Low temperature superplasticity of the ECAP Al7075‑based alloy

Abstract

A sub-microcrystalline structure was prepared in the Al7075 alloy modified by the addition of 0.2 wt. % Sc and 0.11 wt. % Zr using the equal channel angular pressing technique (ECAP). The ECAP temperature influenced not only the grain size but also its stability at elevated temperatures and thus the deformation behaviour. Higher ECAP temperature (170 °C) resulted in slightly coarser grains with a better temperature stability. High strain rate superplasticity was observed in this material at temperatures above 450 °C. Lower ECAP temperature (120 °C) resulted in finer grains, however, a grain growth started already at 300 °C. Low temperature superplasticity with elongation exceeding 300 % was observed in this material already at 200 °C. A microstructure investigation after low temperature superplastic deformation performed using scanning electron microscopy (SEM), electron back scatter diffraction (EBSD), transmission electron microscopy (TEM), and atom force microscopy (AFM) revealed that the deformation mechanism of low temperature superplasticity is similar to that observed in “true” superplasticity. Grain boundary sliding plays an active role. The elongation of individual grains was observed to be much smaller than that of the overall sample elongation. Despite of this, it suggests the contribution of dislocation slip to the deformation mechanism.