Mechanical properties of Mg and Mg alloys during and after high current density pulsesWednesday (08.11.2017) 12:00 - 12:20 Part of:
The electroplastic effect may enable room-temperature deformation processes of magnesium alloys, which is hardly feasible otherwise.
The challenges faced here are on the one hand of technical nature, i.e. producing sufficiently high current densities in short pulses and ensuring consistent, low resistance contacts between sample and testing machine. On the other hand they consist of differentiating macroscopic thermal effects due to Joule heating from microscopic electrical effects. The measurements were monitored by infrared thermography to determine temperature changes in the sample and to check for excessive heating at the contact surfaces.
To investigate the electroplastic effect, extruded Mg, AZ 31, WE 43 polycrystals as well as differently oriented single crystals of pure Mg were subjected to pulses with current densities exceeding 1kA/mm² while being compressed significantly below the yield point. Macroscopic mechanical response to the pulses was observed as a deviation from the elastic stress-strain relationship. Microstructure evolution was investigated by EBSD.
Mechanical stress and electrical pulses were applied in different directions relative to the extrusion direction of the polycrystalline material and to differently oriented single crystals. Depending on the orientation, a variation of mechanical properties could be observed during and after the pulse. Independent of the material, the pulse caused a sharp drop in stress, which lasts significantly longer than the pulse itself. When straining continued, hardening of the material was observed. Both the softening and the subsequent hardening were influenced by the amplitude and length of the pulses, the stress at the onset of the pulse and composition and orientation of the material.
Differences between the sample behaviour upon loading in different crystallographic directions were most pronounced in pure Magnesium and significantly smaller in the alloys, which can be understood in terms of a higher anisotropy owing to a sharper texture of the former.
Heating of the samples to the same temperatures as that observed during the pulse did not lead to a similar drop in yield stress. This indicates that the electroplastic effect is due to more than just a macroscopic heating of the sample.
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