Search

In this work, it is investigated how grain boundaries influence the local strain determination by the microstructural feature tracking (MFT) algorithm. In this method, tetrahedra are used as the strain calculation unit. We apply the MFT processing procedure on data obtained by a crystal plasticity finite element modeling (CPFEM) simulation to explore the uncertainties in the calculated strains caused by grain boundaries. Effects of tetrahedron types and radius ratios are discussed.

Nucleation of recrystallization at large second phase particles during annealing is essential in many industrially relevant aluminum alloys. To adequately quantify the spatial distribution of such particles and their impact on nucleation in bulk materials, 3D investigations are needed. This study presents a multimodal characterization method to investigate particle stimulated nucleation in a heavily deformed AA5182 alloy. To map the spatial distribution of large second phase particles, conventional laboratory absorption contrast X-ray tomography is used, while the recrystallization nuclei are revealed by Laue micro-diffraction using synchrotron X-rays. The registration of data obtained by the two different non-destructive high-resolution experimental techniques is described. Finally, some preliminary results are presented.

The penetration behavior of liquid gallium in aluminum is characterized using laboratory X-ray attenuation tomography and related to grain boundary properties obtained from the 3D grain map reconstructed by laboratory diffraction contrast tomography (LabDCT). The data is unique because more than 100 grain boundaries are analyzed. It is suggested that it is the grain boundary energy which determines if a boundary is wetted or not: low energy boundaries are much more resistant to liquid gallium than higher energy ones. The potentials of using laboratory diffraction contract tomography for statistical studies of grain boundaries are thereby demonstrated.

The recovery dynamics of 498 individual subgrains within the non-recrystallized matrix of a 50% recrystallized aluminum sample is studied using high resolution X-ray diffraction at a synchrotron source. The subgrains’ change in size, orientation, axial strain and internal disorder are monitored as functions of temperature. The subgrains are surprisingly active, and the individual subgrains exhibit markedly different growth kinetics. Contradictory to common knowledge we find that the individual subgrains’ sizes change significantly even though the average size remain constant. Furthermore we observe no correlation between subgrain size and growth rate. An analysis of the volume changes of the individual subgrains indicates a broad spectrum of critical temperatures above which the subgrains grow or shrink. Possible mechanisms are discussed and related to relaxation of local stresses.

A flexible and standalone forward simulation model has been developed to compute the diffraction projections for laboratory diffraction contrast tomography (LabDCT). The outputs are expected to be of great value for all present users of LabDCT as well as interested new users., Laboratory X-ray diffraction contrast tomography (LabDCT) has recently been developed as a powerful technique for non-destructive mapping of grain microstructures in bulk materials. As the grain reconstruction relies on segmentation of diffraction spots, it is essential to understand the physics of the diffraction process and resolve all the spot features in detail. To this aim, a flexible and standalone forward simulation model has been developed to compute the diffraction projections from polycrystalline samples with any crystal structure. The accuracy of the forward simulation model is demonstrated by good agreements in grain orientations, boundary positions and shapes between a virtual input structure and that reconstructed based on the forward simulated diffraction projections of the input structure. Further experimental verification is made by comparisons of diffraction spots between simulations and experiments for a partially recrystallized Al sample, where a satisfactory agreement is found for the spot positions, sizes and intensities. Finally, applications of this model to analyze specific spot features are presented.

Three-dimensional orientation mapping of samples of aluminum prepared by spark plasma sintering (SPS) with average grain sizes of 5 μm and 1 μm has been carried out using high-brightness synchrotron radiation, from which the geometrically necessary dislocation (GND) density has been determined. The low average measured GND density values confirm that the SPS process can be used to produce samples containing grains with dislocation density similar to that of fully recrystallized coarse-grained samples. Values of GND density are also compared to those obtained from electron back-scatter diffraction studies on the same material, highlighting the significantly higher angular resolution of the synchrotron data. For the 5 μm grain-size sample the measured GND density can account for a large fraction of the previously observed positive Hall-Petch deviation of this material. For the 1 μm grain-size sample, however, the GND-based strengthening contribution is much smaller than the reported positive Hall-Petch deviation, such that the additional strength may be reliably associated with dislocation source-limited strengthening.

