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The formation mechanism of a recrystallisation internal friction (IF) peak superimposed on a grain boundary (GB) IF peak for deformed copper was studied. The P1 peak, associated with the recrystallisation process, was observed at ∼ 599 K (2 K/min) during the first heating process. Subsequently, the P2 peak, located at ∼ 510 K (0.2 Hz), corresponded to GB relaxation during the cooling process. The P1 peak showed minor relaxation at higher frequencies owing to the influence of GB relaxation. To accurately determine the temperature of the P1 peak in this scenario, two methods were used: fitting the measured IF spectra and performing IF measurements at lower frequencies and/or with faster heating rates to minimise the impact of GB relaxation. This work investigated the formation mechanism of the recrystallisation peak and supplemented the method for determining the recrystallisation temperature through IF measurements when the GB peak occurs at a nearby temperature. [ABSTRACT FROM AUTHOR]

The Geometrically Necessary Dislocation (GND) density was estimated from Electron Backscatter Diffraction (EBSD) data for an AZ31/Mg-0.6Gd (wt.%) hybrid material fabricated by high-pressure torsion (HPT) at room temperature through an equivalent strain range of ϵeq = 0.3–144 using Kernel Average Misorientation (KAM) and the Nye tensor approaches. The results show that generally the GND densities are significant at the beginning of the deformation (ϵeq = 0.3) and decrease in both alloys when ϵeq increases. The Mg-0.6Gd alloy exhibits a lower GND density due to rapid dynamic recrystallization. These results were compared to the GND densities measured in AZ31 and Mg-0.6Gd mono-materials processed separately by HPT under the same experimental conditions. In these mono-materials the GND densities increase with increasing equivalent strain up to 7 and then decrease with further straining. The Mg-0.6Gd and AZ31 regions of the hybrid material exhibit higher GND densities than the mono-materials particularly at low strain where the disc thickness and the bonding of the AZ31/Mg-0.6Gd interfaces cause more deformation heterogeneity in the hybrid material. It is shown that the GND density evolution as a function of ϵeq has the same tendency for the KAM and the Nye approaches but the average values are significantly higher with the Nye approach. An analysis suggests that the Nye approach overestimates the GND density of the Mg-based alloys. [ABSTRACT FROM AUTHOR]

Oxide nanoparticles are quintessential for ensuring the extraordinary properties of oxide dispersion strengthened (ODS) steels. In this study, the crystallographic structure of oxide nanoparticles, and their interface with the ferritic steel matrix in an Al-alloyed ODS steel, i.e. PM2000, were systematically investigated by high-resolution transmission electron microscopy. The majority of oxide nanoparticles were identified to be orthorhombic YAlO3. During hot consolidation and extrusion, they develop a coherent interface and a near cuboid-on-cube orientation relationship with the ferrite matrix in the material. After annealing at 1200 °C for 1 h, however, the orientation relationship between the oxide nanoparticles and the matrix becomes arbitrary, and their interface mostly incoherent. Annealing at 1300 °C leads to considerable coarsening of oxide nanoparticles, and a new orientation relationship of pseudo-cube-on-cube between oxide nanoparticles and ferrite matrix develops. The reason for the developing interfaces and orientation relationships between oxide nanoparticles and ferrite matrix under different conditions is discussed. [ABSTRACT FROM AUTHOR]

Voigt and Reuss models do not make any assumption regarding composite microstructure, but they are only able to estimate bounds rather than values of composite properties. To enable the models to predict elastic properties of particulate composites, we isotropized Voigt and Reuss models based on the equivalence of normal and shear strain energy respectively in a representative volume element, and derived a set of simple and uncoupled equations for the prediction of composite Young's modulus and shear modulus. Composite bulk modulus and Poisson's ratio can then be determined either by the elasticity relationships or by generalization of the isotropized Voigt-Reuss equations. Validation with reported experimental data show that, for composites with phase properties of either small or large contrast, isotropized Voigt-Reuss model has similar accuracy as the widely used micromechanical models, for the prediction of Young's modulus and shear modulus. [ABSTRACT FROM AUTHOR]

