Directional solidification of Al-1.5Â wt% Ni alloys under diffusion transport in space and fluid-flow localisation on earth

We present results of a comparative experimental study in Space (diffusion transport) and on Earth (convection and diffusion transport) of solute macrosegregation and directional solidification microstructure in bulk Al-1.5 wt% Ni alloys, which possess a stabilising solutal effect in directional upward solidification on ground. The growth parameters are in the cell-dendrite transition so that cells (no sidebranches) and dendrites (sidebranches) are simultaneously observed, the later having larger spacing. In the benchmark microstructures solidified in microgravity, it is found for the first time that the conditions for the analogy with viscous fingering are satisfied (spacing Peclet number ≪1 and tip constitutional supercooling of order one). The experimental primary spacing is about half the numerical prediction by Lu and Hunt's model. It is suggested that the full panorama of the symmetric and non-symmetric cellular and dendritic branches may explain the misfit. Also, the longitudinal solute macrosegregation across the mushy zone is discussed and modelled, which imposes to depart from the classical Flemings profile. Due to fluid flow driven by the radial temperature gradient, the ground samples show a large macroscopic deformation of the growth front concomitant with microstructure localisation. Cells then appear in the core and dendrites at the periphery. In addition, the measured primary spacing in the centre is smaller than in the corresponding microgravity experiments, which enables to estimate the downward component of the flow velocity in this region.