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CARACTERIZAÇÃO MICROESTRUTURAL DE MAGNÉSIO PROCESSADO POR ECAE USANDO EBSD
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The evolution of the new microstructures produced by two types of dynamic recrystallization is reviewed, including those brought about by severe plastic deformation (SPD). The microstructural changes taking place under these conditions and the associated mechanical behaviors are described. During the conventional discontinuous dynamic recrystallization (dDRX) that takes place at elevated temperatures, the new grains evolve by nucleation and growth in materials with low to medium stacking fault energies (SFE). On the other hand, new ultrafine grains can be produced in any material irrespective of the SFE by means of SPD at relatively low temperatures. These result from the gradual transformation of the dislocation sub-boundaries produced at low strains into ultrafine grains with high angle boundaries at large strains. This process, termed in situ or continuous dynamic recrystallization (cDRX), is still not perfectly understood. This is because many SPD methods provide data concerning the microstructural changes that take place but little information regarding the flow stress behavior. By contrast, multi-directional forging (MDF) provides both types of data concurrently. Recent studies of the deformation behavior of metals and alloys under SPD conditions, carried out using MDF as well as other SPD methods, are synthesized and the links between the microstructural and mechanical observations are examined carefully. Some models for grain formation under SPD conditions are discussed. Next, the post-dynamic recrystallization behavior, i.e. that of annealing after both dDRX and cDRX, is described. The differing annealing behaviors result from the differences in the natures of the deformed microstructures. Finally, an integrated recrystallization model for these phenomena, i.e. dynamic and static recrystallization of both the continuous and discontinuous types, is presented and discussed.
Aluminum and magnesium are two highly important lightweight metals used in automotive applications to reduce vehicle weight. Crystallographic texture engineering through a combination of intelligent processing and alloying is a powerful and effective tool to obtain superior aluminum and magnesium alloys with optimized strength and ductility for automotive applications. In the present article the basic mechanisms of texture formation of aluminum and magnesium alloys during wrought processing are described and the major aspects and differences in deformation and recrystallization mechanisms are discussed. In addition to the crystal structure, the resulting properties can vary significantly, depending on the alloy composition and processing conditions, which can cause drastic texture and microstructure changes. The elementary mechanisms of plastic deformation and recrystallization comprising nucleation and growth and their orientation dependence, either within the homogeneously formed microstructure or due to inhomogeneous deformation, are described along with their impact on texture formation, and the resulting forming behavior. The typical face-centered cubic and hexag-onal close-packed rolling and recrystallization textures, and related mechanical anisotropy and forming conditions are analyzed and compared for standard aluminum and magnesium alloys. New aspects for their modification and advanced strategies of alloy design and microstructure to improve material properties are derived.
Asymmetric rolling of commercially pure magnesium was carried out at three different temperatures: room temperature, 200°C and 350°C. Systematic analysis of microstructures, grain size distributions, texture and misorientation distributions were performed using electron backscattered diffraction in a field emission gun scanning electron microscope. The results were compared with conventional (symmetric) rolling carried out under the same conditions of temperature and strain rate. Simulations of deformation texture evolution were performed using the viscoplastic self-consistent polycrystal plasticity model. The main trends of texture evolution are faithfully reproduced by the simulations for the tests at room temperature. The deviations that appear for the textures obtained at high temperature can be explained by the occurrence of dynamic recrystallization. Finally, the mechanisms of texture evolution in magnesium during asymmetric and symmetric rolling are explained with the help of ideal orientations, grain velocity fields and divergence maps displayed in orientation space.
Commercially Pure Magnesium initially hot rolled and having a basal texture was deformed by Equal Channel Angular Extrusion (ECAE). ECAE was carried out upto 8 passes in a 90° die following routes A and Bc through a processing sequence involving two temperatures, namely 523 and 473 K. Texture and microstructure formed were studied using electron back scatter diffraction (EBSD) technique. In addition to significant reduction in grain size, strong <0002> fiber texture inclined at an angle ~ 45o from the extrusion axis formed in the material. Texture was also analyzed by orientation distribution function (ODF) and compared vis-à-vis shear texture. A significant amount of dynamic recrystallization occurred during ECAE, which apparently did not influence texture.
The MATLAB™ toolbox MTEX provides a unique way to represent, analyse and interpret crystallographic preferred orientation, i.e. texture, based on integral (“pole figure”) or individual orientation (“EBSD”) measurements. In particular, MTEX comprises functions to import, analyse and visualize diffraction pole figure data as well as EBSD data, to estimate an orientation density function from either kind of data, to compute texture characteristics, to model orientation density functions in terms of model functions or Fourier coefficients, to simulate pole figure or EBSD data, to create publication ready plots, to write scripts for multiple use, and others. Thus MTEX is a versatile free and open-source software toolbox for texture analysis and modeling.
