Low-Temperature and High-Strain-Rate Superplasticity of Ultrafine-Grained A7075 Processed by High-Pressure Torsion

An A7075 alloy was processed by high-pressure torsion (HPT) under an applied pressure of 6 GPa for 1, 3 and 5 revolutions with a rotation speed of 1 rpm at room temperature. Vickers microhardness saturated to a level of 220 Hv after the HPT processing and the grain size was refined to similar to 120 nm at the state of the hardness saturation. Tensile tests were conducted with initial strain rates from 2.0 x 10(-2) to 2.0 x 10(-4) s(-1) at a temperature in the range of 200-400 degrees C. It was shown that high-strain rate superplasticity (>1.0 x 10(-2) s(-1)) and/or low-temperature superplasticity (0.5-0.6(Tm)) occurred in the HPT-processed A7075 alloy, where the superplasticity deformation is controlled by grain boundary sliding through grain boundary diffusion.