Fundamentals of Deformation and Annealing

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Authors: Tetsuo Sakai, Kohki Mori, Hiroshi Utsunomiya
Abstract: It is believed that the shear deformation superimposed on rolling deformation accelerates grain refinement. However, it has not yet been completely understood whether the grain refinement is due to the increase in amount of equivalent strain, or the change in strain path. In the present study, three different strain paths in plane strain - (1) simple shear, (2) compression and (3) the combination of simple shear and compression - are introduced into 1100 aluminum sheet. The recrystallization behaviours are compared. Plane-strain compression was achieved by a normal rolling, while the simple shear was achieved by a continuous ECAE (conshearing). The combined strain path was achieved by the conshearing subsequently followed by the rolling. The same amount of the equivalent strain of 1.28 was accumulated in the three paths. The ratio of shear strain to compressive strain was varied by three levels in the combined strain process. After heat treatment, the material processed by the combined strain path gave a finer recrystallized grain size than both of the monotonic strain paths at either annealing temperature. The finest recrystallized grain size was obtained at the shear strain ratio of 0.6 to the total equivalent strain. It was found that the change in strain path was effective for introducing more new high-angle grain boundaries.
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Authors: Lena Ryde, Joacim Hagström, W. Bevis Hutchinson
Abstract: Work has been carried out at KIMAB to determine the best, industrially feasible, conditions for producing TRIP steel sheet with the aim of obtaining a tensile strength of 600 MPa and a ductility of 30% through a hot-dip galvanising process schedule. Two of the most promising steels were selected to study the microstructures and to follow the transformation from ferrite to austenite in more detail, as well as to examine the stability of the austenite during deformation. This investigation has been performed mainly with the aid of the EBSD technique.
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Authors: Taku Sakai, Hiromi Miura
Abstract: Annealing behaviour was studied in deformed copper developed by continuous or discontinuous dynamic recrystallization (cDRX or dDRX). Pure copper was deformed to large strains by multi-directional forging at room temperature, resulting in an ultra-fine grained structure due to operation of cDRX. Subsequent annealing of such a fine-grained copper can be controlled mainly by grain growth accompanied with recovery and no texture change, that is continuous static recrystallization (cSRX). On the other hand, 4 kinds of static restoration processes operate during annealing of dDRXed copper, i.e. metadaynamic recovery and recystallization (mDRV and mDRX), and classical static recovery and recrystallization. The stable existence of mDRVed grains containing moderate dislocations leads to incomplete recrystallization even after a long period of annealing time. It is discussed how such various types of annealing processes, occurring in cDRXed or dDRXed matrices, can be connected with the characteristic nature of the deformed microstructures.
327
Authors: Sandra Piazolo, David J. Prior, M.D. Holness, Andreas O. Harstad
Abstract: Annealing is an important mechanism of microstructural modification both in rocks and metals. In order to relate directly changes in crystallographic orientation to migrating boundaries the researcher has the option to investigate either samples where the grain boundary motion can be directly tracked or a series of samples exhibiting successively higher degrees of annealing. Here we present results from rock samples collected from two well characterised contact aureoles (a volume of rock heated by the intrusion of a melt in its vicinity): One quartz sample in which patterns revealed by Cathodoluminescence (CL) indicate the movement of grain boundaries and a series of calcite samples of known temperature history. Electron backscatter diffraction (EBSD) analysis is used to link the movement of grain, twin boundaries and substructures with the crystallographic orientation / misorientation of a respective boundary. Results from the quartz bearing rock show: (a) propagation of substructures and twin boundaries in swept areas both parallel and at an angle to the growth direction, (b) development of slightly different crystallographic orientations and new twin boundaries at both the growth interfaces and within the swept area, and (c) a gradual change in crystallographic orientation in the direction of growth. Observations are compatible with a growth mechanism where single atoms are attached and detached both at random and at preferential sites i.e. crystallographically controlled sites or kinks in boundary ledges. Strain fields caused by defects and/or trace element incorporation may facilitate nucleation sites for new crystallographic orientations at distinct growth interfaces but also at continuously migrating boundaries. Calcite samples show with increasing duration and temperature of annealing: (a) systematic decrease of the relative frequency of low angle grain boundaries (gbs), (b) decrease in lattice distortion within grains, (c) development of distinct subgrains with little internal lattice distortion, (d) change in lobateness of gbs and frequency of facet parallel gbs and (e) change in position of second phase particles. These observations point to an increasing influence of grain boundary anisotropy with increasing annealing temperature, while at the same time the influence of second phase particles and subtle driving-force variations decrease. This study illustrates the usefulness of using samples from natural laboratories and combining different analysis techniques in microprocess analysis.
