Columnar dendritic solidification of TiAl under diffusive and hypergravity conditions investigated by phase-field simulations

Based on 2D phase-field simulations including fluid flow driven by natural convection, columnar dendritic growth of the beta-solidifying Ti-48 at.%Al alloy is characterised for different gravity levels ranging from 0 to +/- 15 g. Depending on the direction of the gravity g with respect to the growth direction, different flow regimes emerge which show stable or unstable dendritic growth dynamics. When gravity and growth directions are parallel, the dendrite tips experience downward melt flow and individual dendrites grow in a stable manner with a rather small modification of the operating state. When gravity and growth directions are antiparallel, the impact on the operating state is larger. Eventually, at higher gravity levels the upward melt flow around the dendrite tips "destabilises" the dendritic morphology resulting in tip splitting, branching and local changes in the apparent dendrite growth direction which is an alternative mechanism for the adjustment of the primary dendrite arm spacing in addition to tertiary arm formation.