Showing papers on "Grain boundary strengthening published in 2016"
TL;DR: In this paper, the authors gather the grain-size strengthening data from the Hall-Petch studies on pure metals and use this aggregated data to calculate best estimates of these metals' Hall-petch parameters.
Abstract: Refining a metal’s grain size can result in dramatic increases in strength, and the magnitude of this strengthening increment can be estimated using the Hall–Petch equation. Since the Hall–Petch equation was proposed, there have been many experimental studies supporting its applicability to pure metals, intermetallics and multi-phase alloys. In this article, we gather the grain-size strengthening data from the Hall–Petch studies on pure metals and use this aggregated data to calculate best estimates of these metals’ Hall–Petch parameters. We also use this aggregated data to re-evaluate the various models developed to physically support the Hall–Petch scaling.
523 citations
TL;DR: In this paper, the design, phase formation, microstructure, mechanical behavior and strengthening mechanisms of a single-phase Co 25 Ni 25 Fe 25 Al 7.5 Cu 17.5 (at.%) high-entropy alloy (HEA) were investigated.
331 citations
TL;DR: In this paper, the authors show that coupling of dislocations and precipitates within the ultrafine grains has a beneficial impact on the mechanical behavior and results in an extremely high strength, i.e., ultimate tensile strength ∼878 MPa, with uniform elongation of 4.1% strain at fracture.
171 citations
TL;DR: The HfNbTaTiZr refractory high-entropy alloy was investigated on the grain growth kinetics and tensile properties in this paper, where the activation energy was 389 kJ/mol and the growth exponent was 3.5.
162 citations
TL;DR: In this paper, the microstructure, texture, and deformation behavior of a carbon nanotube reinforced aluminum composite via electron backscatter diffraction, X-ray diffraction (XRD), scanning and transmission electron microscopy was characterized.
150 citations
TL;DR: In this article, the evolution of constitutional phases and microstructural features of the milled powders were investigated as a function of milling time, and the results showed that milled powder particles experienced significant cold-welding during the entire milling process, with a wide size distribution.
142 citations
TL;DR: In this article, the authors study the effect of transition metals on the grain boundary strengthening of tungsten GBs and uncover its dependence on the GB structures and the radius of the solute itself.
131 citations
TL;DR: In this paper, the authors investigated the connectivity of high energy random boundaries on the basis of the fractal analyses of grain boundary microstructures in SUS316L stainless steel, to prove the usefulness of a refined approach to grain boundary engineering (GBE) for more precise prediction and control of intergranular corrosion in polycrystalline materials.
127 citations
TL;DR: In situ electron microscopy reveals the dynamic processes of individual dislocation interaction with grain boundaries and indicates that simultaneous consideration of both the geometric effects and the stabilization effects is necessary to quantitatively understand the dislocation impediment processes at grain boundaries.
Abstract: In deformation processes, the presence of grain boundaries has a crucial influence on dislocation behavior; these boundaries drastically change the mechanical properties of polycrystalline materials. It has been considered that grain boundaries act as effective barriers for dislocation glide, but the origin of this barrier-like behavior has been a matter of conjecture for many years. We directly observe how the motion of individual dislocations is impeded at well-defined high-angle and low-angle grain boundaries in SrTiO3, via in situ nanoindentation experiments inside a transmission electron microscope. Our in situ observations show that both the high-angle and low-angle grain boundaries impede dislocation glide across them and that the impediment of dislocation glide does not simply originate from the geometric effects; it arises as a result of the local structural stabilization effects at grain boundary cores as well, especially for low-angle grain boundaries. The present findings indicate that simultaneous consideration of both the geometric effects and the stabilization effects is necessary to quantitatively understand the dislocation impediment processes at grain boundaries.
122 citations
TL;DR: An equiatomic FeCoCrNi high alloy (HEA) with both high tensile strength and ductility was produced by a powder metallurgy (P/M) method as discussed by the authors.
119 citations
TL;DR: In this article, the effect of texture and grain size on mechanical properties of AZ80 magnesium alloy at lower temperatures was investigated for 1, 2 and 4 passes at 523 K. The results showed that a significant grain refinement took place and the original extrusion fiber texture evolved into a new preferred crystal orientation, featuring a favorable alignment of the basal planes along shear planes.
TL;DR: In this paper, the grain boundaries are better described as a collection of dislocations rather than perfectly scattering interfaces, and the grain boundary dislocation strain model of Klemens is used to measure the conductivities of polycrystalline thermoelectric materials.
Abstract: Thermal conductivities of polycrystalline thermoelectric materials are satisfactorily calculated by replacing the commonly used Casimir model (freqeuncy-independent) with grain boundary dislocation strain model (frequency-dependent) of Klemens. It is demonstrated that the grain boundaries are better described as a collection of dislocations rather than perfectly scattering interfaces.
