Evolution of the microstructure, texture and creep properties of the 7075 aluminium alloy during hot accumulative roll bonding

The 7075 Al alloy was severely deformed at 350 degrees C by a 3:1 thickness reduction per pass accumulative roll bonding (ARB) process up to six passes. It was found that discontinuous recrystallisation occurs during the inter-pass annealing stages from the third pass on, attributable to the increment of the mean particle size during processing. As a consequence, the mean crystallite size did not decrease, but remained approximately constant at 440 nm along the present ARB process and the mean boundary misorientation angle reached a maximum of 30 degrees for the 3-passes sample. However, since nucleation of new grains takes place at the pre-existing grain boundaries, discontinuous recrystallisation results in slight changes in texture throughout the processing, being the orientations in the ARBed samples predominantly located along the typical rolling beta-fibre. Uniaxial tests conducted at 300 degrees C and 350 degrees C revealed that the operating deformation mechanism in the processed alloy at such temperatures was grain boundary sliding; the optimum superplastic strain rate being 3 x 10(-3)-10(-2) s(-1). Boundary misorientation and thermal stability are the two main factors that contribute to high elongations to failure. (C) 2014 Elsevier B.V. All rights reserved.