In this paper, we discuss the impact that non-destructive 3D (x,y,z) and 4D (x,y,z,time) X-ray experimental methods have had on the understanding of recrystallization of deformed metals. Well-planned experiments on nucleation and grain boundary migration are reviewed with focus on information gained, which could not have been obtained by traditional 2D or destructive 3D methods. It has also been found that the 3D/4D X-ray measurements sometimes happen to reveal phenomena, which were not expected and therefore not planned to investigate. Examples of this are also reviewed here. Finally, the implications of the new findings for modelling of recrystallization are discussed, key experiments for the future are suggested and the needs for technique advancements are summarized, all seen from the recrystallization perspective.

Switches and crossings (S&Cs) in rail networks suffer from damage and deformation due to severe loading of their complex geometric shapes. This article presents a comparative study of the cyclic deformation characteristics and fatigue behavior of two typical crossing materials, namely head-hardened pearlitic steel and Hadfield manganese steel, as well as the associated microstructural changes. Both uniaxial and biaxial loadings (proportional and non-proportional) are studied. The pearlitic steel endured more cycles to failure for a given strain amplitude compared to the manganese steel. The cyclic response of manganese steel shows significant hardening whereas softening was observed for the pearlitic steel except under biaxial non-proportional loading. The microstructures developed in the two types of steels after deformation are characterized by optical (OM) and transmission electron microscopy (TEM). TEM micrographs reveal that the deformed microstructure of the pearlitic steel consists of threading dislocations and dislocation tangles in the ferrite lamellae. The microstructures in the manganese steel after the different loadings are composed of dislocation cells with dislocation tangles inside, intersected by straight stacking fault lamellae. The observed microstructures are related to the results of the mechanical test, and the suitability of the steels for use in S&Cs is discussed.

The migration of recrystallization boundaries into spatially varying deformation energy fields are simulated based on a two-dimensional phase-field model. The energy fields are chosen as idealized versions of deformation microstructures subdivided by two sets of intersecting geometrically necessary dislocation boundaries (GNBs), and effects of the sharpness of the GNBs are investigated. The simulations show that the shape of the recrystallizing grain as well as the recrystallization kinetics are considerably affected by the wall characteristics of the GNBs. Recrystallization occurs faster in the deformed matrix with "sharper" walls. The simulation results highlight the importance of the deformation microstructure characteristics on the recrystallization kinetics, and as the deformation microstructures depend on the initial grain orientations, the results also indicate tight relations between initial, deformation and recrystallization textures. ispartof: IOP Conference Series : Materials Science and Engineering vol:1121 status: published

The volumetric growth rate of individual grains has been found to be directly proportional to the individual grain volume V as dV/dt = β(Vo -V). This simple result is explained through the DeHoff relationship dV/dt = −kMs between growth rate and the integral mean curvature of individual grains, M s , combined with the experimentally observed relationship between M s and grain volume, M s = α(Vo -V). These relationships have now been observed consistently in both 3D grain growth simulations and experiments. This paper describes the relationships among these kinetic and geometric grain characteristics that provide this simple description of 3D grain growth.

The annealing kinetics of two aluminum alloys AA5xxx is investigated. The volume fraction of recrystallized material, Vv, is determined for a series of partly recrystallized samples both by hardness measurements and by electron back scattering diffraction (EBSD). Because of extended recovery overlapping with recrystallization, in particular in AA5182, it is not straightforward to estimate Vv from the hardness measurements. Two methods are suggested: one relying solely on the hardness data, and one in which EBSD data are used to estimate the hardness at the onset of recrystallization. Effects of the choice of method and of parameters on the resulting annealing kinetics are analysed, and the potential and limitations of the two methods are discussed.

The microstructure and residual elastic strain at graphite nodules (GNs) in ductile cast iron produced using either a fast or slow cooling rate have been characterized using synchrotron 3D X-ray Laue microdiffraction. The results show that thermal stress is introduced during cooling and that part of this stress is relaxed by plastic deformation of the polycrystalline ferrite matrix. It is found that the plastic deformation is accommodated by the formation of dislocations and dislocation boundaries, which are organized in a cell structure. The dislocation density quantified based on the microstructure is most pronounced at the GN/matrix interface around small GNs in the fast cooled sample. Residual elastic strain is also present, which is mainly compressive with a maximum of 6.0–9.9 × 10−4 near the GNs. Gradients of plastic deformation and elastic strain field around the GNs are observed. The results document for the first time that both the elastic strain field and the plastic strain field averaged over the grains around the GNs is approximately scaling with GN size and not affected by the cooling rate. The experimental data are compared with simulations by a finite element method, and agreement and disagreement are discussed in detail.

