LENGTH SCALE EFFECTS ON DEFORMATION MICROSTRUCTURE FORMATION IN THE NEAR-MICROMETRE GRAIN SIZE REGIME

摘要

The effect of grain size on deformation microstructure formation in the near-micrometre grain size regime has been studied using samples of aluminium prepared using a spark plasma sintering (SPS) technique. By careful control of the SPS process samples have been prepared with a fully recrystallized grain structure with average grain sizes ranging from 6 μm to 0.7 μm. Examination in the transmission electron microscope of samples compressed at room temperature to approximately 20% reduction reveals that deformation results in a process of subdivision by dislocation rotation boundaries down to a grain size of approx. 500nm. A number of transitions in the appearance of the deformation microstructure are seen, however, with decreasing grain size, from well defined cell-blocks in grains of size down to approximately 5 μm, to dislocation cell structures in grains of size less than 1 μm, suggesting that cell block formation becomes progressively more difficult with decreasing grain size. Observations of dislocations within individual boundaries, together with stress-strain curves for the samples, provide further evidence for changes in the slip pattern in the near- micrometre grain size regime.