Superplasticity and High Strength in Al-Zn-Mg-Zr Alloy with Ultrafine Grains

Herein, the ultrafine-grained (UFG) 7xxx series alloy Al-4.8Zn-1.2Mg-0.14Zr demonstrates superplasticity at unusually low temperatures of 120-170 degrees C while maintaining its high-strength state. The UFG structure is formed by high pressure torsion (HPT) at room temperature (RT), which leads to a considerable increase in the strength characteristics by approximate to 60% compared with the material after conventional heat treatment T6. It is found that the UFG alloy exhibits thermostability when testing or annealing up to 170 degrees C. Deforming by tensile test at a strain rate of 10(-4) s(-1) and 10(-3) s(-1), the elongation to failure at 120 and 170 degrees C exceeds 250% and 500%, respectively, whereas the strain rate sensitivity reaches 0.45, which is a typical value characterizing superplastic deformation. After superplastic deformation, the UFG alloy maintains 25-50% higher strength characteristics at RT than that after conventional heat treatment T6. The origin of such superior behavior of the UFG alloy is discussed.