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 °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° 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 β-fibre. Uniaxial tests conducted at 300 °C and 350 °C revealed that the operating deformation mechanism in the processed alloy at such temperatures was grain boundary sliding; the optimum superplastic strain rate being 3×10−3-10−2 s−1. Boundary misorientation and thermal stability are the two main factors that contribute to high elongations to failure.