The dislocation pattern of cold deformed FCC metals follows an orientation-dependent rule. However, only final orientation after deformation has been considered in related studies, i.e. without considering lattice rotation effect, although metals have undergone heavy lattice rotation during plastic deformation. In this work, the lattice rotation effect on the dislocation structures of Cu is investigated by combining electron backscatter diffraction, focused ion beam lift-out and scanning transmission electron microscopy techniques. The dislocation patterns of [110]-[110], [111]-[111] and [110]-[111] type grains deformed in tension under room temperature are compared. The notation [h1k1l1]-[h2k2l2] represents the lattice rotation history of a grain, meaning that the grain rotates from [h1k1l1] corner to [h2k2l2] corner during tension. It is found that for a given strain range, the dislocation cell structure is observed in [111]-[111] grain, while the dislocation cell block structure is dominant in [110]-[111] grain. This demonstrates that for Cu grains with a final orientation near [111] corner, dislocation pattern type is influenced by the lattice rotation feature. Same with [110]-[111] type grain, cell block structure is the main feature found in [110]-[110] grain, indicating that for grain with initial orientation near [110] corner, lattice rotation has a marginal effect on the dislocation structure. The relation between the dislocation structure, lattice rotation, slip system and strain level of Cu is discussed. [ABSTRACT FROM AUTHOR]

The correct description of the mechanical behaviour of metallic materials after deformation to large strains requires the quantitative determination of substructure characteristics on different length scales. Important tools for this purpose are transmission electron microscopy (TEM), electron backscattering diffraction (EBSD) and X-ray diffraction (XRD), which lead to quantities such as the mean total dislocation density, the density of excess dislocations stored in dislocation walls, the distributions and the averages of local lattice rotations and the size (chord length) of the lattice regions (cell blocks, fragments) of the mesoscale substructure. This present paper illustrates the application of EBSD and XRD for the analysis of the deformation substructure occurring in polycrystalline Al during uniaxial compression up to strains ε ≈ 2. [ABSTRACT FROM AUTHOR]

The stored energy and activation energy for recrystallization were investigated for a Cu-Ni-Si alloy after high-pressure torsion processing for N = ½, 1, 5 and 10 turns at room temperature. The contributions of geometrically necessary dislocations (GNDs), statistically stored dislocations (SSDs) and vacancies to the stored energy were calculated through the Vickers microhardness measurements, kernel average misorientation (KAM) measurements and an analysis by differential scanning calorimetry (DSC). The results show that the total stored energy decreases rapidly after equivalent strain of εeq ∼ 9 and then saturates through εeq ∼ 86 at ∼70 J/mol. Concurrently, the local stored energy in GNDs and SSDs was found to depend strongly on the radial distance from the centre of the disc and increase with increasing equivalent strain at εeq ∼ 16 and saturate with further straining. Accordingly, the results indicate that the GNDs and vacancies are responsible for the high stored energy in the initial stage of deformation at equivalent strain range of εeq = 8.6–16 and thereafter their contribution decreases slightly due to the occurrence of dynamic recrystallization and the formation of fine grains. At the same time, the contribution of the SSDs is similar to that of the GNDs only in high strain deformation as at εeq = 49.3 to accommodate the deformation process. An activation energy for recrystallization was estimated in the range of ∼ 89.7–98.7 kJ/mol, thereby suggesting poor thermal stability. [ABSTRACT FROM AUTHOR]

Single-crystalline cast aluminium microwires with a diameter near 15 are characterised by Laue microdiffraction. A microwire in the as-cast condition exhibits a misorientation below over a length of 500. The measured density of geometrically necessary dislocations is low, m−2, though local maxima up to one order of magnitude higher are found. After tensile deformation to failure, the dislocation density is significantly increased in microwires that have mostly deformed in single slip (m−2), and yet higher when deformation has occurred by multiple slip (m−2). In deformed single slip oriented microwires, the streaking directions of Laue spots show that dislocations are stored (though not exclusively) on the primary slip system. Results are consistent with a deformation mechanism governed by rotating, likely single-arm, sources. [ABSTRACT FROM AUTHOR]

