Continuous recrystallisation of lamellar deformation structures produced by severe deformation

The annealing behaviour of two single-phase aluminium alloys (Al-3%Mg and Al-0.13%Mg) with directional lamellar grain structures, formed by severe deformation, has been compared with predictions from 3-d Monte Carlo-Potts and 2-d vertex network models. The alloys were processed by ECAE (at 20 °C) to a total effective strain of 10, without rotation of the billet. This produced thin ribbon grains in the 3%Mg alloy (width ∼150 nm and aspect ratio ∼7) and coarser lower aspect ratio grains in the 0.13%Mg alloy (width ∼500 nm and aspect ratio ∼2.5). The ribbon grains were cross-linked by transverse low angle boundaries (LAGBs), resulting in a 'bamboo' grain structure. On annealing, no signs of discontinuous recrystallisation were observed. The simulations showed close agreement to the experimental observations and that the early stages of annealing were very sensitive to the density of the less mobile low angle boundaries. The materials were found to rapidly develop an ultra-fine equiaxed grain structure, by the break-up and spheroidisation of the high aspect ratio ribbon grains during the early stages of grain coarsening. Deviation from the ideal grain coarsening exponent was found during this initial period. The models demonstrated that grain break-up resulted from the surface tension of the transverse LAGBs. It is concluded that this occurs dynamically to some extent during severe deformation processing of low solute Al-alloys, resulting in the lower aspect ratio grains found after deformation. © 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.