Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The dendrite coarsening kinetics and dendrite morphology have been of great interest in the solidification science and casting industry. A detailed analysis of particular solidification phenomena (coalescence, fragmentation etc.) requires X-ray techniques with a high spatial and temporal resolution. High resolution experimental data are also very important for the verification of the existing microstructural models. The synchrotron radiography experiments with solidifying Ga - In alloys were performed at BM20 and ID19 (ESRF, Grenoble) at a spatial resolution of < 1 µm. The temporal dynamics of morphological transitions such as retraction, coalescence and pinch-off of the sidearms were studied in-situ. Recently, we showed that the combination of numerical modelling [1] and experiments [2,3] performed at the ESFR synchrotron X-ray source in Grenoble has allowed to improve the understanding of the pinch-off process of dendritic sidearms and to obtain material information that is relevant for quantitative modelling.
In this work, a Ga–In alloy was solidified in vertical direction starting from the top of the solidification cell at a controlled cooling rate of 0.002 K/s and at a vertical temperature gradient of ~2 K/mm. In general, all fragmentation events are located in the deceleration zone that is formed during the initial phase of solidification. Behind an advancing growth front, under slow growth conditions that are almost close to steady state conditions, the coarsening in the mushy zone does not involve a significant detachment of sidearms.
A detailed and advanced image analysis in combination with the high temporal and spatial resolution data of the experiment, allowed us to identify an additional migration process that is influenced by the existing temperature gradient. This Temperature Gradient Zone Melting (TGZM) process is characterized by a sidearm migration rate of 0.01 µm/s. Interestingly, the results of our analysis suggest that this process does not play a significant role for the sidearm detachment process itself.
References
1. H. Neumann-Heyme, et al. PHYS. REV. E, 92 (2015) 060401
2. Shevchenko et al., IOP Conf. Series: Mat. Sci. and Eng. 228 (2017), 012005
3. Neumann-Heyme, Shevchenko et al., Acta Mater. 146 (2018) 176