Edgar F. Rauch

Plastic flow for non-monotonic loading conditions of an aluminum alloy sheet sample

Abstract Non-linear deformation paths obtained using uniaxial tension followed by simple shear tests were performed for a 1050-O aluminum alloy sheet sample in different specimen orientations with respect to the material symmetry axes. In order to eliminate the time influence, the time interval between the first and second loading steps was kept constant for all the tests. Monotonic uniaxial tension tests interrupted during loading were used to assess the recovery that takes place during this time. In order to eliminate the influence of the initial plastic anisotropy and to compare the results as if the material hardening was isotropic, the flow stress was represented as a function of the plastic work. The behavior of the material after reloading was analyzed in terms of dislocation microstructure and crystallographic texture evolutions. For more quantitative assessment, the full constraints [Int. J. Plasticity 13 (1997) 75] and visco-plastic self-consistent [Acta Metall. Mater. 41 (1993) 2611] polycrystal models were used to simulate the material behavior in the non-linear deformation paths. Based on experimental and simulation results, the relative contributions of the crystallographic texture and dislocation microstructure evolution to the anisotropic hardening behavior of the material were discussed.

Effect of texture and microstructure on strain hardening anisotropy for aluminum deformed in uniaxial tension and simple shear

Abstract Uniaxial and simple shear stress–strain curves were obtained for a 1050-O aluminum alloy sheet sample in different specimen orientations with respect to the material symmetry axes. For uniaxial tension, a strong anisotropy of strain hardening was observed leading to about 30% difference in uniform tensile elongation between the extreme conditions. For simple shear, the hardening was also significantly different. These results were rationalized with an analysis that accounts for dislocation substructure observations, crystallographic texture measurements and polycrystal modeling of texture-induced strength evolution.

Hardening behavior and structural evolution upon strain reversal of aluminum alloys

Abstract The mechanical behavior and the dislocation structure change upon strain reversal are analyzed for 1050-O and 6022-T4 aluminum alloys. Dissolution of the dislocation structure and related transient hardening rate are observed in the AA1050-O but impeded by the numerous precipitates and the high solute content in the AA6022-T4.

Precession Electron Diffraction Assisted Orientation Mapping in the Transmission Electron Microscope

Precession electron diffraction (PED) is a new promising technique for electron diffraction pattern collection under quasi-kinematical conditions (as in X-ray Diffraction), which enables “ab-initio” solving of crystalline structures of nanocrystals. The PED technique may be used in TEM instruments of voltages 100 to 400 kV and is an effective upgrade of the TEM instrument to a true electron diffractometer. The PED technique, when combined with fast electron diffraction acquisition and pattern matching software techniques, may also be used for the high magnification ultra-fast mapping of variable crystal orientations and phases, similarly to what is achieved with the Electron Backscattered Diffraction (EBSD) technique in Scanning Electron Microscopes (SEM) at lower magnifications and longer acquisition times.

Orientation Maps Derived from TEM Diffraction Patterns Collected with an External CCD Camera

An automatic crystallographic orientation indexing procedure is developed for transmission electron microscopes. The numerical identification is performed by mapping the spot diffraction patterns with pre-calculated templates. The diffraction patterns are acquired thanks to an external CCD camera that points to the fluorescent screen through the TEM window. Orientation maps with spatial resolution better than 10 nm were obtained with this low cost equipment.

Fast electron diffraction tomography

A fast and fully automatic procedure for collecting electron diffraction tomography data is presented. In the case of a very stable goniometer it is demonstrated how, by variation of the tilting speed and the CCD detector parameters, it is possible to obtain fully automatic precession-assisted electron diffraction tomography data collections, rotation electron diffraction tomography data collections or new integrated electron diffraction tomography data collections, in which the missing wedge of the reciprocal space between the patterns is recorded by longer exposures during the crystal tilt. It is shown how automatic data collection of limited tilt range can be used to determine the unit-cell parameters, while data of larger tilt range are suitable to solve the crystal structure ab initio with direct methods. The crystal structure of monoclinic MgMoO4 has been solved in this way as a test structure. In the case where the goniometer is not stable enough to guarantee a steady position of the crystal over large tilt ranges, an automatic method for tracking the crystal during continuous rotation of the sample is proposed.

Textural vs structural plastic instabilities in sheet metal forming

Abstract Negative work-hardening rates are precursors of plastic instabilities. The objective of the work is to determine the origin of the stress decrease in the case of prestrained sheet metals. Samples of low carbon steel and pure aluminium deformed in rolling and reloaded in simple shear exhibit softening for specific shear directions with respect to the rolling direction. Structural and textural evolutions are analysed with transmission electron microscopy and X-ray measurements, respectively. The low carbon steel shows clear evidence of intragranular structural destabilization while the Taylor factor, estimated by self-consistent calculations, remains nearly constant. It is deduced that for steel, the softening has a structural origin. By contrast the aluminium samples present identical structural changes whatever the sign of the work-hardening rate. It is shown that the latter is strongly correlated with Taylor factor evolutions. A textural instability is obtained in this case.

Structural and mechanical properties in AA 5083 processed by ECAE

Abstract Equal channel angular extrusion has been used to analyse refining of grains in an industrial 5083 aluminium alloy during severe plastic deformation. The influence of the total strain as well as of the processing route were studied by tensile tests and TEM. The room temperature behaviour and the high temperature properties suggest that large strains increase the density of high angle boundaries in the material. The optimal processing route to achieve grain refinement appears to be route B for the present investigation.

Cold work hardening of Al from shear deformation up to large strains

Several deformation modes have been applied so far which exhibited stage IV and stage V hardening in large strain cold working. However, some deformation modes especially if applied to single crystals failed (1) because of limited deformation (tensile test and compression), (2) inhomogeneous deformation (torsion), (3) iterative deformation (wire drawing, rolling) allowing for recovery processes in between small deformation steps. Moreover, except for torsion test, none of the deformation modes is capable of measuring the strain rate sensitivity up to large strains at low deformation temperatures. Thus it was the aim of the present work to deform single and polycrystalline samples of Al 99.99% in a simple shear test which has been shown to achieve also the late stages of deformation. Moreover, it should make possible strain rate sensitivity measurements in parallel to the shear stress-strain characteristics. For single crystals at room temperature the shear test seems to be the only method which can provide such data.

Comments on`On the reliability of fully automatic indexing of electron diffraction patterns obtained in a transmission electron microscope'by Morawiec & Bouzy (2006)

The limitation of automatic indexing of electron diffraction patterns raised by Morawiec & Bouzy [(2006). J. Appl. Cryst. 39, 101-103] is discussed. The theoretical problem related to the famous 180° ambiguity may be surmounted by adequate technical improvements. Three solutions to avoid misindexing are briefly described.