Friction stir welding (FSW), a highly efficient solid-state joining technique, has been termed as “green” technology due to its energy efficiency and environment friendliness. It is an enabling technology for joining metallic materials, in particular lightweight high-strength aluminum and magnesium alloys which were classified as unweldable by traditional fusion welding. It is thus considered to be the most significant development in the area of material joining over the past two decades. Friction stir processing (FSP) was later developed based on the basic principles of FSW. FSP has been proven to be an effective and versatile metal-working technique for modifying and fabricating metallic materials. FSW/FSP of aluminum alloys has prompted considerable scientific and technological interest since it has a potential for revolutionizing the manufacturing process in the aerospace, defense, marine, automotive, and railway industries. To promote widespread applications of FSW/FSP technology and ensure the structural integrity, safety and durability of the FSW/FSP components, it is essential to optimize the process parameters, and to evaluate thoroughly the microstructural changes and mechanical properties of the welded/processed samples. This review article is thus aimed at summarizing recent advances in the microstructural evolution and mechanical properties of FSW/FSP aluminum alloys. Particular attention is paid to recrystallization mechanism, grain boundary characteristics, phase transformation, texture evolution, characteristic microstructures, and the effect of these factors on the hardness, tensile and fatigue properties as well as superplastic behavior of FSW/FSP aluminum alloys. [ABSTRACT FROM AUTHOR]

Polycrystalline bulk materials are ubiquitous in everyday life, including biological, geological, and engineered structural and functional materials. Their fundamental units are individual grains, which are characterized by their microstructure; i.e., the arrangement of lattice defects. The microstructure usually influences the materials properties critically. [ABSTRACT FROM AUTHOR]

High-energy X-ray diffraction (HEXD) experiments, which include real-time measurements of micromechanical material response using in-situ loading and the non-destructive creation of three-dimensional maps of polycrystalline microstructure, are very rapidly replacing traditional macroscopic mechanical tests and forensic metallurgical characterization methods for structural materials. The center for Integrated Simulation and X-ray Interrogation Tools and Training for Micromechanics at the Cornell High Energy Synchrotron Source (InSitμ@CHESS) was created to facilitate the use of HEXD experiments on structural materials; more notably, metallic alloys such as steel, titanium, aluminum, and nickel. The Office of Naval Research (ONR) has financially supported InSitμ, specifically enabling enhanced industrial user support. This article describes the experimental considerations associated with using HEXD on structural materials and, through a set of examples, illustrates how InSitμ addresses these considerations. [ABSTRACT FROM AUTHOR]

Ti-based materials are one of the most important materials used in biomedical engineering. Recently, commercially pure (CP) Ti has attracted significant attention of research community due to its full biocompatibility with human body, and there is an ongoing demand to improve its mechanical properties without sacrificing other beneficial properties. A possible approach to achieving this aim is to use the method of severe plastic deformation (SPD) to obtain an ultra-fine grained (UFG) microstructure. Significant grain refinement by SPD processing leads to an improvement in mechanical and functional properties. This paper gives an overview of a range of the properties that can be achieved in the UFG CP Ti including microstructural features, mechanical properties, corrosion performance and biocompatibility. Attention is paid to the main SPD processing techniques developed for the fabrication of UFG CP Ti. Current and potential applications of UFG CP Ti in biomedical engineering are also considered. [ABSTRACT FROM PUBLISHER]

This work revisits the classical subject of recrystallization of cold-rolled copper. Two characterization techniques are combined: three-dimensional X-ray diffraction using synchrotron X-rays, which is used to measure the growth kinetics of individual grainsin situ, and electron backscatter diffraction, which is used for statistical analysis of the microstructural evolution. As the most striking result, the strong cube texture after recrystallization is found to be related to a few super large cube grains, which were named supercube grains. These few supercube grains become large due to higher growth rates. However, most other cube grains do not grow preferentially. Because of the few supercube grains, the grain size distribution after recrystallization is broad. Reasons for the higher growth rates of supercube grains are discussed, and are related to the local deformed microstructure. [ABSTRACT FROM PUBLISHER]

