Superplasticity in a 7055 aluminum alloy processed by ECAE and subsequent isothermal rolling

The superplastic properties and microstructural evolution of a 7055 aluminum alloy was examined in tension at temperatures ranging from 300 to 450 °C and strain rates ranging from 2.7 × 10−5 to 5.6 × 10−2 s−1. A refined microstructure with an average grain size of ∼1.4 μm and a portion of high-angle grain boundaries (HAGBs) of ∼67% was produced in sheets by equal channel angular extrusion (ECAE) followed by isothermal rolling (IR) at 250 °C. The alloy exhibited a maximum elongation-to-failure of ∼820% at a temperature of 450 °C and an initial strain rate of 5.6 × 10−3 s−1. Superplastic elongation of ∼670% was recorded at strain rates higher than 10−2 s−1, where the strain rate sensitivity coefficient was around 0.44. The relationship between superplastic ductilities and microstructure stability was analyzed. It was shown that increasing the portion of HAGBs by IR results in enhancement of superplastic properties.