Strain Localization Observed during Shearing of Some Aluminium Alloys and Texture Softening Predicted by FC Taylor and Advanced Lamel Model

Abstract

In-situ SEM shearing tests were performed on samples from the heavily cold rolled (Extrahard) aluminium alloys, where the parallelepiped test sample was cut as to let shear direction (SD) have an angle α with the rolling direction (RD). This shear angle ranges from 0o to 165o with an interval of 15o. These include three heavily cold rolled non heat-treatable aluminium alloys AA1200, AA3004 and AA5182. During these tests, strain localization (macro-shearbands) was bserved. This phenomenon is found to be anisotropic and depends on the angle α. The strain localization or macro-shearbands are believed to be related with strain softening, where the flow stress decreases with strain. According to the crystal plastic theory, the strain softening is considered as resulting from the joint effects of texture and evolution of microstructure, in particular the dislocation patterns. Focusing on texture softening, simple and advanced Taylor type micro-mechanical simulations (Full-constraint Taylor (FC Taylor) and Advanced Lamel models (Alamel)) are performed to calculate the texture and average Taylor factor evolution with the increment of shear strain, on the basis of the measured rolling textures. After the simulations, the shear strain at which texture softening happens is recorded for each alloy and each shear angle. For alloys AA3004 and AA5182, it is found the texture-softening trend is similar to the experimental observations, which showed that the strain localization starts at smaller strains at shear angles of around 30-60o and 120-150o, finally leading to early failure. On the contrary, for alloy AA1200, the calculated average Taylor factor evolution does not resemble the flow behaviour. Furthermore the conclusions for alloys AA3004 and AA5182 are only qualitative, as the value of texture-softening strains predicted by simulation seems different from the observations. This shows that the importance of other effects such as possible microstructural softening mechanisms, especially the one due to the change of strain path (rolling/shear). Then for future models, it will be necessary to incorporate both the texture effects and microstructural effects comprehensively in order to precisely predict the strain localization behaviour of materials.