For the specific slip geometry of two sets of coplanar systems (a total of four systems) in fcc metals, the range of dislocation networks in boundaries aligned with one of the two active slip planes is predicted from the Frank equation for boundaries free of long-range elastic stresses. Detailed comparison with experimental data for eight dislocation boundaries in cold-rolled aluminium grains of the 45° ND rotated Cube orientation is conducted. It is concluded that the boundaries are Low-Energy Dislocation Structures, which are in good agreement with the Frank equation while also lowering the energy by dislocation reactions. Cross slip plays a role in the boundary formation process. [ABSTRACT FROM PUBLISHER]

Novel forming processes for light metal alloys utilize recovery and recrystallization to extend their total elongation and enhance formability. To attain optimum efficiency in such processes, it is necessary to understand and quantify the kinetics of recovery and recrystallization in work-hardened metal alloys. An electron backscatter diffraction based method, using local average orientation spread, is shown to identify the end of recovery as well as the onset of recrystallization. Local average orientation spread results from dislocation flux and storage during plastic deformation and hence, captures the evolution of static recovery process. The method has been demonstrated using pre-strained Al–Mg alloys. The recovery kinetics is shown to be consistent with results from dislocation density based recovery models. In addition, a direct observation of the coexistence of static recrystallization and recovery illustrates competing processes for energy minimization. [ABSTRACT FROM AUTHOR]

Electrodeposited copper samples composed of columnar grains subdivided by alternating twin/matrix (T/M) lamellae have been cold rolled to 30–85% reduction in thickness. The thickness of the T/M lamellae varies over a wide range from a few nanometres to about 1??m. The deformation microstructure has been characterized systematically. In thin T/M lamellae (below 50–100?nm) the deformation behaviours differ significantly from that of thick T/M lamellae, as the dislocation activity is concentrated at the T/M boundaries illustrated by the observations of stacking faults and Shockley partial dislocations. In thick T/M lamellae (100–1000?nm), the deformation microstructure is related to the grain orientation as also observed previously in deformed single crystals and polycrystals with a grain size at the micrometre scale. The experiment therefore suggests that the universal structural characteristics of deformation microstructure can be extended one order of magnitude from about 5??m to the sub-micrometre scale (about 0.5??m). [ABSTRACT FROM AUTHOR]

Previous studies have revealed that dislocation structures in metals with medium-to-high stacking fault energy, depend on the grain orientation and therefore on the slip systems. In the present work, the dislocations in eight slip-plane-aligned geometrically necessary boundaries (GNBs) in three grains of near 45° ND rotated cube orientation in lightly rolled pure aluminium are characterized in great detail using transmission electron microscopy. Dislocations with all six Burgers vectors of the ½⟨1 1 0⟩ type expected for fcc crystals were observed but dislocations from the four slip systems expected active dominate. The dislocations predicted inactive are primarily attributed to dislocation reactions in the boundary. Two main types of dislocation networks in the boundaries were identified: (1) a hexagonal network of the three dislocations in the slip plane with which the boundary was aligned; two of these come from the active slip systems, the third is attributed to dislocation reactions (2) a network of three dislocations from both of the active slip planes; two of these react to form Lomer locks. The results indicate a systematic boundary formation process for the GNBs. Redundant dislocations are not observed in significant densities. [ABSTRACT FROM AUTHOR]

The annealing response of AA5182 Al–Mg alloy deformed to an effective prestrain of = 0.15 via plane strain and equibiaxial strain paths is compared. The comparison is done at two temperatures namely, 623 and 673 K. The recovery, recrystallization and grain growth behaviour of this alloy is studied by electron backscatter diffraction and dislocation density estimation using X-ray line broadening analysis. It is found that recrystallization is slower in equibiaxial deformation condition compared to that in plane strain deformation condition during annealing. Significant recrystallization is observed after annealing for 60 s at 673 K and for 480 s at 623 K following plane strain deformation. Furthermore, significant recrystallization is associated with lower grain growth at 673 K (∼55 μm) as opposed to that at 623 K (∼75 μm). The results are explained on the basis of differences in both the strain paths. [ABSTRACT FROM AUTHOR]

