Plastic Deformation and Work Hardening of Al and AA5754 Al Alloys

Abstract

<p>This research has been carried out to understand the relation between work hardening, deformation behaviour, dislocation substructure and crystallographic texture developed during plastic flow in pure Al and continuous cast and direct chill cast AA5754 AI-Mg alloys. Tensile and strain rate sensitivity tests are performed in the temperature range 4.2K-295K; details of the process of dislocation accumulation, the dislocation substructure development and texture evolution are followed using a range of techniques such as TEM, in-situ resistivity measurements and X-ray diffraction. The tensile deformation results show an unprecedented level of strength and unusual work hardening behaviour of AI-alloys at 4.2K. The electrical resistivity data suggest that fracture is initiated by the collapse of the dislocation network at places where dislocations develop a critical spacing for spontaneous annihilation. This spacing is estimated at approximately 8nm at 4.2K for both pure Al and Al alloys and rv12nm at 78K for pure AI. Strain rate sensitivity measurements suggest that deformation of high purity Al is governed by dislocation-dislocation interactions in a broad range of temperatures, whereas Al alloys exhibit a larger thermal component of flow stress due to the presence of solute atoms in the matrix. The texture studies show that, independently of initial texture, the tensile axis of deformed samples rotates to (111) stable end orientation, and the intensity of the (111) peak increases parabolically with the flow stress, also strongly affected by the temperature. The experiments carried out on samples oriented at different angles to the rolling direction indicate that the crystallographic texture is the major factor responsible for the anisotropy of the alloys' mechanical properties. This is attributed to the effect of the operating slip systems activated during tensile deformation. The Visco-Plastic Self Consistent modeling has been carried out to predict mechanical behaviours of AA5754 alloys and to provide insight into operating mechanisms of plastic flow as well as the origin of the anisotropy of mechanical properties. These results indicate that the more effective dislocation storage in the substructure at a low temperature significantly increases the work hardening rate at the later stage of deformation, and the crystallographic texture is the major cause of the anisotropy in the flow stress behaviour.</p>