Lab-based diffraction contrast tomography (LabDCT) has recently enabled nondestructive 3D characterization of the crystallographic orientations and grain morphologies in bulk materials. Despite the wide accessibility and availability of lab-based X-rays, the current spatial resolution of LabDCT is only about 20 μm and has to be improved to make this technique a more versatile tool. Conventional LabDCT takes advantage of the Laue focusing effect, i.e. the diffracted beam from a sample illuminated by an incoming polychromatic beam, is focused when the source and the detector are symmetrically positioned. In this study we have investigated the possibility of increasing the spatial resolution of LabDCT by placing the detector at a distance beyond that required for Laue focusing. We first developed a forward projection simulation tool to elucidate the effects of placing the detector at a larger distance. Next, we performed LabDCT measurements on a partially recrystallized pure aluminum sample at different sample-to-detector distances. The results show that the diffraction spots can be magnified by a factor of about three compared to the current LabDCT setup. The benefits and limitations resulting from increasing the sample-to-detector distance are discussed.

Pure tungsten is considered as armor material for the most critical parts of fusion reactors (i.e. the divertor and the first wall), among other reasons due to its high melting point (3422 °C) and recrystallization temperature. The thermal stability of a pure tungsten plate warm-rolled to a high plastic strain by 90% thickness reduction was investigated by isothermal annealing for up to 190 h in the temperature range between 1100 °C and 1250 °C. Vickers hardness testing allowed tracking the changes in mechanical properties caused by recovery and recrystallization. The hardness evolution could be rationalized in terms of a logarithmic recovery kinetics and a Johnson–Mehl–Avrami–Kolmogorov recrystallization kinetics accounting for an incubation time of recrystallization. The observed time spans for recrystallization and the corresponding recrystallization activation energy for this highly deformed plate suggest that large plastic deformations (e.g. applied during shaping) are only suitable to produce tungsten components to be used at relatively low temperatures (up to 900 °C for a 2 years lifespan). Higher operation temperatures will lead to fast degradation of the microstructure during operation.

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 grains in 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.

Three Cu 100− x Ni x alloys, with x = 23, 33 and 45 at.%Ni, have been evaluated for use as substrates for coated conductors on the basis of measurements of their microstructure, crystallographic texture and hardness. It is found that high-temperature annealing after heavy rolling generates strong cube textures in each investigated alloy. For all of these alloys an increase in the annealing temperature from 800 to 1000 °C strengthens the cube texture and reduces the fraction of high angle grain boundaries. In the Cu–23 at.%Ni and Cu–33 at.%Ni alloys annealed at 1000 °C for 1 h, the fraction of the cube texture approaches 100% and the fraction of high angle boundaries is less than 4%. These two alloys are however very soft in the annealed condition. The cube texture in the Cu–45 at.%Ni substrate is slightly weaker than in the two other alloys, but this substrate is considerably harder, which makes it better suited for large scale production of superconducting tapes.

Rail wheel contact at switches and crossings (S&Cs) induces impact stresses along with rolling contact stresses, resulting in plastic deformation and eventually crack formation. Damaged and deformed wing rails of a manganese steel crossing are studied and the microstructure, hardness and 3D crack network within the steel are characterized. It is found that the surface of the rail receives the maximum deformation resulting in a hardened top layer. The deformation is manifested by a high density of twins and dislocation boundaries in the microstructure. A complex crack network is revealed in high resolution by X-ray tomography.

The effects of aging on the microstructural development and mechanical properties of a 3D-printed maraging steel have been investigated. The microstructures within printed, solution treated and aged samples are characterized by X-ray tomography and optical microscopy, as well as by scanning and transmission electron microscopy. The mechanical properties are estimated based on hardness measurements. The distribution of voids is found to be inhomogeneous, and even though the samples are 99.9% dense in the sample interior, the remaining voids affect significantly the hardness in the aged condition. In addition to precipitation, the aging treatment leads to formation of about 12% of reverted austenite. Together this leads to a strengthening of the material. Solution treatment results in homogenization of the microstructures such that after aging the solution treated samples are found to have higher strength than the samples that are just aged after printing. These results are discussed and compared to the aging behaviour of conventionally manufactured samples.

