4D synchrotron X-ray tomographic quantification of the transition from cellular to dendrite growth during directional solidification

Solidification morphology directly impacts the mechanical properties of materials; hence many models of the morphological evolution of dendritic structures have been formulated. However, there is a paucity of validation data for directional solidification models, especially the direct observations of metallic alloys, both for cellular and dendritic structures. In this study, we performed 4D synchrotron X-ray tomographic imaging (three spatial directions plus time), to study the transition from cellular to a columnar dendritic morphology and the subsequent growth of columnar dendrite in a temperature gradient stage. The cellular morphology was found to be highly complex, with frequent lateral bridging. Protrusions growing out of the cellular front with the onset of morphological instabilities were captured, together with the subsequent development of these protrusions into established dendrites. Other mechanisms affecting the solidification microstructure, including dendrite fragmentation/pinch-off were also captured and the quantitative results were compared to proposed mechanisms. The results demonstrate that 4D imaging can provide new data to both inform and validate solidification models

In situ experimental observation of the time evolution of a dendritic mushy zone in a fixed temperature gradient

10th International Meeting on Thermodiffusion (IMT), Univ Libre Bruxelles (ULB), Brussels, BELGIUM, JUN 04-08, 2012International audienceThis paper describes a series of experiments performed on BM05 at the European Synchrotron Radiation Facility (ESRF), dedicated to the analysis of a mushy zone evolution in a fixed temperature gradient. A mushy zone is the partially solid/partially liquid zone that is formed when solidification proceeds with the development of dendrites, and it has been recently shown that synchrotron X-ray radiography is a powerful technique, perfectly adapted for such a study. In situ and real-time characterisation clearly evidences the microstructural changes of the mushy zone during the holding stage, and measurements of the mushy zone boundary positions using image processing enable us to analyse the successive regimes. Each regime is directly related to solute diffusion mechanisms, namely temperature gradient zone melting, solute diffusion in the inter-dendritic liquid channels due to the solid-fraction gradient and solute diffusion in the melt