Grain growth kinetics was studied for commercially pure magnesium subjected to equal channel angular extrusion (ECAE). The
specimens were ECAE processed upto 4 passes at 523 K following all the three important routes, namely A, Bc and C. Texture and microstructures of the samples were studied using Electron Back Scattered Diffraction (EBSD) technique
in a Field Emission Gun – Scanning Electron Microscope (FEG-SEM). It was observed that the grain size significantly reduces
after ECAE. ECAE process produces a slightly rotated B and C2 fiber. Static annealing leads to normal grain growth with unimodal distribution of grains through out the temperature range.
Average activation energy for grain growth in the temperature range studied is found to be less than the activation energy
for lattice diffusion and grain boundary diffusion of magnesium. No significant change in texture during isochronal annealing
for 1 hour i.e., the predominant deformation texture remains same.
In this paper, we demonstrate a way to impart severe plastic deformation to magnesium at room temperature to produce ultrafine grain size of ∼250 nm through equal channel angular extrusion (ECAE). The strategy to deform magnesium at lower temperature or to achieve such grain sizes has been proposed as: (i) to obtain a suitable initial orientation with high Schmid factor for basal slip and low Schmid factor for pyramidal/prismatic slip; (ii) to take advantage of low stacking fault energy of basal and high stacking fault energies of prismatic/pyramidal planes in order to relatively work-harden the basal plane with respect to the pyramidal/prismatic plane; and (iii) to lower the temperature of deformation in steps, leading to continual refinement of grains, resulting in finer grain size. The experimental as well as simulated texture of ECAE-processed samples indicate that the deformation mechanism leading to ultrafine grain size is slip-dominated. The recrystallization mechanism during ECAE has been found to be orientation-dependent.
Related Annealing Phenomena fulfils the information needs of materials scientists in both industry and academia. The subjects treated in the book are all active research areas, forming a major part of at least four regular international conference series. This new 2nd edition ensures the reader has access to the latest findings, essential to those working at the forefront of research in universities and laboratories. For those in industry, the book highlights applications of the research and technologically important examples. In particular, the 2nd edition builds on the significant progress made recently in the following key areas: Deformed state, including deformation to very large strainsâ Characterisation of microstructures by electron backscatter diffraction (EBSD) Modelling and simulation of annealing. Continuous recrystallization.
A two-step equal channel angular extrusion (ECAE) procedure was used to process pure Mg. The effects of ECAE processing temperature on the microstructure, mechanical properties, and corrosion behavior of pure Mg were studied. The results show that the average grain size of pure Mg decreases with decreasing extrusion temperature. After ECAE processing at 473 K, fine and equiaxed grains (~9 μm) are obtained. The sample processed at 473 K exhibits the excellent mechanical properties, whereas the sample processed at 633 K has the lowest corrosion rate. The improved corrosion resistance and mechanical properties of pure Mg by ECAE are ascribed to grain refinement and microstructural modification.
This paper deals with dynamic recrystallization (DRX), static recrystallization, and grain growth phenomena of pure magnesium after equal channel angular pressing (ECAP) by route A and BC at 523 K (250 °C) followed by 80 pct cold rolling. The ECAP-deformed and the subsequently rolled samples were annealed at 373 K and 773 K (100 °C and 500 °C). The associated changes in the microstructure and texture were studied using electron back-scattered diffraction. ECAP produced an average grain size of ~12 to 18 µm with B and C2 fiber textures. Subsequent rolling led to an average grain size ~8 to 10 µm with basal texture fiber parallel to ND. There was no noticeable increase in the average grain size on annealing at 373 K (100 °C). However, significant increase in the average grain size occurred at 773 K (500 °C). The occurrence of different DRX mechanisms was detected: discontinuous dynamic recrystallization was attributed to basal slip activity and continuous dynamic recovery and recrystallization to prismatic/pyramidal slip systems. Only continuous static recrystallization could be observed on annealing.
The Schmid factors for four slip systems (basal 〈a〉, prismatic 〈a〉, pyramidal 〈a〉 and 〈c + a〉 slips) and two twinning patterns ({101¯2} and {101¯1} twins) were systematically calculated in magnesium as a function of angles between the c-axis and the loading direction. The results generated in this study can be effectively used to interpret deformation behaviors, including describing the flow stress and characterizing deformation behaviors, in magnesium and some hexagonal metals whose c/a axial ratios approximate to that of Mg (1.624).
