Effect of sub-boundaries on primary spacing dynamics during 3D directional solidification conducted on DECLIC-DSI

To characterize the dynamical formation of three-dimensional (3D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of a series of experiments on a transparent succinonitrile-0.24 wt% camphor model alloy was carried out under low gravity in the DECLIC Directional Solidification Insert onboard the International Space Station. The continuous interface observation enables to construct space-time evolution maps of cell location and primary spacing. Both convergent and divergent sub-boundaries are identified and new insights on their effects on the spatiotemporal evolution of the pattern are thus evidenced. 3D phase-field simulations that reproduce the experimental sub-boundary configurations are performed to support the analyses. Even for the low angle sub-boundaries studied, the primary spacing increases or decreases in the vicinity of the boundary respectively for divergent and convergent sub-boundary. This effect may extend on a long distance within the different sub-grains and its magnitude depends on the average primary spacing and its positioning relative to the limits of the stability band. On the sample scale, the primary spacing profile is also influenced by the presence of sources and sinks at the crucible wall due to the pattern drift. Their type and distance from the sub-boundaries give rise to complex spatial distributions of primary spacing over the entire sample.