MICROSTRUCTURE AND FLOW-STRESS OF POLYCRYSTALS AND SINGLE-CRYSTALS

The relation between polycrystal deformation and single crystal deform ation has been studied in this work. A pure aluminium polycrystal havi ng an average grain size of 300 mu m has been strained in tension at r oom temperature. The flow stress has been determined at four different strains (0.05, 0.14, 0.22 and 0.34) and deformation microstructures h ave been characterized qualitatively and quantitatively by transmissio n electron microscopy. Improved experimental techniques have allowed l arge foil areas to be characterized and in total 89 grains have been e xamined. At the four strains examined a classification of the deformat ion microstructures into three different types has shown a correlation between the grain orientation and the type of deformation microstruct ure which develops during straining. The dislocation density has been calculated at each strain and by assuming that the shear stress is pro portional to the square root of the dislocation density the shear stre ss-strain relationship has been derived for each of the three groups o f grains showing different deformation microstructures. The stress-str ain curves show a strain hardening behaviour which depends on the orie ntation of the grain. The behaviour of the grains embedded in the poly crystal is compared with the behaviour of single crystals and the stre ss-strain curve of the polycrystal is estimated with good accuracy fro m single crystal data, which are weighted based on a quantitative text ure analysis of the polycrystal. (C) 1998 Acta Metallurgica Inc.