The generation of large numbers of tangled dislocations and sub-grain boundaries, via dislocation pile-up, was observed in stir-cast alloy by means of transmission electron microscopy. The misorientations between grains were measured by means of selected-area diffraction and convergent-beam electron diffraction. The density and degree of entanglement of the dislocations increased with increasing solid fraction, shear stress and shear rate, as a result of increased plastic strain. Longer isothermal stirring times gave rise to a higher dislocation density and a smaller sub-grain size. The melting of high-angle boundaries, as suggested by the Vogel-Doherty-Cantor grain-boundary melting model, was supported by the observation of sub-grain boundaries that were due to dislocation pile-up during semi-solid stirring.

Generation of Dislocations and Sub-Grains in Stir-Cast Al-6.2Si Alloy during Semi-Solid State Processing J.I.Lee, G.H.Kim, H.I.Lee: Materials Science and Technology, 1998, 14[8], 770-6