Showing papers on "Grain boundary strengthening published in 1979"
TL;DR: In this article, a curve separates the two domains in the σ, T plane and moves to higher temperatures and stresses for crystals of higher impurity content; for natural crystals, only rotation recrystallization can occur.
Abstract: Single crystals of pure and impure halite have been dynamically recrystallized during compression creep at temperatures between 250° and 790°C and stresses between 1.5 and 120 bars. Recrystallization was found to occur by two different mechanisms: at lower temperatures and stresses the new grains result from the rotation of subgrains without grain boundary migration (rotation recrystallization), and at higher temperatures and stresses the final texture results from the migration of the high-angle grain boundaries of the rotated subgrains. Migration recrystallization was shown to occur for critical stress and temperature conditions, allowing rapid grain boundary migration. A curve separates the two domains in the σ, T plane and moves to higher temperatures and stresses for crystals of higher impurity content; for natural crystals, only rotation recrystallization can occur. In each recrystallization regime the recrystallized grain size is uniquely related to the applied stress, thus yielding two different geopiezometers, which should not be applied indiscriminately to natural tectonites to determine lithospheric or mantle deviatoric stresses. The experimental results are interpreted by the Lucke et Stuwe theory for impurity-controlled grain boundary migration.
343 citations
TL;DR: In this article, the deformation behavior of a superplastic Ti-6A1-4V alloy at 927°C has been characterized by means of constant strain-rate tensile tests up to large plastic strain.
Abstract: The deformation behavior of a superplastic Ti-6A1-4V alloy at 927°C has been characterized by means of constant strain-rate tensile tests up to large plastic strain. Significant hardening has been recorded in the course of deformation. Microstructural studies on deformed samples indicate the occurrence of simultaneous strain-rate induced grain growth, which explains nearly all of the hardening. A small amount of hardening may also be expected from grain elongation or grain clustering effects. As a result of concurrent grain growth, the strain-rate sensitivity is found to decrease with strain, thus indicating that stress-strain rate behavior determined initially may not be applicable after large amounts of plastic strain. The stressJstrain-rate data obtained from step strain-rate test for a variety of grain sizes, together with the grain growth kinetics plots, provide a means for developing a constitutive description for this material at large strains.
250 citations
TL;DR: In this paper, the flow stress of polycrystalline Al, Cu, Cu-13 at% Al and Fe have been investigated as a function of the grain size and the specimen thickness.
242 citations
TL;DR: This paper showed that grain boundaries and interphase boundaries provide high diffusivity paths in solids, but little bulk composition change can occur at temperatures where lattice diffusion is frozen out unless the boundaries migrate.
190 citations
TL;DR: In this paper, the influence of grain boundary sliding (GBS) on the nature of deformation maps for aluminum and nickel is considered, and it is shown that large portions of such deformation map are dominated by GBS.
158 citations
TL;DR: Grain boundaries in polycrystalline aluminium have been examined using transmission electron microscopy (TEM) in order to determine the nature of grain boundary dislocations and the interactions that occur when crystal dislocation enter the boundary region as mentioned in this paper.
148 citations
TL;DR: In this article, a combined optical and electron microscopy study of microstructural variations across a quartz mylonite zone with increasing shear strain was performed, and the results indicated that estimates based on grain size are unreliable if phyllosilicates inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are unlikely to give meaningful estimates.
Abstract: The results are reported of a combined optical and electron microscopy study of microstructural variations across a quartz mylonite zone with increasing shear strain. The mylonite developed by recrystallization of the deformed quartz grains with increasing shear strain. It was found in a given specimen that the size of recrystallized grains and of sub-grains were always smaller in electron micrographs. The possible reasons for this are discussed. The size of both features decreased with increasing shear strain irrespective of the microscope used. However the density of unbound dislocations remained constant. A marked grain size reduction occurred in phyllosilicate rich layers. Variations in sub-grain size were observed within the relict old grains which remained at low shear strains. These are thought to reflect stress intensification adjacent to grain boundaries during deformation. The relict grains recrystallized at higher strains. Stresses were estimated from grain and sub-grain sizes and from the dislocation density. The results indicate that estimates based on grain size are unreliable if phyllosilicates inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are unlikely to give meaningful estimates. It is also concluded that the microstructures reflect stress gradients present during the formation of the mylonite, that is the initiation and propagation of the shear zone and that these were subsequently replaced by strain rate gradients.
137 citations
TL;DR: The grain boundaries in snow are generally unstable when the pore space is filled with liquid water (i.e., liquid-saturated snow) and the grains must be arranged in clusters to achieve local force equilibrium as discussed by the authors.
125 citations
TL;DR: In this article, the energies of formation for various defects in a two-dimensional electron lattice were calculated, including vacancies and interstitials, dislocation pairs, and grain boundaries.
