TEM studies on recovery and recrystallisation in Equal Channel Angular Extrusion processed Al-3%Mg alloy

Abstract

Equal Channel Angular Extrusion (ECAE) is a promising severe plastic deformation (SPD) process which can produce polycrystalline materials with ultra-fine grains (UFG) of sub micrometer range or nanometer range. Large plastic shear deformation induced by the high applied pressure in ECAE material processing is the prime reason behind the grain refinement. The focus of the present work is to study the evolution of dislocation microstructure during dynamic recovery (due to intense strain deformation) and static recovery (due to static annealing after deformation) in commercial Al-3%Mg alloy processed by ECAE. It is observed that local concentrations of shear strain can take place and high angle boundary (HAGB) segments are formed initially at random locations. When thermal energy is provided, during static annealing, the boundary segments get further defined and extended. This leads to the formation of very fine size grains with high mis-orientations which subsequently develop into an ultra-fine grain distribution in the material. Also, it appears dynamic recrystallisation (DRX) occurring during the deformation itself is a general phenomenon leading to refinement of grains. Transmission Electron Microscopy (TEM) is the characterizing tool used in the present study. The influence of precipitates/second phase particles on the deformation characteristics and on the increased degree of grain fragmentation is also detailed.