Mg alloy was investigated, and the texture variations were facilitated by changing the uniaxial tensile loading orientation with respect to the normal direction of the rolled Mg plate.

294 citations


Journal ArticleDOI
TL;DR: In this article, a linear trend between carbon segregation and the misorientation angle ω was found for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime.
Abstract: Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation.

284 citations


Journal ArticleDOI
TL;DR: In this article, a review of recent progress in understanding dislocation interactions with grain boundaries and interfaces in metallic systems via static and in situ dynamic experimental approaches is reviewed, as well as a comparison of different experimental approaches.
Abstract: Recent progress in understanding dislocation interactions with grain boundaries and interfaces in metallic systems via static and in situ dynamic experimental approaches is reviewed.

274 citations


Journal ArticleDOI
TL;DR: This study presents dislocation motion, glide and climb, leading to grain boundary migration in a tungsten disulphide monolayer, and demonstrates how dislocations introduce considerable strain along the grain boundaries and at the dislocation cores.
Abstract: Dislocations have a significant effect on mechanical, electronic, magnetic and optical properties of crystals. For a dislocation to migrate in bulk crystals, collective and simultaneous movement of several atoms is needed. In two-dimensional crystals, in contrast, dislocations occur on the surface and can exhibit unique migration dynamics. Dislocation migration has recently been studied in graphene, but no studies have been reported on dislocation dynamics for two-dimensional transition metal dichalcogenides with unique metal-ligand bonding and a three-atom thickness. This study presents dislocation motion, glide and climb, leading to grain boundary migration in a tungsten disulphide monolayer. Direct atomic-scale imaging coupled with atomistic simulations reveals a strikingly low-energy barrier for glide, leading to significant grain boundary reconstruction in tungsten disulphide. The observed dynamics are unique and different from those reported for graphene. Through strain field mapping, we also demonstrate how dislocations introduce considerable strain along the grain boundaries and at the dislocation cores.

195 citations


Journal ArticleDOI
TL;DR: The authors showed that the Hall-petch effect is not another size effect sui generis but is the same size effect as that observed in epitaxial thin film growth and in micromechanical testing of small specimens.

163 citations


Journal ArticleDOI
TL;DR: Hall-Petch analysis has been connected to the macro-scale description of the fracture mechanics stress intensity parameter as mentioned in this paper, and the pile-up model description has been more definitely associated with the Griffith theory of achieving a critical stress concentration at the tip of a crack.
Abstract: Pioneering research results reported in the early 1950’s by E. O. Hall and N. J. Petch on iron and steel materials have led to an expanded description of the grain size dependence of the complete stress­strain behavior of a wider range of materials and including assessments of other mechanical properties such as the ductile to brittle transition behavior and the hardness of materials, particularly, of nanocrystalline materials. The dislocation pile-up model that was presented originally for the inverse square root of grain diameter dependence of material strength has endured. Most recently, the pile-up model description has been more definitely associated with the Griffith theory of achieving a critical stress concentration at the tip of a crack; and, the Hall-Petch analysis has been connected to the macro-scale description of the fracture mechanics stress intensity parameter. These topics and other “60 years of Hall-Petch” type researches are tracked over time in the present report while giving special emphasis to current order-of-magnitude strength improvements that are reported for metals with nanopolycrystalline grain diameters.

159 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the dominant factor for extra grain refinement by alloying was due to the effect of solute-matrix atomic-size mismatch and modulus interaction on the mobility of edge dislocations.

157 citations


Journal ArticleDOI
Pauli Lehto1, Heikki Remes1, Tapio Saukkonen1, Hannu Hänninen1, Jani Romanoff1 
TL;DR: In this paper, a rule of mixtures based approach for determining the characteristic length of the microstructure for heterogeneous weld metal was introduced, and the proposed grain size parameter, the volume-weighted average grain size, was measured experimentally for nine structural steel weld metals and two base materials.
Abstract: The strength of polycrystalline metals increases with a decrease in grain size according to the Hall–Petch relationship. However, heterogeneous microstructures deviate from this relationship depending on the distribution of grain sizes. This paper introduces a rule of mixtures based approach for determining the characteristic length of the microstructure for heterogeneous weld metal. The proposed grain size parameter, the volume-weighted average grain size, is measured experimentally for nine structural steel weld metals and two base materials. The weld metals are found to have a large variety of grain size distributions that are noticeably broader than those of the base material due to differences in phase contents. The results show that the volume-weighted average grain size is able to capture the influence of grain size distribution on the strength of welded structural steel. Based on the experimental results, a modified Hall–Petch relationship is formulated for the strength prediction of heterogeneous microstructures. The modified relationship is also found to be applicable to data from the literature.

