Combined in situ X-ray radiographic observations and post-solidification metallographic characterisation of eutectic transformations in Al-Cu alloy systems

In this study thin Al-Cu hypoeutectic alloys were mounted in a Bridgman furnace and solidified near isothermally from above the liquidus to below the eutectic temperature. The eutectic transformation is easily identified from the X-ray absorption contrast difference between the semi-solid mush and the fully solid phase in the field of view (FOV). By virtue of real time in situ X-radiography, and subsequent image analysis techniques, it was possible to directly observe and accurately measure eutectic nucleation, as well as transformation rate across the FOV. Post-solidified samples were then subjected to further microstructural analysis, whereby the lamellar eutectic spacings were measured at random locations across the FOV region. The lamellar spacings were correlated with the eutectic transformation rate and showed reasonable agreement with theoretical predictions. Some interesting observations, such as, for example, in situ observations of solidification shrinkage and the effect of liquid feeding through the semi-solid mush, are also discussed.