Uniaxial tension tests and elastoplastic self-consistent (EPSC) simulations were conducted to study the effect of texture on the microyielding of highly textured AZ31 plate. The onset of microyielding is always associated with the activation of basal 〈a〉 slip irrespective of starting texture, while the value of the microyield stress does depend on the texture. The macroscopic yield stress is largely affected by the critical resolved shear stress of the deformation mode with the highest Schmid factor. An inverse relation is found between the microyield strength normalized by the macroscopic yield strength and the average Schmid factor of prism 〈a〉 slip, which is useful for a rough estimate of the microyield strength.
A magnesium alloy was subjected to severe plastic deformation via an unconventional equal channel angular extrusion route at decreasing temperatures. This method facilitates incremental grain refinement and enhances formability by activating dynamic recrystallization in the initial steps and suppressing deformation twinning. Compression experiments in three orthogonal directions demonstrated high strength levels in the processed sample, up to 350 MPa in yield and 500 MPa in ultimate strengths. Notable flow stress anisotropy is correlated with the processing texture and microstructure. (C) 2010 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
The focus of this article is texture development in metals of fcc, bcc, and hcp crystal structure processed by a severe plastic deformation (SPD) technique called equal-channel angular extrusion (ECAE) or equal-channel angular pressing (ECAP). The ECAE process involves very large plastic strains and is well known for its ability to refine the grain size of a polycrystalline metal to submicron or even nano-size lengthscales depending on the material. During this process, the texture also changes substantially. While the strength, microstructure and formability of ECAE-deformed metals have received much attention, texture evolution and its connection with these properties have not. In this article, we cover a multitude of factors that can influence texture evolution, such as applied strain path, die geometry, processing conditions, deformation inhomogeneities, accumulated strain, crystal structure, material plastic behavior, initial texture, dynamic recrystallization, substructure, and deformation twinning. We evaluate current constitutive models for texture evolution based on the physics they include and their agreement with measurements. Last, we discuss the influence of texture on post-processed mechanical response, plastic anisotropy, and grain refinement, properties which have made ECAE, as well as other SPD processes, attractive. It is our intent to make SPD researchers aware of the importance of texture development in SPD and provide the background, guidance, and methodologies necessary for incorporating texture analyses in their studies.
Texture development in magnesium during equal channel angular extrusion (ECAE) with an angle of 90° between the intersecting channels has been studied. Textures were simulated using polycrystal plasticity models as well as determined experimentally for routes A, Bc and C up to four passes. Based on a previous study on texture evolution during the simple shear of materials with hexagonal crystal structures [Beausir B, Tóth LS, Neale KW. Acta Mater 2007;2695:2705], the ideal orientations for ECAE of hexagonal metals were identified considering the shear to be at the 45° intersection plane of the two channels. Although dynamic recrystallization (DRX) was experimentally observed during the ECAE experiments, the polycrystal modeling using the viscoplastic self-consistent (VPSC) model was able to reproduce the experimental textures successfully. This was attributed to the fact that the textures were c-type fibers with their axis of rotation parallel to the c-axis, whereas the nucleation of grains during DRX takes place in positions simply rotated around the c-axis.
Microstructural analysis by advanced and automated methods has allowed deformation microstructures to be quantified in terms
of common structural parameters. This quantification has shown for a variety of materials and processing conditions that the
microstructural evolution follows a universal pattern of grain subdivision from the macroscale to the nanometer scale. This
microstructural evolution has been described empirically and in theoretical models based on general principles for the formation
of dislocation structures during plastic deformation by slip. The similarity between the behavior of materials undergoing
different deformation patterns forms the basis for future research and development encompassing traditional as well as new
materials and processes.
The role of geometrically necessary dislocations in providing macroscopic strengthening is reviewed with particle reinforcement as an example. The general relation between the Nye tensor and slip along a curvilinear slip line net is derived to emphasise the importance of curvature of slip lines upon the rate of dislocation storage.
