Papers by Author: Hans Jørgen Roven

Abstract: In the present work, a peak-aged 6061 Al-Mg-Si aluminum alloy was subjected to equal channel angular pressing (ECAP) at 110 °C. The microstructure of the sample was characterized by high-resolution transmission electron microscope and weak-beam dark-field method. It was shown that the dislocation density in some local areas is much lower than the average dislocation density expected in the usual alloys processed by severe plastic deformation. High-resolution transmission electron microscope observations indicated that many full dislocations were dissociated into partial dislocations connected by stacking faults. In addition, a Z-shaped defect (i.e., a type of dislocation locks) probably formed by the reactions of the partials in different {111} planes was first observed in the ECAPed alloy. Furthermore, the precipitation behavior and sequence in the present ECAPed sample were identified by high-resolution transmission electron microscopy.

Abstract: Three alloys of the magnesium AZ-series (AZ31, AZ61 and AZ91) were processed by multi-temperature equal channel angular pressing (ECAP) with five passes using route B_C. The ECAP temperature was decreased from 275 °C to 250 °C during the process for better grain refinement. The mechanical behaviour was investigated over a wide range of strain rates (10^-3 s^-1 up to 10³ s^-1) under compressive loading at room temperature. The investigations show that significant grain refinement takes place during the ECAP-process. The initial grain size of 12 μm, 9 μm and 5.8 μm for extruded AZ31, AZ61 and AZ91, respectively, could be refined to 2.5 μm. The grain refinement occurs by dynamic recrystallisation. Compared to extruded initial Mg-alloys, the yield stress increases slightly for all selected alloys after ECAP processing, while the amount of strain hardening decreases at the same time, due to reduced grain size and texture effects. Furthermore, the flow stress of extruded and ECAPed material is less affected by strain rates within a range of 10^-3 s^-1 to 10^-1 s^-1.

Abstract: The production of primary aluminum is an energy costly process. With the global warming being of concern, the secondary aluminum stream is becoming an even more important component of aluminum production and is attractive due to its economic and environmental benefits. Recycling of aluminum by new solid state recycling techniques instead of conventional remelting and subsequent refining processing can result in significant energy savings. Severe Plastic Deformation (SPD) techniques have been applied for consolidating nano particles into fully dense materials with good mechanical properties. However, solid state recycling of scraps by SPD is only in the beginning. In the present study, degreasing of aluminum chips from the machine workshop was investigated by a thermal method and chemical treatment. Thereafter, the decoated chips were recycled by Cyclic Extrusion Compression (CEC) at deformation temperatures between 400 and 500 °C. The microstructure and mechanical properties of the recycled aluminum scrap processed by SPD were subsequently investigated. The results show that SPD technology provides a promising alternative for recycling of aluminum scrap. Thermal degreasing of aluminum scrap resulted in more oxidization of aluminum scrap particles. Visible interfaces between chips were observed even at a low magnification.

Abstract: Nanostructures and microhardness of a commercial purity Al, three binary Al–Mg alloys and a commercial AA5182 alloy subjected to high pressure torsion (HPT) at room temperature were comparatively investigated using high-resolution transmission electron microscopy, X-ray diffraction (XRD) and high-resolution XRD line profile analysis. The hardness values of HPT samples are twice to three times larger than that of the undeformed counterparts. Grain sizes measured by XRD are in the range 10–200 nm with typical average values ranging from 46 to 120 nm. The hardness values and the dislocation densities increased, whereas, the average grain size decreased significantly with increasing Mg contents. Typical dislocation densities are in the range 1.7 × 1014 m-2 – 2.3 × 1015 m-2. However, local densities in grain boundary and triple junction areas might be as high as 1017 m-2. The strengthening mechanisms contributing to high hardness may primarily be attributed to the cooperative interactions of high dislocation densities, grain boundaries and planar interfaces.

Abstract: The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.