High-energy synchrotron X-rays are well suited to study engineering (structural) materials due to their small wavelength, adjustable energy and beam size, high flux, and ability to penetrate bulk polycrystalline samples up to centimeters in thickness. Recent advances in the use of high-speed, high-resolution detectors are making it possible to characterize microstructures at both the single grain and ensemble levels and to characterize the micromechanical responses of polycrystalline aggregates in three dimensions. These capabilities open new avenues of “in-situ” research that augments traditional forensic evidence with real-time data on functioning, evolving systems. X-ray scattering data are extremely rich, but taking the best advantage requires a continued refinement of experimental methods and analysis and a closer coupling of material models to detected intensities. [ABSTRACT FROM AUTHOR]

The observed mechanical behaviour of micron-sized samples raises fundamental questions about the influence of size on the underlying dislocation plasticity. In situ µLaue diffraction on a single crystalline copper bending beam was performed to study the feasibility of bending tests and their contribution to our understanding of size-dependent dislocation plasticity. Theoretical considerations lead to a minimum sample size where in situ µLaue experiments are useable. A critical size is evidenced below which, depending on Young's modulus and maximum stress, the elastic and plastic contributions to the lattice curvature cannot be separated. The experiment shows the increase in geometrically necessary dislocations during plastic deformation followed by a decrease during unloading. This can be explained by the formation and dissolution of a dislocation pile-up at the neutral axis of the bending cantilever. The dissolution of the dislocation pile-up is caused by the back stress of the pile-up and a direct observation of the Bauschinger effect, which is consistent with the non-purely elastic mechanical behaviour when unloading the sample. [ABSTRACT FROM AUTHOR]

Theory predicts that deformation that occurs by emission of strain bursts falls into two regimes, one in which the burst emission remains a stochastic process as strain increases, and another in which the emission of bursts settles into a deterministic process for large strains. The stochastic regime occurs when the burst emission rate decreases with strain, and in this case, large statistical scatter persists in the stress-strain response on repeated measurements. The deterministic regime occurs when the emission rate increases with strain, and the scatter in the corresponding stress-stress behaviour diminishes at large strains. The strength at the same strain in the stochastic regime is also higher than in the deterministic regime. Factors that affect the burst emission rate include the number of sources as well as the stress dependence of the efficiency of the sources. [ABSTRACT FROM AUTHOR]

Mapping of residual stresses at the mesoscale is increasingly practical thanks to technological developments in electron backscatter diffraction (EBSD) and X-ray microdiffraction using high brilliance synchrotron sources. An analysis is presented of a Cu single crystal deformed in compression to about 10% macroscopic strain. Local orientation measurements were made on sectioned and polished specimens using EBSD and X-ray microdiffraction. In broad strokes, the results are similar to each other with orientations being observed that are on the order of 5° misoriented from that of the original crystallite. At the fine scale it is apparent that the X-ray technique can distinguish features in the structure that are much finer in detail than those observed using EBSD even though the spatial resolution of EBSD is superior to that of X-ray diffraction by approximately two orders of magnitude. The results are explained by the sensitivity of the EBSD technique to the specimen surface condition. Dislocation dynamics simulations show that there is a relaxation of the dislocation structure near the free surface of the specimen that extends approximately 650 Å into the specimen. The high spatial resolution of the EBSD technique is detrimental in this respect as the information volume extends only 200 Å or so into the specimen. The X-rays probe a volume on the order of 2 µm in diameter, thus measuring the structure that is relatively unaffected by the near-surface relaxation. [ABSTRACT FROM AUTHOR]

