Abstrakt
Using equal channel angular pressing (ECAP) at 120 degrees C, a sub-microcrystalline structure was prepared in the Al7075 alloy modified by the addition of 0.2 wt. % Sc and 0.11 wt. % Zr. This relatively low ECAP temperature resulted in a very fine grain size of 0.5 mu m, however, this microstructure was not stable and a grain growth started already at 300 degrees C.
Consequently, the superplastic behaviour was not observed at temperatures above 400 degrees C which are typical for the Al7075-type alloys. The tensile tests performed at temperatures below 300 degrees C revealed enhanced ductility -elongation exceeding 300 % was observed in this material already at 200 degrees C.
In order to verify whether such behaviour can be considered as so called "low temperature superplasticity", a microstructure investigation was performed using scanning electron microscopy, electron back scatter diffraction, transmission electron microscopy, and atom force microscopy. These experiments revealed that the deformation mechanism was similar to that observed in "true" superplasticity.
Grain boundary sliding played still an active role. The elongation of individual grains during straining was significantly lower in comparison with the overall sample elongation, nevertheless it documented the contribution of dislocation slip to the deformation mechanism.