Abstract: We have calculated the energies of formation for various defects in a two-dimensional electron lattice. Included are vacancies and interstitials, dislocation pairs, and grain boundaries. From the results, we have extracted the dislocation core energy, and an approximate formula for the grain boundary energy as a function of angle. We discuss some possible implications for melting, and for dynamic processes such as electron diffusion and dislocation migration.
117 citations
TL;DR: In this article, the microstructure of two-phase α β alloys such as Ti-6A1-4V was evaluated in terms of their fully characterized microstructures.
Abstract: The strong dependence of the superplastic behavior of metals and alloys on grain size has been demonstrated, and it is now well known that a fine grain size is normally a requirement for superplasticity. However, the microstructure of certain alloy systems such as Ti-6A1-4V cannot always be adequately characterized by a single parameter such as grain size. In two-phase α β alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, grain aspect ratio, grain size distribution and crystallographic texture may also influence superplasticity. For example, if “grain switching” is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as grain aspect ratio and range of grain sizes would be expected to have an effect on superplastic behavior. In this study, these microstructural features were determined for several different heats of Ti-6Al-4V, and the corresponding superplastic properties were evaluated in terms of their fully characterized microstructure. The flow stress as a function of strain rate, strain rate sensitivity exponent (m) as a function of strain rate and total elongation on properties were found to be strongly influenced by microstructural parameters such as grain aspect ratios, grain size and grain size distribution.
104 citations
TL;DR: In this article, a phenomenological viscoelastic model that includes the grain-size effect was developed to analyze the contribution of grain-boundary sliding strain to the total strain, and its dependence on stress, time, temperature and grain diameter.
Abstract: Delayed elastic phenomenon during high–temperature creep of polycrystalline materials is correlated with strain due to grain boundary sliding. This correlation has been used to develop a phenomenological viscoelastic model that includes the grain–size effect. With ice as a reference material, it is shown that the contribution of the grain-boundary sliding strain to the total strain, and its dependence on stress, time, temperature and grain diameter can be systematically analysed by the proposed model. The results appear to agree with the observed trends in other materials.
TL;DR: In this paper, the authors considered the rate of point defect emission and absorption at high angle grain boundaries in single phase materials by means of an (intrinsic) boundary dislocation model and found that the emission rate depends on the boundary structure and the applied stress and/or point defect supersaturation.
TL;DR: In this paper, the influence of grain size on the fatigue crack propagation behavior of age-hardened Al-Zn-Mg-Cu and Ti-Al alloys has been investigated in vacuum and 3.5% NaCl solution.
TL;DR: In this article, the misorientation dependence of grain boundary sliding and effects of grain boundaries structure and crystal deformation on sliding have been studied by experiments using 〈1010〉 tilt zinc bicrystals with different misorientations.
Abstract: The misorientation dependence of grain boundary sliding and effects of grain boundary structure and crystal deformation on sliding have been studied by experiments using 〈1010〉 tilt zinc bicrystals with different misorientations. The results have shown that grain boundary sliding strongly depends on the misorientation and is difficult in exact- or slightly off-coincidence boundaries. A cusp found near a tilt angle of 55° is explained by the existence of a 56·6°〈1010〉/〉/Σ9 near-coincidence boundary. The observations are interpreted on a dislocation mechanism for sliding which is based on the movement of crystal lattice dislocations along the grain boundary by a combination of climb and glide.
TL;DR: In this article, a plausible correlation is advanced between the grain boundary structure and its transgranular ionic resistivity and capacitance, and a physically relevant interpretation of equivalent circuit parameters determined from dispersive type measurements seems not justified.
Abstract: Grain boundary morphology and structure has been examined by means of transmission electron microscope lattice imaging, and a plausible correlation is advanced between the grain boundary structure and its transgranular ionic resistivity and capacitance. The geometrical aspects and resistance of the grain boundaries depend on whether or not dislocations with ab=1 spinel block can accommodate the misorientation. Such dislocations are shown to exist in a wide range of grain boundaries. The wide spread in grain boundary structure and the complexity of the microstructure, making parallel current path considerations necessary, can account qualitatively for the deviation from the ideal Maxwell dispersive behaviour. However, a physically relevant interpretation of equivalent circuit parameters determined from dispersive type measurements seems not justified. Several model circuits are used to illustrate the interpretational difficulties. The structure and effects of intergranular phases also have been studied and are discussed.
TL;DR: In this article, a model is developed for the formation of recrystallization nuclei at grain boundaries by processes involving subgrain coalescence, and the predicted microstructures predicted by the model are in accord with those observed experimentally.