The deformation around a 500-nm deep Berkovich indent in a large grained Fe sample has been studied using high resolution electron back scatter diffraction (EBSD). EBSD patterns were obtained in a two-dimensional map around the indent on the free surface. A cross-correlation-based analysis of small shifts in many sub-regions of the EBSD patterns was used to determine the variation of elastic strain and lattice rotations across the map at a sensitivity of ∼±10-4. Elastic strains were smaller than lattice rotations, with radial strains found to be compressive and hoop strains tensile as expected. Several analyses based on Nye's dislocation tensor were used to estimate the distribution of geometrically necessary dislocations (GNDs) around the indent. The results obtained using different assumed dislocation geometries, optimisation routines and different contributions from the measured lattice rotation and strain fields are compared. Our favoured approach is to seek a combination of GND types which support the six measurable (of a possible nine) gradients of the lattice rotations after correction for the 10 measurable elastic strain gradients, and minimise the total GND line energy using an L1 optimisation method. A lower bound estimate for the noise on the GND density determination is ∼±1012 m-2 for a 200-nm step size, and near the indent densities as high as 1015 m-2 were measured. For comparison, a Hough-based analysis of the EBSD patterns has a much higher noise level of ∼±1014m-2 for the GND density. [ABSTRACT FROM AUTHOR]

To clarify the effect of grain orientation on the evolution of dislocation structures in metals of medium-to-high stacking fault energy, detailed TEM characterization of structures was carried out for more than 350 individual grains in Al and Cu deformed in tension or by cold rolling up to moderate strain levels (εvM ≤ 0.8). Efforts were made to obtain a precise description of the three-dimensional arrangement of the dislocation structures and to determine the crystallographic plane of extended dislocation boundaries (geometrically necessary boundaries). A universal pattern of structural evolution characterized by a formation of three types of structure was found in both metals, irrespective of material parameters (stacking fault energy, grain size and impurity) and deformation conditions (deformation mode, strain and strain rate). The key parameter controlling the formation of the different structural types was found to be grain orientation with respect to the deformation axis (axes) and a clear relationship between the structural type and the grain orientation was established. A review of single crystal data shows a similar relationship. The grain orientation dependence of the structural type and similar structural types observed in tension and rolling suggest a common cause. Part II explores this by relating the structural types to the active slip systems. [ABSTRACT FROM AUTHOR]

The relationship between deformation and dislocation properties has been studied for pure polycrystalline nickel and austenitic stainless steel AISI 316L in stage III. Special care was taken to study statistically the effects of the grain size and grain orientation on dislocation densities and distribution. It is shown that the nature of dislocation cells depends on grain size and crystallographic orientation. The dimensional parameters, which depend on grain size, i.e. the inter-boundary spacing (λ) and boundary thickness (e), define three domains of crystallographic orientation and depend on the grain size. Scaling hypotheses reveal two physical mechanisms which, at this level of plastic strain, are correlated to a specific value of the noise, associated with distribution functions. Similarities between structural parameters and dislocation densities in each phase (walls and inter-walls spacing) are identified and discussed in terms of kinetic equations describing dislocation density evolution and fluctuations of certain physical parameters. This similarity provides physical signification of the scaling distribution obtained on λ and e in terms of a stochastic approach to dislocation distribution. The origin of Hall-Petch behaviour observed at large strain is interpreted in terms of an interaction between inter- and intra-granular long-range internal stresses, which depends on grain size. We conclude that, at high strain, the Hall-Petch phenomenological relationship is a consequence of plastic strain history and strain gradient in grains. From this last point, a length scale arises naturally, which depends on stacking fault energy. [ABSTRACT FROM AUTHOR]

The dynamic internal variables which control plastic flow can only be assessed by dynamic materials testing at any given instance. The testing method championned by our studies has been precision strain rate sensitivity (PSRS) whereby the change in flow stress due to a set change in strain rate is taken to be an operational measure of the activation volume and its product with the flow stress gives rise to the operational activation work. Also, from the work hardening slope, a modelled parameter proportional to the mean slip distance (λ) is simultaneously determined. The deviation from the linear Cottrell–Stokes relation as determined with the Haasen plot indicates the evolution of secondary defects other than monopole dislocations. Hence PSRS can assess the theoretical predictions of the activation distance ( d ) and work as a function of temperature, resulting in quantitative values that are in accord with dislocation theory at temperatures below that where point defects become mobile. A method to calibrate λ using Stage II slope θ II shows that λ/ℓ, where ℓ is the mean forest dislocation spacing, is inversely proportional to θ, the work hardening coefficient. This analysis has led to a new plot of θ II /θ versus b 2 λ/ν where b is the Burgers vector and its slope is directly proportional to d . An example using an alumina-dispersed high conductivity copper shows that geometrically necessary punched out loops are continuously generated. The role of point defect mobility is dramatically illustrated by load drops in [001] aluminium crystals with the formation of slip clusters. [ABSTRACT FROM AUTHOR]