The onset of plastic yielding has been studied in samples of aluminum with near-micrometer average grain size using in situ tensile loading combined with synchrotron radiation. A white-beam Laue 3D micro-diffraction technique was used allowing the collection of diffraction signal from a volume with a resolution of 1 μm in all three dimensions. The orientation data obtained from the micro-diffraction are used to estimate the local variation in geometrically necessary dislocation density, and correction factors are suggested for voxels where neighbors are missing in either one or two dimensions. The results demonstrate a considerable heterogeneity in deformation in the early stages of plastic deformation.

Recent synchrotron measurements have shown the presence of local residual stress within nearly defect-free grains in partially recrystallized samples. A detailed quantification of such local stress is very important for understanding local boundary migration during recrystallization and for tailoring the microstructure of materials. In this study, we utilize high resolution electron backscatter diffraction (HR-EBSD, Wilkinson’s method) to investigate the local residual stress in recrystallized grains in a sample of partially recrystallized iron. The effects of reference selection on the strain distribution are also discussed.

The microstructure and crystallographic texture have been investigated in a Cu–45 at.%Ni alloy after heavy rolling and subsequent annealing at different temperatures. Cold-rolling to a von Mises strain of 5.7 produced a sample with an average boundary spacing along the normal direction of ~70 nm and a large fraction of high-angle boundaries (HABs), ~70 %. Annealing of this sample for 1 h at temperatures ≤450 °C causes structural coarsening, during which the fraction of HABs decreases. Annealing at higher temperatures results in pronounced discontinuous recrystallization accompanied by twinning. Large frequencies of twin boundaries contribute to high HAB fractions measured in the as-recrystallized condition. Cube-oriented grains demonstrate a size advantage compared to grains of other orientations, thus creating a strong cube texture in the recrystallized material. Further annealing of the recrystallized microstructure promotes grain growth, which leads to a significant strengthening of the cube texture and to a dramatic loss of HABs. After 1 h of annealing at 1000 °C the fraction of the cube texture reaches 99 % and the HAB fraction is 12 %.

Columnar grained Ni is used as a model material allowing simultaneous non-surface investigations of the evolution of crystallographic orientations and deformation microstructures within individual grains as a function of rolling strain up to e = 0.7. Electron channelling contrast and electron backscattered diffraction are used to visualise microstructures and crystallographic orientations. It is found that both the microstructural and the textural development depend strongly on the initial grain orientation. A grain size effect is observed on the deformation-induced orientation scatter within the grains. Large grains have microstructure and orientation scatters similar to those observed in single crystals of similar orientation. The observations are interpreted based on a slip system analysis, considering the relative effects of the initial grain orientation and the interaction between neighbouring grains as well.

The growth of individual grains during recrystallization in 96% cold-rolled pure nickel has been followed using electron backscatter pattern maps of the same surface area taken after each of several annealing steps. It was found that the growth is quite complex, with boundaries moving, stopping and moving again. The growth kinetics differ from grain to grain and, on average, cube-oriented grains grow the fastest. The growth of the grains has also been analyzed as a function of boundary misorientation. This analysis shows that there is no significant difference in misorientation distribution between boundaries that move and those that do not. This is contrary to the usual assumption that the boundary mobility and the migration rate depend on the misorientation across a boundary. This observation and the reasons for the faster growth of cube-oriented grains are discussed.

A directionally solidified, high-purity nickel sample with a strong 〈0 0 1〉 fiber texture was cold rolled to 40% reduction. Electron backscattered diffraction (EBSD) was used to identify triple junctions (grain edges) before annealing the sample at 430 °C, after which further EBSD was performed to identify 17 recrystallization nuclei at the initially characterized triple junctions on the rolling plane. In addition, a longitudinal section was cut on which a further seven recrystallization nuclei were found and characterized. This characterization of all nuclei revealed that the majority of nuclei could be broadly associated with a 40° 〈1 1 1〉 misorientation to the matrix in which they formed, while a much lower percentage were found to have orientations as those of the local deformation substructure. However, a substantial fraction (around one-third) could not be associated with either of these types. Mechanisms for the formation of nuclei with new orientations are discussed, as is evidence of reorientation of material ahead of the recrystallization front.

