cut plane as shown in Fig. 1b. In order to record images ofthe shear plane, the samples were cut at a radius of 6mm. Every recorded image will refer to the given coordinate system in Fig. 1b. The axes ofthe coordinate system are denoted as “TOR” for the torsion axis, “RAD” for the radial direction and “TAN” for the tangential direction. Figure 1: Temperature vs time and torque vs time ofthe HPT cooling experiment are shown in (a). A schematic drawing of a HPT-disc which was cut at a certain radius r is shown in (b). The sample is accompanied by a coordinate system which refers to the tangential direction “TAN”, the radial direction “RAD” andthe torsion axis “TOR”. Results Materials Science Forum Vols. 584-586 939 In one part ofthe HPT experiments, the deformation ofthe aluminium alloy was performed at different but constant temperatures and at different rotational speeds. In Fig. 2a, which depicts the results ofthese experiments, the saturation torque is shown as a function ofthe temperature andthe strain rate. In this plot the strain rate is given in s -1 . Usually one cannot deform a torsion sample at a certain strain rate; however the strain rate can be considered as constant when a fixed radius is assumed. Therefore the strain rates in Fig. 2b are referred to a radius of 6mm. Fig. 2a clearly shows a thermaland athermal part ofthe saturation torque. The athermal regime was measured in a temperature range between room temperature and 180°C and in a strain rate range between 0.2s -1 and 0.6s -1 . At strain rates lower than 0.2s -1 the material exhibits a strain rate sensitive behavior. However, the saturation torque at room temperature seems to be independent ofthe rotational speed. Similar results were obtained in the cooling experiments. Fig. 2b shows the influence ofthe strain rate in a temperature range from -196°C (liquid nitrogen) to 450°C. Like in the measurements performed at constant temperatures the material behavior can be separated in a thermaland athermal part. Furthermore, the cooling experiments show through a comparison ofthe individually measured curves that the athermal regime is influenced bythe strain rate.
940 Nanomaterials by Severe Plastic Deformation IV Figure 2: The saturation torque measured at different constant temperatures and rotational speeds is shown in (a). A similar behavior was observed in cooling experiments (b). The strain rate refers to a radius of 6mm. In addition to the mechanical test the microstructure development was measured with a scanning electron microscope. Fig. 3 shows the BSE images ofthe aluminium alloy samples deformed at constant temperatures and at a strain rate of 0.2s -1 at a radius of 6mm. In Fig. 3a the microstructure of a sample which was deformed at -196°C is depicted. The image was recorded in the centre of a cut plane which is located at a radius of 6mm and shows the microstructure ofthe shear plane. The individual grains are elongated and aligned in the tangential direction, which coincides with the shear direction during the HPT-deformation. The grain size that can be achieved at this temperature is somewhat above 100nm. A HPT deformation at room temperature produces a similar microstructure; however an increase ofthegrain size to approx. 250nm was observed. With rising temperature a further increase in grain size is measured, but also a change of microstructure is obvious. At 250°C thegrains become more equiaxed andthe alignment in the tangential direction loses its significance. At even higher temperature a more or less equiaxed microstructure is observed.