TL;DR: In this article, an analytical model is proposed to predict the grain size of hypoeutectic Al-Si alloys, in which the final grain size can be related to the diffusion-hindrance efficiency of NPs.
TL;DR: It is found that yttrium solute atoms preferentially segregate to specific atomic sites at the core of the grain boundary, forming a unique chemically-ordered structure across thegrain boundary.
Abstract: Grain boundary segregation is a critical issue in materials science because it determines the properties of individual grain boundaries and thus governs the macroscopic properties of materials. Recent progress in electron microscopy has greatly improved our understanding of grain boundary segregation phenomena down to atomistic dimensions, but solute segregation is still extremely challenging to experimentally identify at the atomic scale. Here, we report direct observations of atomic-scale yttrium solute segregation behaviours in an yttria-stabilized-zirconia grain boundary using atomic-resolution energy-dispersive X-ray spectroscopy analysis. We found that yttrium solute atoms preferentially segregate to specific atomic sites at the core of the grain boundary, forming a unique chemically-ordered structure across the grain boundary.
TL;DR: In this paper, industrial 2524 aluminium alloy plates with various grain sizes (0.8-298 µm) were prepared by cold rolling and heat treatment and the fatigue-crack growth rate was studied as a function of grain size through fatigue tests and microstructural observations.
TL;DR: In this article, the carbon solution energies and cohesive properties of three different grain boundaries in presence of carbon were determined and it was demonstrated that the most stable segregation sites possess the greatest coordination number and maximum Fe-C nearest neighbor distance.
01 Jan 2016-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the structural and twin boundary evolution of alloy 617B during dynamic recrystallization was investigated by optical microscope, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) technique.
Abstract: Substructure and twin boundary evolution of alloy 617B during dynamic recrystallization (DRX) was investigated by optical microscope, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) technique. Simulated compression tests were carried out at different true strains in the temperature range of 1120–1210 °C with a strain rate of 0.001 s −1 . The results show that discontinuous dynamic recrystallization (DDRX) featured by original grain boundary bulging is the dominant nucleation mechanism for alloy 617B. The progressive subgrain rotation, which is a characterization of continuous dynamic recrystallization (CDRX) can be detected at the early stage of hot deformation at lower temperature, which can just be considered as an assistant mechanism. The evolution of substructure and twin boundaries have a significant effect on DRX process of alloy 617B. Twinning formation can active the DRX process by accelerating original grain boundary bulging and separation of bulging grain boundaries. The formation of twin steps resulting from twin slipping provide additional DRX nucleation locations. The effort of twins gets weaker with the increase of temperature as the DRX grain growth process associated with grain boundary area reduction gradually becomes a preferential mechanism for energy minimization. Different from previous study, the fraction of twin boundaries decrease with the increase of temperature, which can be attributed to the twin boundary accelerated prior grain growth process. Such process also results in the serious bulging of grain boundaries into adjacent grains.
TL;DR: In this article, the grain boundary was found to contribute to plastic deformation through the process of dislocation absorption, transmission or nucleation at boundary plane, as well as grain boundary accommodation mechanisms such as GB sliding and GB migration.
TL;DR: In this paper, the authors incorporated grain boundary sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials.
Abstract: When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall–Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall–Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d1/2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.
TL;DR: In this paper, the grain size effect on the stress hysteresis of a nanocrystalline NiTi alloy during superelastic cycling was investigated by testing two types of samples with 39% and 75% cold-drawn area reduction.
TL;DR: In this article, the effect of grain boundary character on intergranular film formation was studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using Cu-Zr as a model system.
Abstract: Segregation-induced structural transitions in metallic grain boundaries are studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using Cu-Zr as a model system, with a specific emphasis on understanding the effect of grain boundary character. With increasing global composition, the six grain boundary types chosen for this paper first form ordered complexions, with the local segregation pattern depending on the grain boundary core structure, then transform into disordered complexions when the grain boundary composition reaches a critical value that is temperature dependent. The tendency for this transition to a disordered interfacial structure consistently depends on the relative solute excess, instead of the grain boundary energy or misorientation angle. Grain boundaries with high relative solute excess go through gradual disordering transitions, whereas those with low relative solute excess remain ordered until high global Zr concentrations but then abruptly transform into thick disordered films. The results presented here provide a clear picture of the effect of interface character on both dopant segregation patterns and disordered intergranular film formation, showing that all grain boundaries are not equal when discussing complexion transitions.