153 citations


Journal ArticleDOI
TL;DR: In this paper, the authors use crystal plasticity finite element (CPFE) models of 2D and 3D polycrystalline microstructures to elucidate 3D topological effects on microstructural evolution during rolling deformation.

148 citations


Journal ArticleDOI
TL;DR: In this paper, the empirical Hall-Petch relationship mathematically describes grain boundary strengthening and provides guidance for a straightforward way to produce stronger materials by increasing the average crystallite grain size.

148 citations


Journal ArticleDOI
TL;DR: In this paper, the strengthening contributions in medium-carbon tempered martensite are revealed by using transmission electron microscopy and synchrotron radiation X-ray diffraction, the different microstructural features have been captured; these include precipitation, grain boundary, solid solution and dislocation forest strengthening.

Book ChapterDOI
Dierk Raabe1
01 Jan 2014
TL;DR: In this paper, the main mechanisms, lattice defects, and driving forces associated with recovery, recrystallization and grain growth are reviewed and an introduction to the simulation of these phenomena is provided.
Abstract: Recovery, recrystallization and grain growth are among the most important metallurgical heat treatment processes to soften cold worked metals and design desired microstructures and textures. Specifically the reduction in grain size can be efficiently achieved by recrystallization. While plastic cold working increases the stored energy of metals, mainly through dislocation accumulation, recovery and specifically recrystallization lead to it reduction. While recovery describes the gradual re-ordering and annihilation of the stored dislocations, primary recrystallization proceeds discontinuously by the formation and motion of high angle grain boundaries which discontinuously sweep the deformation substructure. Grain growth describes the process of competitive capillary driven coarsening of the average grain size. This chapter reviews the main mechanisms, lattice defects, and driving forces associated with recovery, recrystallization and grain growth and provides an introduction to the simulation of these phenomena.

Journal ArticleDOI
TL;DR: In this paper, the effects of high pressure torsion (HPT) processing on an Al-Mg-Si alloy (AA6060) have been investigated comprehensively, and it was shown that the processing temperature has complex effects on the strength, grain refinement and solute nanostructures of the alloy.

Journal ArticleDOI
TL;DR: In this article, the effect of grain size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to 2.25mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined.
Abstract: Crack initiation in uniaxial compressive loading of rocks occurs well before the peak strength is reached. The factors that may influence the onset of cracking and possible initiating mechanisms were explored using a discrete element numerical approach. The numerical approach was based on grain-based model that utilized the Voronoi tessellation scheme to represent low porosity crystalline rocks such as granite. The effect of grain size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to 2.25 mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined. The modelling revealed that crack initiation appears to be a tensile mechanism in low porosity rocks, and that shear cracking along grain boundaries is only a prominent mechanism near the peak strength. It was also shown that the heterogeneity introduced by the grain size distribution had the most significant effect on peak strength and crack initiation stress. The peak strength ranges from 140 to 208 MPa as the grain size distribution varies from heterogeneous to uniform, respectively. However, the ratio of crack initiation to peak stress showed only minor variation, as the heterogeneity decreases. The other factors investigated had only minor effects on crack initiation and peak strength, and crack initiation ratio.

Journal ArticleDOI
TL;DR: In this article, the authors used focused ion beam fabricated micrometer-sized bicrystalline Cu pillars including either a large-angle grain boundary (LAGB) or a coherent twin boundary (CTB) parallel to the compression axis and additionally on single-crystalline reference samples.

Journal ArticleDOI
TL;DR: In this article, the grain boundary energy is derived and systematically studied in terms of temperature, grain size, concentration and solute segregation for binary systems of 44 solvents and 52 solutes, using readily available elemental data, such as moduli and liquid enthalpy of mixing.

