Generation of dislocations and subgrains in stir cast AI-6.2Si alloy during semisolid state processing

Abstract

The generation of a large number of tangled dislocations and subgrain boundaries by dislocation pileups in the stir cast AI-6'2 wt-%Si alloy was observed by a transmission electron microscopy (TEM) study. Misorientations between grains were measured by selected area diffraction (SAD) and convergent beam electron diffraction (CBED) patterns. The density and entanglement of dislocations increased with higher solid fraction (or shear stress) and higher shear rate as a result of increased plastic strain. Longer isothermal stirring time gave rise to higher dislocation density and smaller subgrain size. The meltoff of high angle boundaries, which was suggested by Vogel-Doherty-Cantor's grain boundary melting model, is supported by the observation of subgrain boundaries owing to dislocation pileups during semisolid stirring.