Real Time Imaging Of Dendrite Growth In Metallic Alloy By Synchrotron Radiation

Dendrite growth in metallic alloy during solidification was observed by synchrotron radiation technique, which has high energy, high brightness, good monochromaticity, worked with high read and write speed and high resolution CCD system. Dendrite growth, dendrite coarsening, and dendrite growth under DC of Sn-12wt.%Bi alloy were studied at Beijing synchrotron radiation facility and Shanghai synchrotron radiation facility with an updated synchrotron radiation imaging technique. A series of growth behavior and morphology evolution of dendrite have been in situ observed, such as columnar-to-equiaxed transition, dendrite competition, and dendrite fragmentation & floating etc. The images obtained in 100μm thick samples reveal the dynamic process of dendrite growth and allow quantitative measurement and analysis.It is shown by in situ observations that different coarsening mechanisms of secondary dendrite arm might operate under different cooling conditions or solidification stages. Coarsening mainly happened in early solidification stage realized by the competitions, and therefore, secondary dendrite arm spacing (DAS) is mainly determined in this stage. Coalescence was the dominant coarsening mechanism in late solidification stage. In particular, at lower cooling rate, e.g.1.5℃/min, dendrite fragmentation frequently occurred, and played an important role during dendrites coarsening. When alloy solidified at the same cooling rate with different temperature gradients, the time taken from the beginning of solidification to forming the final secondary arm spacing was different, but the final secondary arm spacing were almost same. The real time observations in real alloy can give direct proofs to verify the dendrite arm coarsening models.Synchrotron radiation imaging technique was used to in situ observe the dendrite growth of a solidifying Sn-Bi binary alloy under a direct current (DC) electric field. By applying a DC (7-32 A/cm2), the dendrite branching was suppressed, the dendrite tip was modified to be round or flat, and no tertiary dendrite was found. With increasing DC density, the dendrite morphology was changed from columnar dendritic to equiaxed cellular to equiaxed dendritic. In particular, the primary dendrite branched following a tip-split manner in a higher intensity DC. These results can offer the direct proofs to verify or improve the solidification theories of metallic alloy. This research opens a novel window to the study of alloy solidification and enables the unambiguous understanding of solidification processes in optically opaque, metallic alloys.