Abstract: An Al–0.5 Mg alloy and a commercial AA5182 alloy were subjected to high pressure torsion (HPT) to five turns under pressure of 6 GPa at room temperature. The grain boundary structure and deformation defects were investigated after HPT using high-resolution transmission electron microscopy (HRTEM). Low-angle, high-angle, equilibrium and non-equilibrium grain/subgrain boundaries, twin boundaries, full dislocations, dipoles, microtwins and stacking faults were identified by HRTEM. Extrinsic 60° dislocations in the form of dipoles were frequently observed in non-equilibrium grain/subgrain boundaries. In addition subgrain size distributions and dislocation densities were quantified by x-ray line profile analysis. It was observed that the average grain size decreased from about 120 nm to 55 nm as the Mg content increased from 0.5 to 4.1 wt%. Concomitantly the average stored dislocation density increased from 1.7 to 12.8  1014 m-2. Based on the HRTEM investigations and the x-ray line profile analyses, the deformation mechanism associated with the typical grain boundaries and deformation defects in the aluminium alloys were discussed.

Abstract: Cyclic extrusion compression (CEC) is an effective severe plastic deformation (SPD) process which can be used for fabricating ultrafine grained light materials such as magnesium alloys. This method introduces three-dimensional compression and shear stresses and the process can be repeated for a certain number of passes until the desired accumulated strain has been introduced. In order to reveal the effect of second phases on the microstructure developed in magnesium alloys during CEC, three different alloys (AZ31, AZ31-1wt.%Si and AZ91) were investigated after CEC 7 passes performed at 225°C. The experimental results show that the CEC process can effectively refine the microstructures of these alloys and the mean grain size achieved is 1.3µm, 1.5µm and 1.4µm, respectively. It is revealed that the grain size, grain shape and grain boundary structures are little affected by coarse phase Mg2Si but strongly affected by the fine phase Mg17Al12. The fine phase Mg17Al12 seems to increase the relative grain misorientations, hence enhancing the formation of high angle grain boundaries (HAGBs). It is expected that such changes are improving mechanical properties, subsequent forming behavior and surface quality.

Abstract: High-resolution transmission electron microscopy investigations revealed different types of deformation structures in a nanostructured commercial Al–Mg alloy processed by high pressure torsion at room temperature. Microtwins and stacking faults were detected within both nanocrystalline grains and ultrafine grains. Full dislocations in the form of dipoles were observed within grains and near the grain boundaries. Two twinning mechanisms previously predicted by molecular-dynamics simulations were directly verified including the heterogeneous twins nucleated by the successive emission of Shockley partials from grain boundaries and homogeneous twins formed in the grain interiors by the dynamic overlapping of stacking faults. Hence, the formation of full dislocations, stacking faults and twins in the present aluminum alloy subjected to severe plastic deformation may be interpreted in terms of molecular-dynamics simulations based on generalized planar fault energy curves for pure metal systems.

Abstract: In-situ synchrotron X-ray diffraction has been applied in order to study grain growth in an ultra-fine grained (D~400 nm) 6060 aluminium alloy at 270°C. The submicron grain structure was produced by Equal Channel Angular Pressing (ECAP) to an effective strain of ~6 without rotation of the billet. As the material was textured after ECAP, the initial stages of grain growth were seldom detected, but in the grain size interval available for studies a grain growth exponent of 3.6±0.3 was obtained. By interpolation of the grain growth curves to D=D0 (determined by EBSD) the effect of growth on the softening of the alloy was estimated. The interpolated average curve indicates that the initial stages of softening are not due to uniform grain growth, but rather reconfiguration and annihilation of dislocations as well as overaging of hardening precipitates.

Abstract: The crystallographic slip activity in several grains deformed by simple tension is determined by use of in-situ deformation in combination with Electron Back Scattering Diffraction (EBSD)-investigations and Secondary Electron (SE) imaging. This technique is also used to determine grain lattice rotation paths of grains with different initial orientation, providing information on basic deformation mechanisms of grains present in texture gradients. Both slip activity and grain lattice rotation paths depend on the initial orientation and are influenced by the neighbouring grain orientations. This indicates that predictions of the forming behaviour of extruded profiles with a strong through thickness texture gradient relate to a very complex nature.