Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion-compression

Polycrystalline pure aluminium (99.99%) has been deformed at room temperature by the Cyclic-Extrusion-Compression (CEC)-method to strains in the range 0.9-60 (1-67 cycles). At different strains, the microstructure and local crystallography have been characterised in particular by transmission electron microscopy. It has been found that the microstructure develops from a cell block structure into an almost equiaxed structure of cells and subgrains, that the spacing between the boundaries subdividing the structure is almost unaffected by the strain and that the misorientation across these boundaries increases with the strain over the whole strain range. At the largest strain, the average misorientation across the deformation induced boundaries is similar to 25 degrees. The flow stress in compression is measured after the cyclic deformation and it is found that the flow stress increases with strain towards a saturation level which is reached at a relatively low strain. The discussion comprises the effect of deformation mode and plastic strain over a large strain range on the microstructural evolution and mechanical behaviour of aluminium. (C) 1999 Elsevier Science S.A. All rights reserved.