The development of texture and microstructure during recrystallization of pure nickel rolled to very high strains has been investigated. Low-temperature annealing was chosen in order to separate the effects of recrystallization and grain growth. Particular attention was focused on the nucleation and growth of the cube orientation texture component, and on the effect of twinning in producing texture components of other orientations. The growth rates for recrystallizing grains of cube and non-cube orientations were analyzed from the EBSP data using an extended Cahn–Hagel method. Within the estimated experimental errors the growth rates of both orientation classes remain constant during recrystallization, with the cube orientation grains having a higher growth rate by a factor of ∼3. Some minor recrystallization texture components result from twinning, with the role of twinning being most important in the early stages of recrystallization.

Three-dimensional (3-D) microstructural characterization has proven to be indispensable for the thorough understanding of the often highly complex microstructures studied in materials science. However, most 3-D characterization techniques of opaque materials such as metals and ceramics are destructive and therefore prohibit 3-D studies of the dynamic microstructural evolution processes. In this paper we describe two complimentary techniques capable of nondestructive 3-D characterization and provide examples of the application of these techniques to investigate microstructural evolution processes in metallic systems.

Three-dimensional X-ray diffraction (3DXRD) microscopy offers the possibility of time-resolved mapping of structures down to the micrometer scale 1 , 2 , 3 , 4 , 5 , 6 , i.e. four-dimensional studies. In this review, the principles of the 3DXRD microscope are described and various examples of its applications are presented.

Microstructure and texture are analyzed through the thickness of two aluminum plates cold-rolled 40% with different roll gap geometries. It is found that both texture and microstructure are strongly affected by the rolling geometry. After rolling with intermediate-size draughts a rolling-type texture is developed throughout the plate thickness. In this case, grains are subdivided by extended planar dislocation boundaries preferentially aligned at an angle of 40 ± 15° to the rolling direction. In the plate rolled with small draughts, shear texture components appear in the intermediate layers. In these layers, extended planar dislocation boundaries are frequently found to be inclined closely to the rolling direction. The subsurface and central layers of this plate exhibit microstructures similar to those in the plate rolled with intermediate draughts. It is suggested that the development of different textures and microstructures at different depths is related to the activation of different slip systems due to through-thickness strain gradients.

We observed the in situ growth of a grain during recrystallization in the bulk of a deformed sample. We used the three-dimensional x-ray diffraction microscope located at the European Synchrotron Radiation Facility in Grenoble, France. The results showed a very heterogeneous growth pattern, contradicting the classical assumption of smooth and spherical growth of new grains during recrystallization. This type of in situ bulk measurement opens up the possibility of obtaining experimental data on scientific topics that before could only be analyzed theoretically on the basis of the statistical characterization of microstructures. For recrystallization, the in situ method includes direct measurements of nucleation and boundary migration through a deformed matrix.

The recrystallization kinetics of a 90% cold-rolled commercial aluminium alloy AA1050 annealed at 270 °C has been investigated by use of 3-dimensional X-ray diffraction (3DXRD) microscopy. For the first time growth curves of a large number of individual bulk grains have been measured in situ during recrystallization providing unique information on the nucleation and growth behaviour of the individual grains. From observations of 244 individual growth curves, it is found that each grain has its own growth kinetics. The orientation dependencies of the recrystallization kinetics are investigated by grouping the measured growth curves into cube, rolling and other orientation classes. Based on analysis of the growth curves, distributions of nucleation time, grain size and growth rate has been derived, and are used for a discussion of the recrystallization kinetics of aluminium AA1050.

Pure aluminium has been cold rolled and partially annealed to yield recrystallization nuclei. The orientations present around a number of triple junctions were characterised using the Electron Back Scattered Diffraction (EBSD) technique before and after the annealing, so that a direct comparison could be made between the orientations in the local deformed structure and the nuclei. It was found that triple junctions were particularly good nucleation sites within the microstructure. Approximately half of the nuclei found had orientations that fell within the scatter from one of the original grains and the remainder were rotated about a pole close to relative to a deformed grain.

