In-situ X-ray radiographic observations of eutectic transformations in Al-Cu alloys

In-situ X-ray radiography is currently being used to study solidification processes in a wide range of binary and ternary alloy systems. In this study, thin samples of hypoeutectic Al-Cu alloys were mounted in a Bridgman furnace and solidified near-isothermally from above the liquidus to below the eutectic. The eutectic transformation was easily identified from a 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-ray radiography, and image analysis techniques, it was possible to directly observe and 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 to the eutectic transformation rate and the results compared to theoretical predictions. Reasonable agreement was observed and some interesting observations are noted, e.g. liquid feeding induced grain motion and the possible precursor to hot tearing.