Showing papers on "Grain boundary strengthening published in 2006"
TL;DR: The mechanical properties of nanocrystalline materials are reviewed in this paper, with emphasis on their constitutive response and on the fundamental physical mechanisms, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, fatigue and creep responses.
3,828 citations
25 Nov 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the microstructure and the strength of lath martensite in Fe 0.2C and Fe 1.2Mn alloys were analyzed as a function of the prior austenite grain size.
Abstract: The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels.
496 citations
TL;DR: The measured hardness of nanocrystalline Cu with grain sizes as small as 10 nm still follows the Hall-Petch relation as discussed by the authors, suggesting grain boundary activities are enhanced but not yet dominant in the plastic deformation.
456 citations
TL;DR: In this article, two different mechanisms for plastic strain-induced grain refinement in pure Cu were identified, corresponding to different levels of strain rate, and the minimum size of refined grains via this process is about 100 nm.
446 citations
TL;DR: In this paper, the deformation kinetics are controlled by the activities of dislocations, and the dominant thermally activated mechanism is suggested to originate from three possible processes, all involving interactions of mobile dislocation with grain boundaries.
416 citations
TL;DR: It is found that the yttrium segregates to very localized regions along the grain boundary and alters the local bonding environment, thereby strengthening the boundary against mechanical creep.
Abstract: Impurity doping often alters or improves the properties of materials. In alumina, grain boundaries play a key role in deformation mechanisms, particularly in the phenomenon of grain boundary sliding during creep at high temperatures. We elucidated the atomic-scale structure in alumina grain boundaries and its relationship to the suppression of creep upon doping with yttrium by using atomic resolution microscopy and high-precision calculations. We find that the yttrium segregates to very localized regions along the grain boundary and alters the local bonding environment, thereby strengthening the boundary against mechanical creep.
385 citations
TL;DR: In this article, the leading and trailing dislocations are nucleated at different regions within a grain boundary or even different grain boundaries, and propagation is hindered by a pinning-depinning mechanism influenced by the relative orientation between the Burgers vector and the ledge geometry of the grain boundary.
371 citations
TL;DR: In this paper, isothermal and constant-grain-size sintering was carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature.
Abstract: Isothermal and constant-grain-size sintering have been carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature. The normalized densification rate follows Herring's scaling law with a universal geometric factor that depends only on density. The frozen grain structure, however, prevents pore relocation commonly assumed in the conventional sintering models, which fail to describe our data. Suppression of grain growth but not densification is consistent with a grain boundary network pinned by triple-point junctions, which have a higher activation energy for migration than grain boundaries. Long transients in sintering and grain growth have provided further evidence of relaxation and threshold processes at the grain boundary/triple point.
338 citations
TL;DR: In this article, microstructural analyses of peridotite mylonites from the oceanic lithosphere indicate that shear localization results from the combined effects of grain size reduction, grain boundary sliding and second phase pinning during deformation.
338 citations
TL;DR: In this paper, a model for the low temperature tensile response of copper polycrystals with grain sizes in the range of 2-50μm was proposed and the initial work hardening behavior was strongly grain size dependent and was considered to arise from a combination of kinematic and isotropic hardening due to dislocation-grain boundary interactions.
265 citations
TL;DR: In this paper, a dislocation density based constitutive model was proposed to incorporate the mechanical interaction between mobile dislocations and grain boundaries into a crystal plasticity finite element framework, and the model was applied to the case of 50% (frictionless) simple shear deformation of bicrystals with either a small, medium or large angle grain boundary parallel to the shear plane.
TL;DR: In this article, the authors investigated deformation-induced grain growth in electrodeposited nc Ni during high pressure torsion (HPT) and showed that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters.
Abstract: Deformation-induced grain growth has been reported in nanocrystalline (nc) materials under indentation and severe cyclic loading, but not under any other deformation mode. This raises an issue on critical conditions for grain growth in nc materials. This study investigates deformation-induced grain growth in electrodeposited nc Ni during high-pressure torsion (HPT). Our results indicate that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters. Also, texture evolution suggests that grain-boundary-mediated mechanisms played a significant role in accommodating HPT strain.
TL;DR: In this article, a series of molecular dynamics simulations of a circular grain embedded in an otherwise single-crystal matrix and monitor both the grain size and the misorientation of the two grains as a function of time were performed.
TL;DR: In this paper, the main contribution to the mechanical strength of the alloys is attributed to the grain boundary strengthening in Cu referring to the Hall-Petch relation, which is quantified.
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TL;DR: In this paper, the authors considered two alternative and complementary rate mechanisms of plasticity: grain boundary shear and dislocation plasticity, each contributing to the overall strain rate in proportion to the volume fraction of the material in which they operate.
