Ralph Jörg Hellmig

Dislocation density-based modeling of deformation behavior of aluminium under equal channel angular pressing

In this study, the deformation behavior of aluminium during equal channel angular pressing (ECAP) was calculated on the basis of a dislocation density-based model. The behavior of the material under ECAP, including the dislocation density and cell size evolution as well as texture development, was simulated using the finite element method (FEM). The simulated stress, strain and cell size were compared with the experimental data, which were obtained by ECAP for several passes in a modified Route C regime. Good agreement between simulation results and experimental data, including strain distribution, dislocation density and cell size evolution, strain hardening and texture development was obtained. As concerns the general trends, the stress was found to increase rapidly in the first ECAP pass, the strain-hardening rate then dropping from the second pass on. Calculations showed a non-uniform strain distribution evolving in the course of ECAP. The simulated cell size is also in good agreement with the experiment, particularly with the observed rapid decrease of the cell size during the first pass slowing down from the second pass onwards. Larger cells were found to form in the upper and the lower parts of the workpiece where the strain is smaller than in the middle part. Due to the accumulation of strain throughout the workpiece and an overall trend to saturation of the cell size, a decrease of the difference in cell size with the number of passes was predicted.

Modeling of deformation behavior of copper under equal channel angular pressing

Abstract The deformation behavior of copper during equal channel angular pressing (ECAP) was calculated using a three-dimensional version of a constitutive model based on the dislocation density evolution. Finite element simulations of the variation of the dislocation density and the dislocation cell size with the number of ECAP passes are reported. The calculated stress, strain and cell size are compared with the experimental data for Cu deformed by ECAP in a modified route-C regime. The results of finite element (FE) analysis were found to be in good agreement with the experiments. After a rapid initial decrease down to about 200 nm in the first ECAP pass, the average cell size was found to change little with further passes. Similarly, the strength increased steeply after the first pass, but tended to saturate with further pressings. The FE simulations also showed strain non-uniformities and the dependence of the resulting strength on the location within the workpiece.

Modeling of texture evolution in copper under equal channel angular pressing

Abstract Texture evolution was analyzed with the full-constraint Taylor model for an idealized perfectly plastic face-centered cubic material as well as for real, strain-hardening copper subjected to equal channel angular pressing (ECAP). For the idealized material, the stress in the plastically deformed part of the billet was shown to be uniform leading to complete filling of the die. Finite element simulations showed that plastic deformation is localized in a narrow shear zone and that the plastic strain and texture in the billet become uniform after ECAP. A simplified recipe for texture calculation akin to that proposed by Gholinia et al. was suggested: it reduces the deformation under ECAP to a combination of two rotations separated by tension-compression. For the case of copper, a strain hardening model based on dislocation density evolution was used. It was shown that due to significant strain hardening during the first ECAP pass, the flowing material does not fill the outside die corner and a strai...

Microstructure and corrosion behaviour of ultrafine-grained copper

Microstructure evolution and corrosion behaviour of ultrafine-grained copper processed by equal channel angular pressing (route Bc) were studied. The results of TEM investigation of the microstructure evolution are presented along with the measurements of the corrosion potential, the corrosion current density and the anodic current density for two aggressive media, viz. 3% NaCl and 1M H2SO4. An important finding and a good news is that the corrosion behaviour of ECAP copper is not inferior to and does not qualitatively differ from that of the coarse grained material. Moreover, it was shown by SEM investigation that the corrosion damage is more homogeneous in ultrafine grained ECAP processed copper than in its coarse grained counterpart.

On the Thermal Stability of ECAP Deformed FCC Metals

Pure Cu, CuZr and an Al-alloy were processed by Equal Channel Angular Pressing (ECAP) at room temperature applying route Bc. Microstructure evolution during ECAP and subsequent annealing was investigated. The deformed and annealed states were characterized by EBSD, TEM and microhardness tests. The microstructure variation was recorded and compared to the behavior of conventional cold rolled material. The study revealed a very low thermal stability of ECAP deformed pure Cu samples compared to cold rolled material with same total strain. However, the thermal stability was significantly improved by alloying with Zr. In contrast, ECAP deformed Al-alloy showed higher thermal stability than cold rolled material.

