Influence of growth velocity on fragmentation during directional solidification of Al-14 wt.% Sn alloy studied by in-situ synchrotron X-radiography

Directional solidification of Al - 14 wt.% Sn alloy was investigated by means of synchrotron X-radiography at the European Synchrotron Radiation Facility (ESRF). The dynamics of the solidification microstructure formation was studied in-situ and in real-time. In the present manuscript, a special attention is given to the influence of increasing growth velocity on the fragmentation phenomenon of dendrite arms, which has an important role and impact on the final grain structure. Fragmentations of both secondary dendrite arms and more strikingly part of primary trunks have been observed. The experiments show that the number of fragments increases concomitantly with the solidification growth rate until it reaches a limit value after which fewer or no more fragments are observed. The possible origins of the fragmentation are discussed in detail and the likelihood of each of these potential mechanisms is estimated to understand the reason of both primary and secondary arm detachment. It is shown that the fragmentation in a transient regime is correlated to the variations of the solute accumulation, latent heat rejection and the curvature of the attachment neck during solidification. Moreover, during these experiments, Columnarto-Equiaxed Transition (CET) induced by the fragmentation phenomenon is observed. The velocity of the dendrite fragments due to the buoyancy force is a major parameter to control the CET and our results show that a reverse Equiaxed-to- Columnar Transition can even be observed. Once the growth velocity is of the same order or higher than the fragment flotation velocity, the fragments are trapped within the columnar dendritic microstructure, and CET cannot take place or the equiaxed regime cannot be sustained. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.