Abstract: The well known break-down of the Hall–Petch effect of the rise of the plastic resistance with decreasing grain size in polycrystalline metals, when the grain size drops into the nanometre range resulting in a peak plastic resistance at a grain size of about 12–15 nm, is explained by considering two alternative and complementary rate mechanisms of plasticity, grain boundary shear and dislocation plasticity, each contributing to the overall strain rate in proportion to the volume fraction of the material in which they operate. In the model for a given applied strain rate it is shown that the plastic resistance reaches a maximum at a grain size of about 12.2 nm in Cu when the two mechanisms contribute to the overall strain rate equally, defining the so-called strongest size.
TL;DR: In this article, simulation results show grain boundary motion in nanocrystalline Ni and support the notion of stress-driven grain boundary migration in Ni materials that could be part of a basic deformation mechanism.
TL;DR: In this paper, the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si) was investigated using synchrotron-based analytical microprobe techniques, electron backscatter diffraction and defect etching.
Abstract: Synchrotron-based analytical microprobe techniques, electron backscatter diffraction, and defect etching are combined to determine the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si). Metal silicide precipitate decoration is observed to increase with decreasing atomic coincidence within the grain boundary plane (increasing Σ values). A few low-Σ boundaries contain anomalously high metal precipitate concentrations, concomitant with heavy dislocation decoration. These results provide direct experimental evidence that the degree of interaction between metals and structural defects in mc-Si can vary as a function of microstructure, with implications for mc-Si device performance and processing.
TL;DR: In this article, tensile tests were carried out on dozens of electrodeposited nanocrystalline Ni-W alloys with grain sizes of 20, 12, 8 and 5 nm.
25 Feb 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the authors present synthesis, characterization and evaluation of hardness and sliding wear resistance of electrodeposited nanocrystalline Ni-W alloys with 9.33 at.
Abstract: The present work involves synthesis, characterization and evaluation of hardness and sliding wear resistance of electrodeposited nanocrystalline Ni–W alloys. Crystallite size reduced with an increase in current density due to an increase in the W content. Ni–W alloy with 9.33 at.% W plated at 75 °C exhibited the maximum hardness of 638 HV. Alloys plated at 75 °C followed direct Hall–Petch relation. However, alloys plated at 85 °C exhibited an inverse Hall–Petch relation below a crystallite size of 15 nm. Wear resistance of alloys plated at 75 °C increased due to an increase in hardness with a reduction in the crystallite size up to 20 nm. It reduced due to brittle fracture of the coating below 20 nm. Wear resistance of alloys plated at 85 °C increased with a reduction in the crystallite size in the direct Hall–Petch region and decreased in the inverse Hall–Petch region. Ni–W coatings with 6–8 at.% W exhibited superior wear resistance.
TL;DR: In this article, the responses of nanocrystalline aluminum powder of different grain sizes, prepared by ball milling and consolidated into bulk specimens by hot pressing, were determined under quasi-static and dynamic compression.
25 Oct 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effect of grain refinement due to controlled extrusion and the fatigue behavior of the extruded materials in three magnesium alloys, AZ31B, AZ61A and AZ80, was investigated.
Abstract: This paper describes the grain refinement due to controlled extrusion and the fatigue behaviour of the extruded materials in three magnesium alloys, AZ31B, AZ61A and AZ80. First, in order to investigate the effect of working temperature, billets of AZ31B and AZ61A were extruded at an extrusion ratio of 67 under controlled conditions with three different working temperatures. It was found that grain refinement was attained in both alloys whose grain size decreased with decreasing working temperature. Rotary bending fatigue tests were performed using smooth specimens. In AZ31B, fatigue strength increased with decreasing grain size, but in AZ61A, it did not depend on grain size. Fatigue crack initiation and subsequent small crack growth were examined in AZ31B. Consequently, grain refinement improved both crack initiation resistance and small crack growth resistance, resulting in the increase in fatigue strength. Furthermore, it was indicated that fatigue strength was expressed properly by the Hall–Petch relationship in AZ31B, but not in AZ61A. Then, in order to investigate the effect of extrusion ratio, billets of AZ61A and AZ80 were extruded at three different extrusion ratios under controlled conditions. It was found that grain size decreased with increasing extrusion ratio in both alloys, but mechanical properties were not affected significantly by extrusion ratio, i.e. grain size, and the grain size dependence of fatigue strength was different between alloys. In AZ61A, fatigue strength was higher in the material extruded at lower extrusion ratio (coarser grain), but in AZ80, in the material extruded at higher extrusion ratio (finer grain). Fatigue crack initiation behaviour was consistent with the tendency of fatigue strength, where crack initiation delayed in the material extruded at lower extrusion ratio in AZ61A and in the material at higher extrusion ratio in AZ80. Furthermore, the observed grain size dependence of fatigue strength in both alloys was discussed on the basis of a texture formed by extrusion and the presence of inclusions from which cracks generated.
15 Mar 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the mechanical behavior of polycrystalline 99.999% Al sheets with a thickness of a single or a few crystallites is investigated experimentally under uniaxial tension.