Journal ArticleDOI
TL;DR: In this paper, the influence of grain boundary segregation on the strength of a nanostructured austenitic stainless steel was investigated, and it was shown that grain boundary segregations can lead to significant enhancement of the yield stress.

Journal ArticleDOI
TL;DR: In this article, the effect of cold rolling on the microstructure evolution and mechanical properties of Fe-23Mn-0.3C-1.5Al twinning-induced plasticity (TWIP) steel was studied.
Abstract: The effect of cold rolling on the microstructure evolution and mechanical properties of Fe–23Mn–0.3C–1.5Al twinning-induced plasticity (TWIP) steel was studied. The extensive mechanical twinning subdivides the initial grains into nanoscale twin lamellas. In addition, the formation of deformation micro bands at e >40% induces the formation of nanostructured bands of localized shear. It is demonstrated that the mechanical twinning is notably important for dislocation storage within the matrix, as the twin boundaries act as equally effective obstacles to dislocation glide as conventional high-angle grain boundaries. However, the contribution of the grain size strengthening to the overall yield stress (YS) is much smaller than that of the deformation strengthening, which plays a major role in the superior work-hardening behavior of TWIP steels. A very high dislocation density of ~2×10 15 m −2 is achieved after plastic deformation with moderate strains. The superposition of deformation strengthening and grain boundary strengthening leads to an increase in the YS from 235 MPa in the initial state to 1400 MPa after 80% rolling.

Journal ArticleDOI
TL;DR: Generic mechanisms of coupled nucleation and evolution of dislocation and HPP structures in the nanograin material under pressure and shear are elucidated utilizing the developed advanced phase field approach (PFA).
Abstract: There are two main challenges in the discovery of new high pressure phases (HPPs) and transforming this discovery into technologies: finding conditions to synthesize new HPPs and finding ways to reduce the phase transformation (PT) pressure to an economically reasonable level. Based on the results of pressure–shear experiments in the rotational diamond anvil cell (RDAC), superposition of plastic shear on high pressure is a promising way to resolve these problems. However, physical mechanisms behind these phenomena are not yet understood. Here, we elucidate generic mechanisms of coupled nucleation and evolution of dislocation and HPP structures in the nanograin material under pressure and shear utilizing the developed advanced phase field approach (PFA). Dislocations are generated at the grain boundaries and are densely piled up near them, creating a strong concentrator of the stress tensor. Averaged shear stress is essentially larger in the nanograin material due to grain boundary strengthening. This leads to the increase in the local thermodynamic driving force for PT, which allows one to significantly reduce the applied pressure. For all cases, the applied pressure is 3–20 times lower than the PT pressure and 2–12.5 times smaller than the phase equilibrium pressure. Interaction between nuclei leads sometimes to their coalescence and growth of the HPP away from stress concentrators. Plasticity plays a dual role: in addition to creating stress concentrators, it may relax stresses at other concentrators, thus competing with PT. Some ways to optimize the loading parameters have been found that lead to methods for controlling PT. Since such a local stress tensor with high shear stress component cannot be created without plastic deformations, this may lead to new transformation paths and phases, which are hidden during pressure induced PTs.

Journal ArticleDOI
TL;DR: In this article, the authors provide a review of both experimental and atomistic simulation efforts focused on the details of slip transmission at grain boundaries in metallic materials and provide a discussion of outstanding challenges for atomistic simulations to advance this field.
Abstract: To fully understand the plastic deformation of metallic polycrystalline materials, the physical mechanisms by which a dislocation interacts with a grain boundary must be identified. Recent atomistic simulations have focused on the discrete atomic scale motions that lead to either dislocation obstruction, dislocation absorption into the grain boundary with subsequent emission at a different site along the grain boundary, or direct dislocation transmission through the grain boundary into the opposing lattice. These atomistic simulations, coupled with foundational experiments performed to study dislocation pile-ups and slip transfer through a grain boundary, have facilitated the development and refinement of a set of criteria for predicting if dislocation transmission will occur and which slip systems will be activated in the adjacent grain by the stress concentration resulting from the dislocation pile-up. This article provides a concise review of both experimental and atomistic simulation efforts focused on the details of slip transmission at grain boundaries in metallic materials and provides a discussion of outstanding challenges for atomistic simulations to advance this field.

