Microstructure and flow stress of polycrystals and single crystals

The relation between polycrystal deformation and single crystal deformation has been studied in this work. A pure aluminium polycrystal having an average grain size of 300 mu m has been strained in tension at room temperature. The flow stress has been determined at four different strains (0.05, 0.14, 0.22 and 0.34) and deformation microstructures have been characterized qualitatively and quantitatively by transmission electron microscopy. Improved experimental techniques have allowed large foil areas to be characterized and in total 89 grains have been examined. At the four strains examined a classification of the deformation microstructures into three different types has shown a correlation between the grain orientation and the type of deformation microstructure which develops during straining. The dislocation density has been calculated at each strain and by assuming that the shear stress is proportional to the square root of the dislocation density the shear stress-strain relationship has been derived for each of the three groups of grains showing different deformation microstructures. The stress-strain curves show a strain hardening behaviour which depends on the orientation of the grain. The behaviour of the grains embedded in the polycrystal is compared with the behaviour of single crystals and the stress-strain curve of the polycrystal is estimated with good accuracy from single crystal data, which are weighted based on a quantitative texture analysis of the polycrystal. (C) 1998 Acta Metallurgica Inc.