Microstructural evolution during formation of ultrafine grain structures by severe deformation

High resolution electron backscattered diffraction analysis has been used to compare the evolution of the deformed state during severe deformation of two model materials: an Al-0.13 wt-%Mg alloy and an interstitial free steel. The alloys were deformed by equal channel angular extrusion up to a total effective strain of 10, at 20 and 500 degreesC, respectively. At strains of <2, new high angle boundaries were formed from primary deformation bands. At strains of 2-5 (corresponding to high conventional strains) the new high angle grain boundaries, associated with the deformation bands, rotated towards the billet axis creating a lamella structure. Narrow bands of fine grains were also formed in unstable crystal orientations. With increasing strain the separation of the lamella high angle boundaries reduced until at very high strains their spacing was equal to the subgrain width and the long ribbon grains broke up into shorter segments, After an effective strain of 10 the microstructures for both materials consisted of submicron grains with an aspect ratio of <similar to>3, In general it was observed that the interstitial free steel refined more rapidly with strain than the aluminium alloy. MST14767.