Serration behavior and shear band characteristics during tensile deformation of an ultrafine-grained 5024 Al alloy

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.