The goal of this study is to develop a unified picture of the role of geometrically necessary dislocations (GNDs) in the evolution of long-range internal stresses and lattice plane misorientations in the heterogeneous dislocation pattern of deformed crystals. For this purpose, X-ray diffraction techniques are considered as the pertinent experimental tools. On the modelling side, the composite models of single/multiple slip serve to interpret the experimentally measured long-range internal stresses quantitatively in terms of densities of GNDs. However, in order to be able to deduce from experiment the evolution of those GNDs that are responsible for the observed lattice plane misorientations, the composite model must be refined. Quite generally, the same GND array can give rise to both long-range internal stresses and lattice plane misorientations. On this basis, available experimental data obtained on cyclically and tensile-deformed copper (and copper-manganese) single crystals were analyzed quantitatively. The stresses acting locally in the ''hard'' dislocation cell walls and in the ''soft'' cell interiors and the magnitude of the internal stresses are found to increase approximately linearly with the applied stress. In spite of the fact that the density of the GNDs always amounts to only a few per cent of the total dislocation density, they are responsible for the long-range internal stresses and/or for the misorientations. An analysis of the evolution of the lattice plane misorientations shows that the kink walls and the dislocation sheets/grids in stage II are geometrically necessary boundaries (GNBs), whereas the dislocation cell walls formed by multiple slip are incidental dislocation boundaries (IDBs). [ABSTRACT FROM AUTHOR]

In face-centred cubic (fcc) crystals, the stress-strain curves of 〈111〉 crystals are remarkably insensitive to dynamic recovery compared to other orientations. This effect is attested by several early experimental studies on copper, aluminium and silver crystals. The inhibition of dynamic recovery is attributed to a reduction in the rate of annihilation of screw dislocations. The underlying mechanisms are investigated in terms of the reversal of the direction of the resolved stress in the cross-slip plane, which occurs when going from 〈001〉 to 〈111〉 orientations. Two distinct processes governing dynamic recovery in 〈111〉 crystals, acute cross-slip and backward annihilation, are examined and discussed. The second process arises because screw dislocation annihilation occurs along the direction opposite to that of the resolved applied stress in the cross-slip plane. It is concluded that, except possibly in aluminium, backward annihilation is always responsible for the inhibition of dynamic recovery in 〈111〉 crystals. [ABSTRACT FROM AUTHOR]

The theory by Stelmashenko et al., De Guzman et al. and Nix and Gao of the size effect in microindentation is outlined and some implicit assumptions are discussed. By an algebraic rearrangement, the slope of the straight-line plot of the square of hardness against the reciprocal of the diameter or depth of an indentation acquires a value which is predictable and in order of magnitude in agreement with experiment. The corresponding scale length, of order 1 micron in copper, is thus a real physical property of the material, although it is not visible in the undeformed material. In the recent experiments of Motz et al. it is observed directly. The two-phase 'hard metal' WC-Co has a microscopic intrinsic scale length, accordingly, the plot is no longer linear. [ABSTRACT FROM AUTHOR]

Dislocation patterns have been characterized by transmission electron microscopy and Kikuchi line analysis in pure, polycrystalline aluminium deformed in tension at room temperature in the strain range 0.05-0.34. The angle strain relationship of the dislocation boundaries, their scaling behaviour and the occurrence of similitude show that two dislocation patterns coexist in all grains, albeit, with very different characteristics, dependent on the grain orientation. An analysis of the hardening behaviour of the grains in the polycrystal and a comparison with single crystal behaviour show a similar strong correlation, pointing to the slip pattern as a dominating factor both behind the microstructural evolution and the hardening. The division of the stereographic triangle representing all possible crystallographic orientations at the tensile axis based on microstructural characterization and hardening behaviour, correlates with a division based on slip pattern characteristics. [ABSTRACT FROM AUTHOR]