Static, isothermal recrystallization at a temperature of 400 °C was studied by means of quantitative microscopy in a well-characterized, commercial purity aluminum-alloy AA1050 that had undergone plane strain deformation at 400 °C at a strain rate of 2.5 s −1 to an equivalent strain of 2. The microstructural properties, V v , the volume fraction recrystallized, S v , the interfacial area density separating recrystallizing grains from deformed volumes and λ >, the mean recrystallized grain free length, were all measured stereologically as a function of time and the reaction kinetics, microstructural path, grain boundary migration rates and nucleation characteristics of the recrystallization were quantified experimentally. The results are compared to a recently published study of recrystallization in the identical pre-deformation starting material but after room temperature deformation by rolling to a comparable strain. Recrystallization kinetics differences between the two materials include: the hot deformed material had a higher, by at least 120 °C, recrystallization temperature; had many fewer recrystallization nuclei leading to a factor of about three larger as-recrystallized grain size; lacked a Cahn-Hagel growth rate transient like the cold deformed exhibited; and required a slightly different impingement model for the microstructural path analysis. In both cases particle stimulated nucleation (PSN) was thought to be operative but it seemed to be much more potent after cold deformation.

Recrystallization in copper deformed by dynamic plastic deformation was investigated using electron backscatter diffraction. The recrystallized grains show a broad size distribution. The kinetics of grains of different sizes is observed to be different: In the beginning of recrystallization, the area fraction of small recrystallized grains increases rapidly. At later stages of recrystallization, the area fraction of small recrystallized grains is stable, while the area fractions of medium and large recrystallized grains increase. Correlation between the broad grain size distribution (and its evolution) and the heterogeneous deformed microstructure is discussed. © Published under licence by IOP Publishing Ltd. Conference code: 114064; Cited By :1; Export Date: 18 November 2021; Conference Paper; References: Leffers, T., Ray, R.K., (2009) Prog. Mater. Sci., 54, p. 351; Bhattacharyya, A., Rittel, D., Ravichandra, G., (2005) Scr. Mater., 52, p. 657; Luo, Z.P., Mishin, O.V., Zhang, Y.B., Zhang, H.W., Lu, K., (2012) Scr. Mater., 66, p. 355; Hong, C.S., Tao, N.R., Lu, K., Huang, X., (2009) Scr. Mater., 61, p. 289; Morri, K., Meching, H., Nakayama, Y., (1985) Acta Mater, 33, p. 379; Lin, F.X., Zhang, Y.B., Tao, N.R., Pantleon, W., Juul Jensen, D., (2014) Acta Mater, 72, p. 252; Wu, G.L., Juul Jensen, D., (2008) Mater. Charact., 59, p. 794; Pande, C.S., (2000) Proc. 21st Risø Inter. Symposium on Mater. Sci., p. 491; Lin, F.X., Zhang, Y.B., Pantleon, W., Juul Jensen, D., Submitted; Li, Y.S., Tao, N.R., Lu, K., (2008) Acta Mater, 56, p. 230; Li, Y.S., Zhang, Y., Tao, N.R., Lu, K., (2008) Scr. Mater., 59, p. 475; Yan, F., Zhang, H.W., (2012) J Mater. Sci. Technol., 28, p. 289

Switches and crossings are an integral part of any railway network. Plastic deformation associated with wear and rolling contact fatigue due to repeated passage of trains cause severe damage leading to the formation of surface and sub-surface cracks which ultimately may result in rail failure. Knowledge of the internal stress distribution adds to the understanding of crack propagation and may thus help to prevent catastrophic rail failures. In this work, the residual strains inside the bulk of a damaged nose of a manganese railway crossing that was in service for five years has been investigated by using differential aperture synchrotron X-ray diffraction. The main purpose of this paper is to describe how this method allows non-destructive measurement of residual strains in selected local volumes in the bulk of the rail. Measurements were conducted on the transverse surface at a position about 6.5 mm from the rail running surface of a crossing nose. The results revealed the presence of significant compressive residual strains along the running direction of the rail.

A sample of Al with grain size of 5.1 μm, prepared by spark plasma sintering, was deformed to a nominal strain of 0.35% under exposure to X-ray synchrotron radiation, allowing spatially resolved orientation measurements to be made during loading by use of a micro-diffraction technique. A significant heterogeneity in the deformation pattern between grains was observed. A statistical analysis shows that grain deformation depends more on crystallographic orientation than on grain size, with grains with tensile axis lying towards the - border of the unit triangle tending to undergo larger deformation. Other possible reasons for the different deformation behaviour between individual grains are briefly discussed.

