Serrated flow in powder metallurgy Al–5%Mg–1.2%Cr alloy processed by equal channel angular pressing

The microstructure, texture and mechanical behavior of the powder metallurgy Al–5 wt.%Mg–1.2 wt.%Cr alloy subjected to equal channel angular pressing (ECAP) has been investigated. The material processed by ECAP, as well as in the homogenized condition, exhibited room temperature serrated flow (SF) up to fracture. The critical stress for the serration onset decreased with increasing strain rate or ECAP temperature. The results indicated that this SF was induced by shear banding. The stress oscillations were attributed to the interaction between shear bands (SBs) and obstacles like second phase particles, and dislocation locks produced by strain hardening. The early stages of the stress–strain σ–ε curves of the ECAP processed samples showed a transition from type B serrations with continuous strain hardening to type B serrations superimposed on a succession of constant stress plateaus when the tensile strain rate was increased from 10− 4 s− 1 to 10− 3 s− 1. The plateaus in σ–ε curves obtained at a strain rate of 10− 3 s− 1 were ascribed to the nucleation of a band at one end of the sample gauge region and subsequent propagation towards the opposite end. At a low strain rate of 10− 4 s− 1 the sites for band nucleation should be randomly distributed along the sample gauge region. The disappearance of the plateaus in the σ–ε curves are attributed to the activation of a new moving band before the completion of the deformation banding cycle of the preceding band.