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 cond itions. For relatively high solidification rates, particular growth morphol ogies were observed. The most common is linked with the formation of feathe ry grains: these are constituted by series of twinned lamellae, in which th e dendrites have [110] trunks cut through by {111} twin planes. These grain s undergo a selection mechanism which is similar to that occurring for regu lar [100] dendritic grains. The dendritic growth along [110] crystallograph ic directions is supposed to be due to a change in the anisotropy of certai n 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 gr owth direction of aluminium alloys was further demonstrated by the observat ion 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. (C) 1998 Acta Metallurgica Inc. Published by Elsevi er Science Ltd. All rights reserved.