Fundamentals of Deformation and Annealing

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Authors: Edgar F. Rauch, G. Shigesato
Abstract: The dislocation substructure that appears in deformed metals and alloys have been extensively investigated in the past by transmission electron microscopy (TEM). They are known to form a broad variety of microstructures. These substructures are characterized by three main parameters, namely the density of the dislocations that are trapped in the tangles, their degree of patterning and the misorientation between the cells. The aim of the present work is to investigate the relationship between these features and the mechanical properties of the material.
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Authors: N. Zaafarani, Franz Roters, Dierk Raabe
Abstract: This work studies the rotations of a (111) Cu single crystal due to the application of a conical nanoindent. With the aid of a joint high-resolution field emission SEM-EBSD set-up coupled with serial sectioning in a focused ion beam (FIB) system in the form of a cross-beam 3D crystal orientation microscope (3D EBSD) a 3D rotation map underneath the indent could be extracted. When analyzing the rotation directions in the cross section planes (11-2) perpendicular to the (111) surface plane below the indenter tip we observe multiple transition regimes with steep orientation gradients and changes in rotation direction. A phenomenological and a physically-based 3D elastic-viscoplastic crystal plasticity model are implemented in two finite element simulations adopting the geometry and boundary conditions of the experiment. While the phenomenological model predicts the general rotation trend it fails to describe the fine details of the rotation patterning with the frequent changes in sign observed in the experiment. The physically-based model, which is a dislocation density based constitutive model, succeeded to precisely predict the crystal rotation map compared with the experiment. Both simulations over-emphasize the magnitude of the rotation field near the indenter relative to that measured directly below the indenter tip. However, out of the two models the physically-based model reveals better crystal rotation angles
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Authors: Aurelie Wauthier, Hélène Réglé, Rénald Brenner
Abstract: The aim of this study is to acquire more quantitative data on the cold-deformed state, in order to determine the key parameters for nucleation (like orientation gradient, dislocation density, degree of fragmentation…) and to correlate these to parameters which can be predicted by a mean field deformation model. Experimentally, a Ti-IF hot band has been deformed by cold rolling up to 51%. The deformed microstructures are finely characterised using EBSD inside the SEM. It appears then that the heterogeneity of deformation inside the grains begins very early. Some grains undergo fragmentation whereas some others deform very homogeneously. Misorientation and degree of fragmentation are found to be orientation and rolling level dependent. These deformation parameters are plotted against the Taylor factor, calculated in each grain.
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Authors: Y. Song, M.D.J. Cross, Mark W. Rainforth, Bradley P. Wynne
Abstract: The effect of interpass time during thermomechanical processing of AA61111 on flow behaviour and microstructure evolution has been investigated. This was achieved using plane strain compression testing undertaken on the Sheffield thermomechanical compression (TMC) facility, using the hit-hold-hit-quench approach. Following solution treatment at 560°C for 1200s, samples were water mist quenched to 320°C and deformed at a constant strain rate of 85s-1 to an initial strain of 0.5, unloaded and held for delay times of 0.019, 6, 60, 600 and 6000s and then given a second deformation for a further strain of 0.5, followed by a water quench to room temperature. Hardening of the alloy was observed, the extent of which was dependent on the hold time. The microstructure of the samples was quantified by TEM in order to determine the extent of strain induced precipitation. TEM identified precipitation, predominantly β and Q phases, on dislocation lines, the size and volume fraction of which were a function of the hold time. The coarsening rate during the hold period of the precipitates was considerably faster than for coarsening following a conventional precipitation treatment. The size of the microband structure at the end of the double deformation was a function of the hold time, suggesting that coarsening of the precipitates during the hold had altered the Zener pinning potential. The implication of these observations on the thermomechanical processing of 6xxx alloys is discussed.
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Authors: Atef S. Hamada, L. Pentti Karjalainen, Mahesh C. Somani, R.M. Ramadan
Abstract: The hot deformation behaviour of two high-Mn (23-24 wt-%) TWIP steels containing 6 and 8 wt-% Al with the fully austenitic and duplex microstructures, respectively, has been investigated at temperatures of 900-1100°C. In addition, tensile properties were determined over the temperature range from -80 to 100°C. It was observed that in spite of the lower Al content, the austenitic steel possessed the hot deformation resistance about twice as high as that of the duplex steel. Whereas the flow stress curves of the austenitic steel exhibited work hardening followed by slight softening due to dynamic recrystallisation, the duplex steel showed the absence of work hardening and discontinuous yielding under similar conditions. Tensile tests at low temperatures revealed that the austenitic grade had a lower yield strength than that of the duplex grade, but much better ductility, the elongation increasing with decreasing temperature, contrary to that for the duplex steel. This can be attributed to the intense mechanical twinning in the austenitic steel, while in the duplex steel, twinning occurred in the ferrite only and the austenite showed dislocation glide.
