PROPAGATIVE PLASTICITY IN COMMERCIAL ALUMINIUM ALLOYS

摘要

An experimental study on the formation and evolution of strain inhomogeneity in the form of deformation bands in polycrystalline sheet metals is presented. A whole field in-plane deformation measurement technique based on laser speckles has been applied for in situ monitoring the development and propagation of plastic deformation patterns in Al-Mg (AA5082), Al-Cu (AA2219) and Al-Zn-Mg (AA7020) sheet metals. This strain inhomogeneity has been characterized for its width, velocity and angle with the tensile axis. Band width is found to increase steadily with increasing elongation during a uniaxial tensile test and decrease as the band becomes stagnant to form a shear band. In stretching the ability of a metal to resist strain localisation and hence local thinning, is the most important forming property. The uniform strain represents in this regard a most critical factor to describe stretching ability - especially when the material under consideration has a negative strain rate sensitivity. The knowledge obtained with the laser Speckle Technique was useful for understanding the underlying mechanisms for the formability limit when negative strain rate sensitivity is active. The goal was to understand the relationship between Portevien - Le Chatelier / dynamic strain ageing and the formability limit physically manifested as shear band formation.