Year

2016

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

Ultra-fine grained (UFG) materials have a number of desirable properties that make them ideally suited to industrial applications. Such materials can be fabricated using severe plastic deformation (SPD) techniques, which have been the subject of extensive research. Accumulative roll bonding (ARB) is a relatively new SPD technique regarded as being suitable for industrial applications. While UFG materials have already been produced by ARB from various pure metals and alloys, aspects such as their deformation behaviour and the evolution of their texture due to changes in the strain path are still not well understood. Also, modelling of the texture evolution during ARB is rarely reported. Therefore a systematic study of the deformation and evolution of the texture of materials subjected to the ARB process is needed, using both numerical and experimental methods.

In the present work, single crystal aluminium samples processed experimentally by ARB offered a unique opportunity to trace texture evolution and deformation behaviour. Electron backscatter diffraction (EBSD) technique was used to measure texture. Slip trace, texture, and disorientation were determined from EBSD maps. It was found that the final slip trace is the competing result of preserved slip traces and newly imposed deformation caused by change in the strain path due to cutting and stacking in ARB. The ARB process was also applied to commercial purity and high purity polycrystal aluminium under different rolling conditions. Texture and grain refinement were determined from EBSD maps. Grain refinement was found to be rapid during the ARB process using a roller with high surface roughness, and texture is a combination of rolling and shear texture. In high purity aluminium, there were large anomalous grains without orientation gradient.

A finite element method (FEM) model of the ARB process was developed. The FEM simulation was carried out up to 5 cycles using an elasto-plastic constitutive relationship. The distribution of strain was obtained, from which hardness was predicted and validated by experimental measurement. A crystal plasticity finite method (CPFEM) model was developed to predict how the texture would evolve. Crystal rotation in a cold rolled single crystal with different initial orientations was investigated. The effects of the rolling condition and initial orientation on texture evolution were also investigated. Simulations of a single crystal model of the ARB process with initial orientations of Cube, C, S, and (15 12 5)[9 10 ̅̅̅̅ 3̅ ] were carried out. The simulations successfully captured the texture measured experimentally. Partial texture reversal was revealed by alternating the activated slip systems; this is consistent with cutting and stacking in ARB. A polycrystal model was developed and implemented into the CPFEM to simulate the ARB process of aluminium polycrystal. The predicted texture agrees with the experimental results. The effects of asymmetrical rolling, initial texture and neighbouring grains on the evolution of texture were investigated.

This thesis is unavailable until Wednesday, October 19, 2022

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