Many two-phase alloys work-harden much faster than do pure single crystals. This is because the two phases are not equally easy to deform. One component (often dispersed as small particles) deforms less than the other, or not at all, so that gradients of deformation form with a wavelength equal to the spacing between the phases or particles. Such alloys are ‘plastically non-homogeneous’, because gradients of plastic deformation are imposed by the microstructure. Dislocations are stored in them to accommodate the deformation gradients, and so allow compatible deformation of the two phases. We call these ‘geometrically-necessary’ dislocations to distinguish them from the ‘statistically-stored’ dislocations which accumulate in pure crystals during straining and are responsible for the normal 3-stage hardening. Polycrystals of pure metals are also plastically non-homogeneous.The density and arrangement of the geometrically-necessary dislocations can be calculated fairly exactly and checked by electron microscopy and x-ray techniques. The rate at which they accumulate with strain is conveniently described by the ‘geometric slip distance’, a characteristic of the microstructure. Their arrangement is quite different from that of the statistically-stored dislocations, which may make them particularly susceptible to recovery effects, even at low temperatures.Geometrically-necessary dislocations control the work hardening of the specimen when their density exceeds that of the statistically-stored ones. They contribute to hardening in two ways: by acting as individual obstacles to slip, and (collectively) by creating a long-range back-stress, with wave-length equal to the particle spacing.With the exception of single-phase single crystals, almost all metals and alloys are plastically non-homogeneous to some extent. The model provides an explanation for the way in which the stress-strain curve is influenced by a dispersion of particles, and by grain size.
From local orientation measurements on planar surfaces by means of electron backscattering diffraction, six components of the lattice curvature tensor can be identified. They allow determination of five components of the dislocation density tensor (thus two more than hitherto reported) and, additionally, one difference between two other components. With this information improved lower bounds for the geometrically necessary dislocation content are obtained by linear optimization. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
The superplastic properties of metallic materials are associated with the length scale and with the thermal stability of their grain structure. Whereas equal-channel angular pressing (ECAP) may be used to produce ultrafine-grained structures in f.c.c. metals through the homogeneous subdivision of the grains, research on two magnesium alloys reveals a different and heterogeneous process of grain refinement which is dependent upon the initial grain structure in the alloys. Experiments demonstrate that different structural features may be achieved using different processing routes and this leads to the development of a processing strategy for achieving an optimum microstructure. It is shown by mechanical testing that the optimum superplastic properties also depend on the processing route and, depending on the structural characteristics, the maximum elongations to failure may occur either in the early stages of processing by ECAP or after processing through large strains.
This work addresses the course of texture evolution during different routes of equal channel angular extrusion (ECAE) of magnesium and the effect of ECAE texture on subsequent deformation. Commercially pure magnesium was deformed at 250 • C through the routes A, B C and C up to four passes. Texture evolution was found to follow a different course for different routes of ECAE. In general, the ECAE textures were asymmetric. An average grain size of 6–8 m could be achieved after four passes. The improved cold formability in the present investigation has been attributed to the initial non-basal texture and grain refinement introduced by ECAE. The textures introduced by ECAE may be beneficial for cold rolling characteristics of magnesium.
Equal-channel angular pressing (ECAP) is a processing method in which a metal is subjected to an intense plastic straining through simple shear without any corresponding change in the cross-sectional dimensions of the sample. This procedure may be used to introduce an ultrafine grain size into polycrystalline materials. The principles of the ECAP process are examined with reference to the distortions introduced into a sample as it passes through an ECAP die and especially the effect of rotating the sample between consecutive presses. Examples are presented showing the microstructure introduced by ECAP and the consequent superplastic ductilities that may be attained at very rapid strain rates.
Microstructure and texture development of an AZ61 Mg alloy during equal channel angular pressing (ECAP) was investigated and correlated with the mechanical properties. The microstructure was effectively refined by ECAP, and the original fiber texture of the extruded AZ61 alloy was disintegrated and a new texture was gradually developed by repetitive ECAP pressing. After 8 ECAP passes following route Bc, the yield stress is lower than for the as-extruded AZ61 alloy, indicating that the texture softening is dominant over the strengthening due to grain refinement. When route A was used, on the other hand, the yield stress slightly increased after 8 passes. This result is primarily due to a difference in texture. The dominant textures after 8 passes were + and ()[] when processed by route Bc and route A, respectively. Tensile ductility increased after ECAP and the effect of ECAP on ductility is more remarkable when the initial grain size is large.
The mechanisms of plastic deformation and dynamic recrystallization (DRX) in a Mg–5.8% Zn–0.65% Zr alloy were studied by compression tests at temperatures between 423 and 723 K and at strain rates ranging from 10−5 to 10−1 s−1. It was shown that the mechanisms of DRX depended on the operating deformation mechanisms which changed with temperature. Low-temperature DRX (LTDRX below 473 K) was associated with the operation of twinning, basal slip and (a+c) dislocation glide. In the intermediate temperature range (473–523 K) continuous DRX (CDRX) was observed and associated with extensive cross-slip due to the Friedel–Escaig mechanism. At temperatures ranging from 573 to 723 K both bulging of original grain boundaries and subgrain growth were the operating DRX mechanisms and controlled by dislocation climb.