The Portevin-Le Chatelier (PLC) effect and shear band formation in an AA5754 alloy

On the sequence of inhomogeneous deformation processes occurring during tensile deformation of strip cast AA5754

In this article, we study the Portevin-Le Chatelier (PLC) bands and their influences on strain localization and fracture in continuous cast (CC) AA5754 aluminum sheets. Three types of tensile tests are conducted: (1) tensile samples are pulled directly to fracture at 223 K, (2) tensile samples are pulled at 223 K to initiate diffuse necking followed by unloading and reloading to fracture at room temperature, and (3) tensile samples are pulled at 223 K to localized necking and unloaded followed by reloading to fracture at room temperature. Furthermore, in situ V-bending test coupled with deformation mapping using digital image correlation is used to study damage at large strains. The results show that PLC bands detect favorable geometrical sites for shear band initiation. The formation of shear bands precedes damage and damage is a consequence of shear band formation.

Shear Localization and Damage in AA5754 Aluminum Alloy Sheets

Two Al–Zn–Mg alloys with recrystallised and fibrous grain morphologies are studied with regards to the microstructure after solution heat treatment, cold water quenching and immediate room temperature deformation. It was found that the dislocation movement was localised in narrow slip bands cutting through the dislocation tangles. This observation is related to dynamic strain ageing and to macroscopic shear bands frequently observed in these alloys.

Strain localisation in solution heat treated Al–Zn–Mg alloys

The main objective of this work is to understand the major deformation mechanisms operative in the entire plastic strain regime of an ultrafine-grained (UFG) 5024 Al alloy. The initial microstructure in both coarse-grained (CG) and UFG conditions was characterized by transmission electron microscopy and orientation imaging microscopy to better correlate the observed deformation behavior with the microstructural elements. The engineering stress–strain curves in both CG and UFG conditions were serrated from onset of yielding to the point of sample failure although the net intensity of the serrations varied in both the conditions. Interrupted tensile testing with corresponding surface analysis using scanning electron microscopy (SEM) was carried out to clearly demarcate the micro-mechanisms operative in various plastic strain regimes. The serration amplitude as a function of time in UFG material was studied and displayed a different behavior when compared with CG material. The reason for higher non-uniform elongation has been explained in terms of micron-sized shear bands and void coalescence mechanism. Dislocation slip was observed to be the major strain accommodation mechanism, along with shear bands and grain rotation. Analysis of failed specimen surfaces using SEM further assisted in understanding and quantifying the damage accumulation and failure mechanism.

K. Narasimhan

Papers

Heterogeneous initiation and growth of sample-scale shear bands during necking of Al-Mg sheet