Journal ArticleDOI
TL;DR: In this paper, the microstructural and mechanical properties of an ultrafine-grained (UFG) Al-Zn alloy processed by high-pressure torsion (HPT) are investigated using depth-sensing indentations, focused ion beam, scanning electron microscopy and scanning transmission electron microscope.
Abstract: The microstructural and mechanical properties of an ultrafine-grained (UFG) Al–Zn alloy processed by high-pressure torsion (HPT) are investigated using depth-sensing indentations, focused ion beam, scanning electron microscopy and scanning transmission electron microscopy. Emphasis is placed on the microstructure and the effects of grain boundaries at room temperature. The experiments show the formation of Zn-rich layers at the Al/Al grain boundaries that enhance the role of grain boundary sliding leading to unique plastic behavior in this UFG material. The occurrence of significant grain boundary sliding at room temperature is demonstrated by deforming micro-pillars. Our results illustrate a potential for using UFG materials as advanced functional materials in electronic micro-devices.

Journal ArticleDOI
TL;DR: In this paper, the effects of grain size and shape on mechanical properties of nanocrystalline copper with mean grain size varying from 2.6 to 53.1 µm were investigated.
Abstract: Molecular dynamics (MD) simulation has been used to study effects of grain size and shape on mechanical properties of nanocrystalline copper with mean grain size varying from 2.6 to 53.1 nm. Three grain size regions are identified according to the plot of flow stress versus mean grain size d . In region I ( d ≈20–53 nm) flow stress increases with the decrease of d . Deformation twinning process and extended dislocation are observed in this region. In region II ( d ≈8–20 nm) detwinning process appears as a competitive deformation mechanism with the twinning process. The flow stress begins to decrease slightly in this region. In region III ( d d −1 in the simulated range of mean grain size, and it directly depends on the volume fraction of grain boundaries. The Young׳s modulus of the grain boundary component is found to be ~25% of that of the grain interior. MD simulations on samples with spherical and cylindrical grain shapes are also carried out. The influence of grain shape on flow stress is hardly observed, indicating that for different grain shapes the plastic deformation mechanism is the same. The grain shape has an obvious effect on Young׳s modulus, which is attributed to the difference of the volume fraction of grain boundaries for samples with the different grain shapes.

Journal ArticleDOI
TL;DR: In this paper, the continuous columnar-grained polycrystalline Cu 71.8 Al 17.8 Mn 10.4 shape memory alloys were prepared and possess a strong 〈0-0-1〉 texture along the solidification direction and straight low-energy grain boundary.

Journal ArticleDOI
TL;DR: Armstrong et al. as mentioned in this paper applied dislocation and continuum mechanics models of the H-P relationship to predict order-of-magnitude increases in strength properties of nanopolycrystalline materials, including description of the strain rate sensitivity dependence on average grain diameter.
Abstract: Hall and Petch had established in the early 1950s a linear inverse square root of grain diameter dependence for yielding and cleavage of polycrystalline iron and steel materials, with ordinate intercept stress, σ 0, and slope value, k. Petch and colleagues extended the relationship in 1962 to the full stress–strain behavior of a diverse number of metals and alloys. Connection with other mechanical properties such as the hardness, fatigue and strain rate sensitivity properties was demonstrated in 1970. In 1983, Weng incorporated the dependence into a micromechanical analysis of material strength by building onto earlier Taylor-initiated work on multiply-coupled grain deformations. More recently, Armstrong, Weng and colleagues have applied dislocation and continuum mechanics models of the H–P relationship to predict order-of-magnitude increases in strength properties of nanopolycrystalline materials, especially including description of the strain rate sensitivity dependence on average grain diameter. These topics are assessed from a dislocation mechanics viewpoint in the present report that provides H–P connection with the Taylor dislocation density-based theory of strength properties, in σ 0 ɛ , and with the Griffith brittle fracture theory by way of pointing to the H–P slope value, k ɛ , being a microstructural stress intensity analogous to the fracture mechanics parameter, K.