TL;DR: In this paper, the authors describe measurements of stress and the corresponding grain structure during electrodeposition of Ni and Cu films, and interpret the results in terms of a kinetic model for stress evolution that focuses on the developing boundary between adjacent grains while the film is deposited.
26 Apr 2016-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a set of internal-state-variable based unified constitutive equations were proposed to model the flow stress and microstructure evolution of a Ni-based superalloy.
Abstract: In this paper, the flow behavior and microstructure evolution of a Ni-based superalloy were investigated by hot compression tests with true strain between 0.223 and 0.916, strain rate between 0.001 and 1 s−1 and deformation temperature between 1223 and 1373 K. Based on the experimental results, a set of internal-state-variable based unified constitutive equations were proposed to model the flow stress and microstructure evolution of the studied superalloy. The evolution models of dislocation density, average grain size, and dynamic recrystallization fraction were developed and embedded into the constitutive law, which was derived from thermal activation theory and composed of athermal and thermal stresses. The proposed model was calibrated using experimental flow stress and dynamic recrystallization fraction. The predicted flow stress and dynamic recrystallization fraction under different deformation conditions agreed well with the experimental results. Additionally, flow stress under step-strain rate condition was also precisely predicted by the model. The contributions of long and short range barriers to the overall flow stress, variation of corresponding stress components, and microstructure evolution with strain were further analyzed using the unified model. The thermal stress only varied with deformation temperature and strain rate, while the grain boundary strengthening component and stress contribution of dislocation interactions varied with the evolution of grain size and dislocation density, respectively.
TL;DR: In this article, the authors employed a high throughput computational approach to determine the atomic structure, formation energy and excess volume of a large number of tilt grain boundaries in Cu and Ni.
TL;DR: In this article, a discrete dislocation plasticity (DDP) model was used to investigate the effect of crystallographic orientations, localised dislocation behaviour and grain combinations on the phenomenon of load shedding.
TL;DR: In this article, the Hall-Petch (H-P) relationship has been used to compare the strength of mild and severe plastic deformation (SPD)-processed materials.
Abstract: Metals processed by severe plastic deformation (SPD) techniques, such as equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), generally have submicrometer grain sizes. Consequently, they exhibit high strength as expected on the basis of the Hall–Petch (H–P) relationship. Examples of this behavior are discussed using experimental data for Ti, Al, and Ni. These materials typically have grain sizes greater than ~50 nm where softening is not expected. An increase in strength is usually accompanied by a decrease in ductility. However, both high strength and high ductility may be achieved simultaneously by imposing high strain to obtain ultrafine-grain sizes and high fractions of high-angle grain boundaries. This facilitates grain boundary sliding, and an example is presented for a cast Al-7 pct Si alloy processed by HPT. In some materials, SPD may result in a weakening even with a very fine grain size, and this is due to microstructural changes during processing. Examples are presented for an Al-7034 alloy processed by ECAP and a Zn-22 pct Al alloy processed by HPT. In some SPD-processed materials, it is possible that grain boundary segregation and other features are present leading to higher strengths than predicted by the H–P relationship.
TL;DR: In this article, the authors used spherical nanoindentation and orientation imaging microscopy (OIM) to quantify the local changes in slip resistances in the grain boundary regions of deformed, polycrystalline aluminum.
31 Oct 2016-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a thin film model for deconvolution of the intrinsic hardness and flow stress of the metallic phase was proposed, and a Hall-Petch strengthening relationship with a slope (ky) of 0.98 MPa m1/2 was found.
Abstract: WC–Co cemented carbides are geometrically complex composites constituted for two interpenetrating networks of the constitutive ceramic and metal phases. Accordingly, assessment of microstructural effects on the local mechanical properties of each phase is a challenging task, especially for the metallic binder. In this work, it is attempted by combining massive nanoindentation, statistical analysis, and implementation of a thin film model for deconvolution of the intrinsic hardness and flow stress of the metallic phase. Plotting of yield stress values as a function of the binder mean free path results in a Hall-Petch strengthening relationship with a slope (ky) of 0.98 MPa m1/2. This value points out the effectiveness of WC–Co phase boundaries as strong obstacles to slip propagation; and thus, for toughening of the brittle phase (WC) by means of crack-bridging ductile (Co) reinforcement.
TL;DR: In this article, the effects of rare earth elements (La, Gd) addition on the microstructure and the mechanical properties of as-extruded Mg-Al-Zn magnesium alloy under conditions of a medium temperature (230°C) and a slow ram speed (0.1mm/s) have been throughly investigated.
TL;DR: In this paper, the authors employed an advanced room temperature method that involves milling in ethanol followed by low temperature atmospheric drying, which is a much less expensive process than traditional high temperature/high vacuum drying.