Using a novel laboratory diffraction contrast tomography (LabDCT) technique, a non-destructive 4D study was conducted to investigate the evolution in 3D of the grain structure during grain growth in an Armco iron sample. The 3D grain morphology and the crystallographic orientations of more than 300 grains were determined at three temporal states during annealing. The correlation between growth of grains and grain orientation is explored. The results demonstrate the capability of the LabDCT technique to allow detailed studies of grain growth, and thereby provide the necessary 4D experimental evidence required for further understanding of grain growth.

AA 3104 aluminium alloy was deformed at a temperature of 510°C and at a strain rate of 5 s −1 to strains of 0.14, 0.53, 1.52, and 2.49 by plane strain compression. The effect of grain orientation on microstructures has been investigated using transmission electron microscopy (TEM). The results show that the grain orientation has a strong effect on the microstructures at low strains. The microstructures can be classified into three types according to the process of grain subdivision and the characteristics of dislocation boundaries. Every type of microstructure is concentrated in a positive district in orientation distribution figure. The microstructures in grains near the brass and S orientations correspond to the Type A I microstructure. The grains near the copper orientation appear as a Type A II microstructure. The Type B microstructure is found in the region near the rotated Goss orientation. The Type C microstructure is concentrated in the cube orientation. At large strains approximately equiaxed subgrains are observed.

The effect of deformation microstructure heterogeneity on recrystallisation behaviour has been studied using channel die deformed single crystals of typical f.c.c. rolling texture components. These orientations exhibit a wide range of both spatial and orientational heterogeneity. The likelihood of recrystallisation nuclei developing from within the deformed microstructure is found to be directly related to the extent of this heterogeneity. The recrystallisation textures comprise both a retained deformation texture component, as well as a component close, but beyond, to one extreme of the deformation texture spread. The growth of recrystallised grains, regardless of orientation, is affected by the degree of heterogeneity of the deformation microstructure. For randomly oriented nucleation, and under equivalent annealing conditions, recrystallisation proceeds two orders of magnitude faster in the lower stored energy homogeneous {110} orientation than in the {123} orientation, with a much more heterogeneous microstructure and higher stored energy, even after recovery.

The isothermal recrystallization of 90% cold-rolled commercial purity aluminum alloy AA1050 was studied by means of quantitative microscopy at four temperatures from 245°C to 280°C. The microstructural properties, V v , the volume fraction recrystallized, S v , the interfacial area density separating recrystallized grains from deformed grains and 〈 λ 〉, the mean recrystallized grain free length, were measured stereologically as a function of time. The kinetics, microstructural path, grain boundary migration rates and temperature dependence of recrystallization were quantified experimentally. Based on analysis of all data and microstructural path modelling, recrystallization was determined to be growth (boundary migration rate) controlled; all nucleation occurred in time periods short compared to the earliest annealing times. The activation energy for grain boundary migration was calculated to be 172–183 kJ/mole suggesting that a solute-limited grain boundary migration rate mechanism was operative in the alloy. The recrystallization microstructural path was found to be isokinetic, i.e. identical at all the annealing temperatures studied. Two stages of recrystallization kinetics were observed; an early transient-like stage characterized by decreasing growth rates and a later stage in which the kinetics approached Avrami behavior and the growth rates were approximately constant. The transient-like behavior is attributed to the steep, deformation-induced stored energy gradients surrrounding precipitate particles where the recrystallized grains are nucleated.

Pure (99.996%) polycrystalline aluminium (grain size about 300 μm) has been cold-rolled to reductions in the range from 5 to 50% ( e vm =0.06–0.80) and the deformation microstructure has been analysed by TEM in a large number of grains. The deformation microstructure is subdivided by dislocation boundaries having different characteristics depending on the orientation of the deformed grain. An analysis of the dislocation boundaries based on Frank's formula shows that the majority of the dislocations in the boundaries originate from active slip systems predicted by a Schmid factor analysis. Thereby a link is created between the microstructural evolution and the macroscopic plastic behaviour. The boundary parameters have also been used in a calculation of the dislocation density and the stored energy for grains of different orientations. These calculations together with measurements of the angular misorientation distribution shows that microstructural differences caused by differences in grain orientation are enhanced as the strain is increased.

