In situ investigation of the Columnar-to-Equiaxed Transition during directional solidification of Al–20 wt.%Cu alloys on Earth and in microgravity

Solidification experiments on Al–20 wt.%Cu alloys with grain refiner were performed in a Bridgman-type furnace to investigate the impact of gravity-driven phenomena on the Columnar-to-Equiaxed Transition (CET) during directional solidification (i) in microgravity, on board the sounding rocket MASER-14 and (ii) on Earth in three growth directions, namely horizontal, vertical upward (counter-gravity direction) and vertical downward (in-gravity direction). The CET was provoked by a step increase of the cooling rate and was visualised in situ and in real-time using X-radiography. This paper reports direct observations of dendritic columnar growth, CET and the subsequent equiaxed regime, and quantitative characterisation of the microstructures. The increase in constitutional undercooling that induces the nucleation of the first grains ahead of the columnar front can be related to the rapid thermal change following the increase in cooling rate. The nucleation distance of equiaxed grains ahead of the columnar front is larger in microgravity than on Earth and possible origins for this difference are discussed. Both mechanical and solutal blocking mechanisms were observed at CET. Mechanical blocking is attributed to the occurrence of solidification-induced shrinkage flow. The relative importance of each blocking mechanism is detailed for each growth configuration. In particular, mechanical blocking is shown to be the principal type of impingement in the microgravity experiment. The final equiaxed grain structure was characterised, and the results confirm that gravity effects become less significant at high growth rates.