333
Authors: Shigeo Saimoto, Hai Ou Jin
Abstract: A nominally pure Al slab was thermo-mechanically treated to result in a near random texture of 90 m grain size. Subsequent cold rolling with intermediate anneals at 230, 275, and 300°C reduced the Fe solute to near equilibrium compositions below 0.5 ppm atomic. The final cold rolled sheet continuously recrystallized; grain growth of this structure is reported. A grain-growth kinetics mapping was generated, correlating the parameters of Fe-in-Al solubility limit, Fe diffusivities in the grain boundaries and the Al lattice and the activation energies for migration rates.
339
Authors: Satoru Kobayashi, Stefan Zaefferer, Dierk Raabe
Abstract: Recrystallisation behavior was studied in two Fe3Al-based alloys containing both large and fine particles with a different fine particle dispersion level using high-resolution SEM and EBSD. High misorientation of 15-30° was created around large particles after a hot rolling process in the two alloys. The kinetics of recrystallisation were, however, considerably retarded in the alloy containing dense fine particles. It was observed that the growth of subgrains created around the large particles was inhibited by the presence of the fine particles. This result clearly suggests that when the particle density (Ns) is high relative to local stored energy (E) around large particles, nucleation can be completely hindered. As the Ns/E level decreases, nucleation may occur and the kinetics of recrystallisation might be determined by both the nucleation rate and the growth of nuclei into a matrix with fine particles.
345
Authors: Hans Erik Ekström, Stian Tangen, O.V. Mishin, Lars Östensson
Abstract: The microstructural evolution during annealing of a commercial Al-Mn alloy cold rolled to a high strain was investigated using EBSD and Gallium Enhanced Microscopy. The precipitation of manganese, coarsening of precipitates and tensile properties were monitored at different stages. It was found that during recovery the subgrains grow until they reach the limiting subgrain size when the driving force has been reduced to the same level as the Zener drag from the dispersoids. New grains are nucleated at constituent particles and a few are able to grow. The softening during recovery and the onset of discontinuous recrystallisation are analyzed and discussed in terms of recent theories of recovery and recrystallisation.
351
Authors: S. Eric Offerman, Henrik Strandlund, Niels H. van Dijk, Jilt Sietsma, Erik M. Lauridsen, L. Margulies, Henning Friis Poulsen, John Ågren, Sybrand van der Zwaag
Abstract: Ferrite formation during austenite decomposition in carbon-manganese steel is studied during slow continuous cooling by three-dimensional x-ray diffraction microscopy at a synchrotron source. The ferrite fraction and nucleation rate are measured simultaneously and independently in real time in the bulk of the specimen. Thermodynamic calculations involving both ortho- and paraequilibrium have been performed to determine the driving force for nucleation. From the experiments and thermodynamic calculations the activation energies are estimated for nucleation and the transfer of iron atoms across the interface of the cluster during ferrite nucleation in steel.
357
Authors: R.L. Higginson
Abstract: The effect of strain path on an aluminium-manganese alloy has been studied using single pass rolling and simple compression tests. Strain paths of 0, 90 and180° have been studied in terms of texture development and recrystallisation behaviour and compared with equivalent positions in the rolled slab. The effects of the individual deformations on the dislocation sub-structure have been studied using transmission electron microscopy. The study has shown that although samples can be deformed to the same strain via nominally the same stain path change the deformation mode can fundamentally influence the sub-structure developed during each deformation stage.
363
Authors: Matthew R. Barnett, Aiden G. Beer
Abstract: An analytical approximation for the steady state dynamic recrystallized grain size is combined with a simple nucleation criterion to assess the propensity for dynamic recrystallization. In line with observation, the criterion predicts dynamic recrystallization in 99.9995% pure Al but not in material 99.5% pure. It also agrees with the observation that zone refined ferrite can display dynamic recrystallization at high temperatures and low strain rates but not at lower hot working temperatures. The criterion is applied here to common wrought magnesium alloys to argue that conventional dynamic recrystallization is expected under "normal" hot working conditions.
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