X-Ray Monitoring of Solidification Phenomena in Al-Cu Alloys

In-situ synchrotron X-ray radiography has been used to study columnar and equiaxed dendritic growth in directional solidification of Al-Cu alloys employing a Bridgman furnace. Nominal spatial and temporal resolutions of 1.5 µm and 150 ms, respectively, were obtained with a 1.3 %1.3 mm2 field of view, and a signal-to-noise above 99.5 %. Dedicated processing software has been developed to allow for quantitative extraction of data such as solid-liquid interface morphology, local propagation velocities and constitutional gradients from the images. The data collected also contain unprecedented in-situ observations on dendrite fragmentation. The limited field of view together with a slight sample position dependency in the heat transfer coefficients made it necessary to impose thermal gradients, G > 10 K/mm, to have reproducible solidification processes. Non-vanishing horizontal G-components contributed to convection that at some occasions resulted in observable effects on growing crystals or on liquid segregates.