217
Authors: M. Lopez-Pedrosa, Bradley P. Wynne, Mark W. Rainforth
Abstract: The effects of strain path reversal on the microstructure in AA5052 have been studied using high resolution EBSD. Deformation was carried out using two equal steps of forward/forward (F/F) or forward/reverse (F/R) torsion at a temperature of 300°C and strain rate of 1s-1 to a total strain of 0.5. In both cases the deformation microstructure in the majority of grains analysed consisted of microband arrays clustering at specific angles to the macroscopic deformation axes. For the F/F condition microbands clustered around -20° and +45° to the maximum principle stress direction, whilst for the F/R condition significantly more spread in microband angle was observed. This suggests that the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path. This leads to the conclusion that instantaneous deformation mode determines the orientation of new microbands formed whilst a non-linear strain path history influences the range of misorientation angle in the material through the dissociation of previously formed microbands and the formation of new microbands at the new straining condition, leading to a lower level of misorientation angle. Analysis of material subjected to static annealing at 400°C for 1 hour appears to correspond with these observations as the F/F material was completely recrystallised with a fine grain structure whilst the F/R material had no major signs of recrystallisation.
223
Authors: Talal Al-Samman, Bashir Ahmad, Günter Gottstein
Abstract: Texture evolution and microstructure development of hot extruded magnesium alloy AZ31 deformed by PSC and uniaxial deformation at select temperatures and a constant strain rate of 10-4 s-1 were investigated and compared using X-ray techniques, electron back scattered diffraction (EBSD) and optical microscopy. At a deformation temperature of 200 °C both deformation routes resulted in a similar crystallographic texture and showed a heterogeneous microstructure consisting of highly deformed zones appearing as huge and/or elongated grains containing twins and shear bands embedded in a very fine-grained microstructure. High temperature deformation (400 °C) gave rise to completely different deformation textures for the two processes. Uniaxial deformation tended to randomize the initial extrusion texture, whereas in PSC a prismtexture {10-10}<11-20> prevailed. The flow stress was found to be strongly dependent on loading conditions and deformation modes.
229
Authors: Jacob R. Bowen, Toshiaki Masui, Nobuhiro Tsuji
Abstract: The effect of large strain deformation on microstructural refinement has been intensely investigated by many deformation processes. Two popular processes are equal channel angular extrusion (ECAE) and accumulative roll-bonding (ARB). This work presents the evolution of microstructure in Al-0.13%Mg as a function of von Mises strain for both ECAE and ARB in terms of high and low angle boundary spacing and relative fraction, as well as the stability of the deformed structures after a strain of 10, thereby allowing a direct comparison of the two processes. It is found that microstructural evolution as a function of strain is similar, and that ARB structures after a strain of 10 are more stable than ECAE on annealing.
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Authors: Yan Huang, Philip B. Prangnell
Abstract: The feasibility of a novel continuous severe plastic deformation (SPD) technique, continuous frictional angular extrusion (CFAE), for producing ultra-fine grained strip material, has been studied. The CFAE technique takes advantage of facets of rolling and equal channel angular extrusion (ECAE) and is designed to produce bulk ultra-fine grained (UFG) metals with high productivity and low cost. A process setup was established through the modification of a standard rolling mill. CFAE processing of commercially pure aluminium AA1050 sheets was successfully carried out at room temperature, using a 120o die angle. A uniform UFG structure with an average grain size of ~0.6μm was achieved after 10 CFAE passes, at an equivalent strain of ~ 6.6. Evolution of the deformation structure and texture during processing was examined as a function of strain and characterized using high resolution EBSD.
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Authors: Paul Olaru, Günter Gottstein, Andre Pineau
Abstract: Eutectic AlNi, AlFe, alloys exhibit plastic strains to failure (usually in the range of 1%- 5%), that those of structural alloys. We have developed a technique to measure strains at the scale of the microstructure and have used this method to assess the variation in failure properties with microstructure. This method is capable of using the grayscale information in the image of a gridded sample to obtain sub-pixel marker displacement, and can therefore accurately determine small strain values. Microstructures that exhibit large variation in local strain distribution tend to have higher variability in tensile properties, particularly tensile ductility, compared to microstructures that accumulate strain more uniformly. Orientation and morphology of lamellar plates in lamellar colonies play, also, a role in influencing the distribution of strain. Local grain orientation, phase distribution and segregation are factors influencing the strain distribution, and therefore the properties of these materials.
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