Athermal and thermal limits of the grain refinement by SPD Hafok ...

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Athermal and thermal limits of the grain refinement by SPD Hafok ...

cut plane as shown in Fig. 1b. In order to record images of the 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

of the 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 of the 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” and the torsion axis “TOR”.

Results

Materials Science Forum Vols. 584-586 939

In one part of the HPT experiments, the deformation of the aluminium alloy was performed at

different but constant temperatures and at different rotational speeds. In Fig. 2a, which depicts the

results of these experiments, the saturation torque is shown as a function of the temperature and the

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 thermal and athermal part of the 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 of

the rotational speed.

Similar results were obtained in the cooling experiments. Fig. 2b shows the influence of the

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 thermal and

athermal part. Furthermore, the cooling experiments show through a comparison of the individually

measured curves that the athermal regime is influenced by the strain rate.

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