A mathematical framework is presented for microtexture derivation and analysis. A six-dimensional space is introduced, defined as the product of direct space ( x , y , z ) and orientation space, parametrized by the Rodrigues vector ( r 1 , r 2 , r 3 ). The framework is adapted to diffraction experiments, where orientation information from many grains or deformation microstructures is recorded in parallel, with the prime aim of allowing fast measurements and thus the ability to study microstructure dynamics. Assuming the use of a monochromatic beam the geometry of the projection surfaces is deduced. The derivation of the microtexture is formulated as an inverse problem similar to reconstruction problems in absorption or emission tomography. The applicability of the two major types of reconstruction algorithm, namely transform methods and algebraic methods, is discussed. An explicit solution in terms of the iterative algebraic reconstruction technique algorithm is given. The choice of basis function is discussed. Furthermore, the framework is applied to a number of important simplifying cases. These include grain-by-grain reconstruction, classical pole figure inversion and the generation of three-dimensional maps of non-deformed grains. [ABSTRACT FROM AUTHOR]

A combined atomic force microscopy (AFM)-nanoindentation system was developed which has a silicon tip for imaging and a diamond indenter for indentation testing. The system makes it possible to conduct indentation tests and to obtain high-resolution topographic images on the same position of a specimen without difficult operations. The quality of AFM images was improved by scanning with a silicon tip instead of a diamond tip. The developed system was applied to materials with fine microstructures. First, the mechanical properties and microstructures of tempered martensitic steel were investigated. The improved image of fine carbide particles can confirm the correlation between the local hardness and the carbide density. Second, indentations created on tungsten, nickel and glass were observed and analysed for calibration of the indenter shape, which is important for any nanoindentation tests. It was found that nickel is suitable for the calibration because of its low and uniform elastic recovery. The area function of the indenter, which is the relationship between the depth and the projected area, was directly estimated using AFM images of indentations on nickel. The area function was approximated with the simple form of A(h) = 28.0(h + 11.8)[sup 2]. Accordingly, the AFM-nanoindentation system has proven to be valuable for studies on materials with fine microstructures and for the calibration process in nanoindentation tests. [ABSTRACT FROM AUTHOR]

It is shown that certain varieties of dipolar debris usually interpreted in terms of a jog-trailing process result from the impact of a dislocation on individual loops or loop strings. A mechanism for spontaneous patterning in single slip is proposed that rests on the capture of dislocations of both signs by a string where they annihilate by pairs adding interlaced loop strings on to the initial embryo and building up a three-dimensional wall gradually. The process should apply to any crystal where the free-flight distance in the cross-slip plane is moderate. A manoeuvre for the conservative elimination of a loop string by a single dislocation is introduced. [ABSTRACT FROM AUTHOR]

Plastic instability associated with formation of narrow flow channels results from dislocation pinning-unpinning by defect clusters. We investigate the dynamics of dislocation interaction with radiation-induced defect clusters, and specifically with, firstly, sessile self-interstitial atom clusters in dislocation decorations and, secondly, stacking-fault tetrahedra (SFTs) in the matrix. It is shown that the critical stress to free trapped dislocations from pinning atmospheres can be a factor of two smaller than values obtained on the basis of rigid dislocation interactions. The unpinning mechanism is a consequence of the growth of morphological instabilities on the dislocation line. Dislocation sources are activated in spatial regions of low SFT density, where their destruction by glide dislocations leads to subsequent growth of localized plasticity in dislocation channels. We show that removal of SFTs is associated with simultaneous dislocation glide and climb. Jogs of atomic dimensions are formed when a fraction of SFT vacancies are absorbed by pipe diffusion. The width of a flow channel is explained in terms of two length scales: the size of an individual SFT, and the dislocation source-to-boundary distance (d of the order of micrometres). While dislocation segments climb by a few atomic planes with each SFT destruction event, d determines the total number of such events. Numerically computed channel widths (about 70-150 nm), and the magnitude of radiation hardening in copper are consistent with experimental observations. [ABSTRACT FROM AUTHOR]