Twinned feathery grains and related morphologies in aluminum alloys

Al-Cu, Al-Mg and Al-Si alloys without grain refiner addition have been directionally solidified under well-controlled thermal and convection conditions. For relatively high solidification rates, several changes in the dendrite growth morphology were observed. One of the most common structure that may appear under such conditions is called `feathery grains': it was demonstrated recently that this lamellar structure is constituted of 〈110〉 dendrites whose trunks are cut through by {111} twin planes. These grains undergo a selection mechanism which is similar to that occurring for regular 〈100〉 dendritic grains. The transition between 〈100〉 and 〈110〉 dendrite forms is progressive and morphologies that develop arms in both type of crystallographic directions can be observed. Moreover, in the case of a 5182 Al-Mg alloy, regular 〈110〉 dendrites, without twins, could also be obtained. This 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. The possibility of growth along other directions than the usual 〈100〉 was further demonstrated by the observation of dendrites developing in an Al-Cu-Mg solidified in a Bridgman device. In this case, 〈112〉 trunks form and progressively change their growth direction, thus showing curved shapes.