An Al-Zn-Mg-Cu alloy was friction stir processed over two kinds of backing anvils, at two different cooling rates. A finer grain size, 0.3 vs 0.5 mu m, was obtained by processing at the highest cooling rate. Both materials showed superplastic behavior with a maximum elongation to fracture of about 510%. Grain boundary sliding was the operative deformation mechanism. Furthermore, the finer grain size material showed high strain rate superplasticity, at 10(-2) s(-1), at lower temperatures, as low as 250 degrees C. (C) 2013 Elsevier B.V. All rights reserved.