The evolution of microstructure and local crystallography has been examined in pure Al single crystals of {110}〈112〉 orientation (brass or B-orientation) deformed by channel die compression at room temperature to logarithmic strains of ϵ =0.5 and ϵ =1.5. A homogeneous cell block deformation microstructure is seen at the low strain. At higher strain the cell blocks are less distinct and some evidence of strain localisation is observed. A shallow texture gradient is seen, corresponding to a lattice rotation about [101] and consistent with slip occuring on just the two most highly stressed slip systems. However an analysis using the Frank formula of the dislocation content for some of the observed boundaries suggests that a significant amount of slip takes place on systems not predicted by a plasticity analysis. This unpredicted slip stabilises the dislocation walls on planes other than those predicted using the Frank formula.

Some of the dislocation boundaries in cold deformed f.c.c. metals at low and intermediate strains lie on crystallographic slip planes and others have a macroscopic direction with respect to the sample axes (i.e. they are non-crystallographic). A model for the occurrence of the former type of dislocation boundaries is proposed. The model combines slip pattern analysis and dislocation theory. It is assumed (i) that the dislocations in the boundaries are generated by slip, (ii) that the deformation temperature is low enough to exclude dislocation climb and (iii) that the driving force for formation of boundaries is minimisation of the energy stored in the boundaries. Formation of crystallographic boundaries is predicted if two active slip systems in the same slip plane account for a large fraction of the total slip. For single crystals the agreement between predicted and experimentally observed crystallographic and non-crystallographic boundaries is excellent. For different grain orientation in poly crystalline aluminium specimens, the agreement between prediction and experiment is satisfactory in view of the complexity of polycrystal studies compared to studies of single crystals.

Recovery and recrystallization were studied in commercial purity aluminum cold rolled to an ultrahigh strain ( e vM = 6.4) and isothermally annealed at 300 °C. The deformed material consists of three layers with similar fractions of high-angle boundaries (HABs) and similar lamellar boundary spacings, but with different textures and different spatial arrangements of the rolling texture components. Annealing leads initially to a coarsening of the lamellar microstructure, accompanied by a reduction in the HAB fraction. Ex-situ experiments using very short annealing times indicate that such microstructural changes are consistent with a process of coarsening via triple junction motion. The recovery proceeds similarly in the center and subsurface layers, but because of the different initial spatial arrangement of the texture components in these layers, the loss of HABs is significantly greater in the subsurface compared with the center layer. Further annealing leads to discontinuous recrystallization, which occurs differently in the center and subsurface layers. In the center layer, recrystallization proceeds more rapidly and with a larger frequency of nuclei, resulting in a smaller recrystallized grain size. In contrast, pronounced recrystallization in the subsurface layers is delayed, and the recrystallized grain size is larger than in the center. It is concluded that the changes taking place during recovery are very significant in determining the subsequent recrystallization behavior in terms of the final grain size and texture.

Non-destructive three-dimensional X-ray diffraction (3DXRD) was used to characterise the coarsening and growth of bulk crystallites in very heavily deformed aluminium in-situ during isothermal annealing. It was found that initially during the annealing, coarsening by recovery dominates. Later recrystallisation starts and, by fast growth of recrystallisation nuclei, some very big grains evolve. This occurs simultaneously with recovery coarsening of other parts of the microstructure. Consequently, very broad grain size distributions are observed. The 3DXRD results show that the nuclei (thosecrystallites that end up being very large by fast recrystallisation growth) do not have an initial size advantage compared to those coarsening much slower by recovery. Kinetics curves for these two categories of grains are determined. Data of this type are considered very important for understanding the thermal response of nanometals and thus also for instructing thermal treatment for optimal mechanical properties.

The statistical analysis of data in the form of orientations is a relatively new discipline and results from the literature on this subject are not yet widely known outside the statistics community. This paper provides an introduction to and the key references for statistical methods for analysing orientation data. More specifically, the problem of estimating an unknown orientation is considered and results on the precision of such an estimated orientation are described. The calculation of average orientation and dispersion parameters for a sample of orientations is also considered. Finally, procedures for generating and testing for random orientations are described. The methodology is illustrated with crystal orientation data obtained from the analysis of electron back-scattering patterns.

Two new computer procedures for analysis of electron back scattering patterns (EBSP) are presented. One is a semiautomatic procedure for on-line analysis of EBSPs. The other is an image processing procedure for computer identification of bands in an EBSP. These two procedures may be combined for fully automatic indexing of EBSPs.