Abstract: The mechanical behaviour under uniaxial tension of polycrystalline 99.999 at.% Al sheet with a thickness of a single or a few crystallites is investigated experimentally. The specimens are cold rolled to a thickness ( t ) of 100 up to 340 μm and the grain size ( d ) is varied by recrystallisation between 75 and 480 μm. All specimens have a similar texture and a regular grain structure. For t / d t / d , independently of t or d itself. For 1 t / d
TL;DR: In this paper, the authors show that grain growth results from rotation of nanograins and propagation of shear bands, together with the unstable migration of grain boundaries via process of coupled motion.
Abstract: The authors show using atomistic simulations that stress-driven grain growth can be obtained in the athermal limit during nanocrystalline aluminum indentation. They find that the grain growth results from rotation of nanograins and propagation of shear bands. Together, these mechanisms are shown to lead to the unstable migration of grain boundaries via process of coupled motion. An analytical model is used to explain this behavior based on the atomic-level shear stress acting on the interfaces during the shear band propagation. This study sheds light on the atomic mechanism at play during the abnormal grain coarsening observed at low temperature in nanocrystalline metals.
TL;DR: In this article, the percolation threshold occurs at a fraction of approximately 70 ± 5% coincident site lattice (CSL) boundaries, and the maximum random boundary cluster length drastically decreases and the frequency of resistant triple junctions increases with increasing frequency of the CSL boundaries near the percation threshold.
TL;DR: In this article, the pinning effect of small incoherent particles on grain growth in two-dimensional polycrystalline systems has been simulated using a phase field model, where the grain size distribution and the number of particles located at grain boundaries were determined as a function of time.
TL;DR: In this paper, an AISI 301 metastable stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes.
Abstract: Nano/submicron austenitic stainless steels have attracted increasing attention over the past few years due to fine structural control for tailoring engineering properties. At the nano/submicron grain scales, grain boundary strengthening can be significant, while ductility remains attractive. To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold-worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes. The effects of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined. Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combination of lath type and dislocation cell-type martensitic structure. For the annealed samples, the average austenite grain size increases from 0.28 µm at 600 °C to 5.85 µm at 900 °C. On the other hand, the amount of reverted austenite exhibits a maximum at 750 °C, where austenite grains with an average grain size of 1.7 µm compose approximately 95 pct of the microstructure. Annealing temperatures above 750 °C show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)23C6 carbides form within the grains and at the grain boundaries.
TL;DR: In this article, the β grain size of the alloy Ti-10V-2Fe-3Al was varied by heat treatment, and the tensile behavior and fracture toughness were evaluated as a function of β grain sizes at room temperature.
Abstract: The β grain size of the alloy Ti-10V-2Fe-3Al was varied by heat treatment, and the tensile behavior and fracture toughness were evaluated as a function of β grain size at room temperature. The alloy showed stress-induced martensitic transformation, and the triggering stress for this transformation varied with grain size. The 0.2 pct yield stress exhibited a Hall-Petch relationship with grain size. The ductile fracture toughness was found to increase with decrease in grain size, and it was also shown to follow a Hall-Petch kind of relationship. The grain boundary and the stress-induced martensitic contribution to fracture toughness were separated out.
TL;DR: In this paper, the authors used EBSD to measure the misorientation between original protolith "parent" grains and recrystallized daughter "daughter" grains, and found that the majority of misorientations between the parents and their neighbouring daughter grains are in the range 10-30° requiring that another process has operated to rotate grains to higher mis orientations with respect to their parents.
TL;DR: In this article, the effect of specimen size on the uniaxial deformation response of planar single crystals and polycrystals using discrete dislocation plasticity is investigated. But the authors focus on the case where the dislocations are modeled as line singularities in a linear elastic material.
Abstract: The effect of specimen size on the uniaxial deformation response of planar single crystals and polycrystals is investigated using discrete dislocation plasticity. The dislocations are all of edge character and modelled as line singularities in a linear elastic material. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated through a set of constitutive rules. Grain boundaries are modelled as impenetrable to dislocations. Two types of polycrystalline materials are considered: one that only has grains with a single orientation while the other has a checker-board arrangement of two types of grains which are rotated 90° with respect to each other. The single crystals display a strong size dependence with the flow strength increasing with decreasing specimen size. In sufficiently small single crystal specimens, the nucleation rate of the dislocations is approximately equal to the rate at which the dislocations exit the specimens so that below a critical specimen size the flow strength is set by the strength of the initially present Frank–Read sources. On the other hand, grain boundaries acting as barriers to plastic deformation in polycrystalline specimens of the same size lead to a more diffuse deformation pattern and to a nearly size-independent response.
TL;DR: Zhu et al. as discussed by the authors explored the effect of grain size and grain size distribution, along with the influence of material parameters, on the mechanical response of nanocrystalline face-centered cubic aggregates.