Journal ArticleDOI
TL;DR: In this paper, the effect of the grain boundary character on the segregation process was examined and the link between RIS and atomistic modeling was explored through molecular dynamic simulations of the interaction of vacancies at different grain boundary structures through defect energy.

Journal ArticleDOI
TL;DR: In this article, the microstructures of the as-received powder and cold spray processed (CSP) ultrafine-grained (UFG) 6061 depositions were characterized by different electron microscopy techniques.
Abstract: Gas-atomized 6061 aluminum powder was used as feedstock for deposition using a high pressure cold-spraying process. The microstructures of the as-received powder and cold spray processed (CSP) ultrafine-grained (UFG) 6061 depositions were characterized by different electron microscopy techniques. It was found that there is segregation of solute elements at the particle grain boundaries, which is increased after cold spraying (CS). Various microstructural features were observed in both directions (parallel and perpendicular) of the CSP layer, including low-angle grain boundaries, clustered-small-cell walls, and dislocation tangle zones. The results also indicated that a combination of different recrystallization mechanisms (i.e., continuous and geometrical) may contribute to the formation of nano and UFG structures during CS.

Journal ArticleDOI
TL;DR: In this paper, it was found that the transfer of dislocations across grain boundaries is governed primarily by the minimization of the magnitude of the Burgers vector of the residual grain boundary dislocation.
Abstract: In situ straining in the transmission electron microscope and diffraction-contrast electron tomography have been applied to the investigation of dislocation/grain boundary and dislocation/twin boundary interactions in α-Ti. It was found that, similar to FCC materials, the transfer of dislocations across grain boundaries is governed primarily by the minimization of the magnitude of the Burgers vector of the residual grain boundary dislocation. That is, grain boundary strain energy density minimization determines the selection of the emitted slip system.

Journal ArticleDOI
TL;DR: In this article, the influence of variations in the processing parameters employed during spark plasma sintering of elemental nickel powders on its microstructure and mechanical properties (i.e., tensile yield strength and ductility) has been investigated in a systematic manner.
Abstract: The influence of variations in the processing parameters employed during spark plasma sintering (SPS) of elemental nickel powders on its microstructure and mechanical properties (i.e., tensile yield strength and ductility) has been investigated in a systematic manner. Bulk polycrystalline nickel samples were fabricated from high purity micron-size powder via SPS for three different pressures (i.e., 50, 65, and 80 MPa) and three different temperatures (i.e., 700, 850, and 1000 °C). These SPS processed nickel samples exhibited an average grain size ranging from 5 to 45 μm, tensile yield strength ranging from 130 to 278 MPa, and ductility ranging from 40% to 60%. The results clearly indicate that as compared to the sintering pressure, the sintering temperature plays a more dominant role in controlling the grain growth and mechanical properties of SPS processed pure nickel. A quantitative Hall–Petch relationship, relating the yield strength to the grain size of SPS processed nickel, has been developed.

Journal ArticleDOI
TL;DR: In this article, a study on grain boundary serration in Ni-based Alloy 690 was conducted through various heat treatments, and it was suggested that the grain boundary migration during serration is closely correlated with the precipitation of intergranular Cr carbides in this alloy.

Journal ArticleDOI
TL;DR: The grain boundary character distribution of polycrystalline silicon was determined from the stereological analysis of electron backscatter diffraction maps as discussed by the authors, and the distribution of grain boundary disorientations is non-random and has peaks at 36°, 39°, 45°, 51°, and 60°.
Abstract: The purpose of this paper is to describe the five-parameter grain boundary character distribution (GBCD) of polycrystalline silicon and compare it to distributions measured in metals and ceramics. The GBCD was determined from the stereological analysis of electron backscatter diffraction maps. The distribution of grain boundary disorientations is non-random and has peaks at 36°, 39°, 45°, 51°, and 60°. The axis-angle distribution reveals that most of the grain boundaries have misorientations around the [111], [110], and [100] axes. The most common grain boundary type (30 % number fraction) has a 60° misorientation around [111] and of these boundaries, the majority are twist boundaries. For other common boundaries, symmetric tilt configurations are preferred. The grain boundary character distribution of Si is distinct from those previously observed for metals and ceramics. The measured grain boundary populations are inversely correlated to calculated grain boundary energies available in the literature.