Downscaling Equal Channel Angular Pressing

ECAP (equal channel angular pressing) is a well-known severe plastic deformation method used to produce ultra-fine grained materials. The dimensions of ECAP specimens are usually in the centimeter range. For producing high strength wires or fibres with diameter in the micrometer/millimeter range, downscaling of the ECAP process may be a viable option. To achieve this, several experiments were carried out. For downscaling to the micrometer range, porous steel discs can be used as processing tools. In this case, a solid state infiltration method as a variant of the forcefill process can be used. Extremely large strain is introduced due to the material flow through the tortuous channels inside a porous pre-form leading to grain refinement depending on processing conditions. To obtain specimens with a typical dimension in the millimeter range, the forcefill approach was altered by using die channels produced by conventional drilling. The tool geometry used is equivalent to conventional ECAP, but with a multi-channel die. Microstructure investigations demonstrating significant grain refinement confirm the viability of this approach.

Mechanical Properties of ECAP Processed Magnesium Alloy AS21X

Typically, magnesium alloys with conventional grain size exhibit microplastic behaviour already at low stresses. This behaviour restricts the technological utilization of these materials. The aim of this study was to investigate whether ECAP can be applied to enlarge the elastic range of AS21X. Cyclic tensile tests at room temperature were carried out to examine the effect of the ECAP induced grain refinement on the elastic properties. The results obtained are compared with the cyclic behaviour of conventional, coarse-grained AS21X. The differences in mechanical properties between the two conditions are discussed.

Strength and Ductility of Ultrafine Grained Copper: Modelling and Experiment

A phase mixture model is employed to describe the deformation behaviour of ult a fine grained materials (down to the nanometer grain size scale). The m ain feature of the model is that the grain boundaries are treated as a separate phase. Dislocation gli de in combination with diffusional creep mechanisms are considered as the mechanisms of plasticity of the crystalline grain interior phase, while the deformation mechanism for the grain boundaries is model led as a purely diffusional flow of matter. The results are compared with litera ture data for nanocrystalline copper and own measurements on ultra fine grained copper prepared by ECAP. Introduction As there is still a great demand in understanding the mechanical properties of ultra fine grained materials, particularly nanostructured metals, intensive researc h is being done in this area. One important issue is the need for advanced models having good descriptive and predictive capabilities. For the explanation of the mechanical properties of nanocrystalline m aterials a phase mixture model based on dislocation density evolution and diffusional flow was applied succe sf lly [1]. Some discrepancies between the model prediction and experiment owed to the f ac that the data were obtained on specimens that contained porosity inherent in the preparation te chniqu s. The aim of this report is to provide some additional experimental data on ultrafi ne grained copper to confirm the capability of the model to predict the deformation behaviour over a br oad grain size range. Fully dense bulk specimens were prepared by equal channel angular pressing (ECAP). Phase mixture model A phase mixture model for the mechanical properties of ultrafine g rained, particularly nanocrystalline, materials was developed [2]. A single phase mater ial is egarded as effectively a two-phase one. Figure 1 shows a schematic of the phase mixture model for a nanocrystalline material. As can be seen, the unit cell of a nanocrystalline material consi sts of several ‘phases’: the grain interior (crystallite), the grain boundaries, the triple lines and the quadruple nodes. In a first approximation, no distinction is made between the last three ‘phases’, i.e. the intercrystalline components: all three are lumped together in a ‘phase’ with a volume fraction of fic. Journal of Metastable and Nanocrystalline Materials Online: 2003-05-01 ISSN: 2297-6620, Vol. 17, pp 29-36 doi:10.4028/www.scientific.net/JMNM.17.29

New Applications of the SPD Concept: μSPD

Extreme grain refinement by severe plastic deformation (SPD) is an established processing approach broadly applied to bulk materials. The dimensions of specimens or workpieces used are typically in the centimeter range. We propose to adopt an analogue of equal channel angular pressing in the context of microforming. It is suggested that using sub-millimeter sized channels (“angular vias”) in a process similar to conventional forcefill technique, fibres or wires with ultrafine grain size can be manufactured. The cross-sectional dimensions of the channels can be in the micrometer range. An alternative technique that may lead to ultrafine grained fibres or wires is pressing of metals through porous “filters” with micrometer or submicrometer scale open porosity. First results demonstrate the viability of this approach which we refer to as μSPD.