Dendrite growth morphologies in aluminium alloys

Different aluminium alloys, in particular Al–Cu, Al–Mg and Al–Si, have been directionally solidified under well-controlled thermal and convection conditions. For relatively high solidification rates, particular growth morphologies were observed. The most common is linked with the formation of feathery grains: these are constituted by series of twinned lamellae, in which the dendrites have 110 trunks cut through by {111} twin planes. These grains undergo a selection mechanism which is similar to that occurring for regular 100 dendritic grains. The dendritic growth along 110 crystallographic directions is supposed to be due to a change in the anisotropy of certain properties of the alloy, such as the solid/liquid interfacial energy and/or the atom attachment kinetics. When solidification conditions become less favourable to 110 growth morphologies, a mixed dendritic form containing 110 trunks and 100 arms may be obtained. In the case of the 5182 Al–Mg type alloy, 110 columnar grains which were not twinned could be observed together with feathery crystals. The possibility of such changes in the growth direction of aluminium alloys was further demonstrated by the observation of dendrites of Al–Cu–Mg solidified in a Bridgman device. In this case, 112 dendrites grow and progressively change their growth direction, thus showing curved shapes.