Microstructural Analysis of Accumulative Roll-Bonded Aluminium

Abstract

This thesis aims to understand microstructural and textural inhomogeneities that develop in the Al layers in accumulative roll bonded sheets during deformation and, then, subsequent annealing. Although extensive research is being underway since the invention of the technique in 1998, our understanding on through thickness inhomogeneities is relatively quite limited. As a result, the existing literature on the crystallographic texture and microstructure sometimes contradict and therefore often become a challenge to achieve a design targeted outcome. In this thesis this area is addressed in a composite sheets consisting of alternating layers of commercially pure (99.8% purity) aluminium and an Al-0.3 wt.% Sc alloy (either in the supersaturated solid solution or age-hardened conditions) that were generated through accumulative roll bonding for up to 5 cycles. Across the transverse sections, the Al layers, based on their location within the composite stack, were examined. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the microstructure and crystallography of the deformed and annealed composites. In the deformation study, Al layers were examined after 1,3 and 5 rolling cycles. An inhomogeneous microstructure and texture developed through the aluminium layers of the sheet thickness; the nature of these inhomogeneities changed as the ARB bonding progressed to higher cycles. Microstructural inhomogeneities remain prominent in the first ARB cycle, but diminish during the subsequent cycles. Texture inhomogeneities appear in different forms as rolling progresses. High frictional shear forces in the sample surface and in-plane shear forces across interlayer bonds generate these inhomogeneities. In the annealing study, the 5-cycle roll-bonded sheets were subjected to annealing at 300 °C for up to 60 minutes. Subsequently, the sheet hardness was measured on the Al layers at different annealing times through Vickers testing. Accordingly, the differences in the recrystallization and grain growth textures in the aluminium layers along the thickness direction were examined after a 30-minute anneal. It was found that during the early stages of annealing the β-fibre rolling textures remain unchanged and then become sustained by a so-called continuous recrystallization process. Then, at the completion of primary recrystallization, orientation selective recrystallization occurs and the Brass orientation becomes eliminated. A strong β-fibre primary recrystallization texture was found to be favourable for secondary recrystallization growth of Cube orientations. Otherwise, conventional grain growth occurs when the primary recrystallization textures have distortions in the β-fibre.