Abstract: An experimental study has been made of recrystallization processes which have been initiated at grain boundaries in aluminium samples. The samples had an initial grain size of between 30 and 130 $\mu $m and were deformed 50% by rolling before annealing at 573 K. The microstructures developed in the region of the grain boundaries, after annealing times chosen just to initiate recrystallization, have been investigated with transmission electron microscopy. A major part of the investigation has been the identification of a number of different types of potential recrystallization nuclei at these sites. It has been established that the formation of most of these recrystallization nuclei involves modifications to the subgrain structures adjacent to grain boundaries. A model is developed for the formation of recrystallization nuclei at grain boundaries by processes involving subgrain coalescence. The microstructures predicted by the model are in accord with those observed experimentally.
TL;DR: In this paper, a model for creep based on the climb of dislocations at grain boundaries is presented, and it is shown that when a sliding interface or slip band intersects a grain boundary, a traction distribution is established on the boundary.
TL;DR: In this paper, a model of grain boundary migration based on the motion of boundary dislocations was proposed, which depends on both the boundary crystallography and the temperature, and may account for experimental measurements of grain migration activation volumes.
Abstract: Some recent discussions have centred on the mechanism by which lattice dislocations are absorbed by grain boundaries at high temperatures. Boundaries in two well-characterized coincidence site lattice systems have been studied using an electron microscope hot stage. The dislocation behaviour was shown to be consistent with a description based on dissociation and reactions involving grain boundary dislocations. A model of grain boundary migration based on the motion of grain boundary dislocations depends on both the boundary crystallography and the temperature, and may account for experimental measurements of grain boundary migration activation volumes.
TL;DR: The early stages of yielding in high purity copper (plastic strains of 1 −10 × 10 −4 ) have been studied by both quantitative transmission electron microscopy (TEM) and tensile data analysis as discussed by the authors.
TL;DR: In this article, the effects of grain size, dislocation cell size and intertwin spacing in shock-loaded nickel have been investigated, and deformation twins were observed to occur preferentially in those grains oriented with [001] parallel to the direction of the wave propagation.
TL;DR: In this paper, the effect of grain boundary carbides on high temperature alloy creep resistance was investigated for a nickel-base superalloy in which carbides were selectively dissolved or precipitated along grain boundaries.
TL;DR: In this paper, conditions under which an interface, possibly containing an adsorbed species, is capable of sustaining an atomistically sharp cleavage crack, rather than having any such crack blunt out via dislocation nucleation, are discussed.
Abstract: Conditions are discussed under which an interface, possibly containing an adsorbed species, is capable of sustaining an atomistically sharp cleavage crack, rather than having any such crack blunt out via dislocation nucleation. Two models for ductile versus brittle interface response are discussed. One of these is applied to model grain boundaries in face-centred-cubic materials, and is used to compare predictions with experimental observations on the embrittlement of Cu polycrystals by the addition of dilute concentrations of Bi. The results are in qualitative agreement.
TL;DR: In this paper, the effect of grain boundary sliding is considered and the sliding boundaries are treated as disk-shaped viscous heterogeneities of a critical volume fraction to result in contiguously connected boundary paths through the material.
TL;DR: In this paper, the authors investigated the effect of texture alterations and their effect on the kinetics of grain growth by computer simulation. And the authors found that the major texture component can strengthen or weaken in grain growth depending on its spread and initial microstructure.
TL;DR: In this article, the authors studied the role of boundary emission in work hardening of poly crystals, especially at large strains, and found that small strains grain boundaries and twin boundaries serve principally as sources of dislocations which are generated in response to the requirement of strain compatibility.
TL;DR: Ionic and electronic conductivity and compressive creep of hot-pressed polycrystalline acceptor-dominated Al2O3 were measured as a function of oxygen partial pressure and grain size varying from 3 to 200 μm as discussed by the authors.
Abstract: Ionic and electronic conductivity and compressive creep of hot-pressed polycrystalline acceptor-dominated Al2O3 were measured as a function of oxygen partial pressure and grain size varying from 3 to 200 μm. Hole conduction shows a slight preference for grainboundaries; ionic conduction is slightly hindered by grain boundaries, indicating that fast oxygen grain-boundary diffusion involving charged species does not occur. Conductivity and creep are accounted for on the basis of a model in which there is fast grain-boundary migration by a neutral oxygen species.
TL;DR: In this article, the structure and properties of grain boundaries are considered during superplastic deformation on the basis of recent concepts of the interaction between lattice dislocations and grain boundaries.
Abstract: The structure and properties of grain boundaries are considered during superplastic deformation on the basis of recent concepts of the interaction between lattice dislocations and grain boundaries. It is shown that the boundaries contain both, lattice and grain boundary dislocations. The presence of these extraneous defects leads to an increase of grain boundary energy; there is an increase of the vacancy concentration and enhancement of the diffusion coefficient in the boundaries with non-equilibrium structure. The results of numerical estimation of dislocation density and of the change in diffusion parameters agree well with the established experimental data.
TL;DR: In this paper, the authors measured the energy-misorientation relationship of grain boundaries in copper by recording the rotation rate of a single crystal sphere of copper sintered on to a flat single crystal of the same material.