Impact of solute flow during directional solidification of a Ni-based alloy: In-situ and real-time X-radiography

Abstract Understanding the impact of thermo-solutal convection during alloy solidification is of central importance in the casting industry. Convective flow generated from density variations in the liquid is at the origin of many casting defects, such as chemical segregations and freckles, to which Ni-based superalloys used in jet engines are known to be highly sensitive. In this article, we present experimental results obtained during directional solidification of a CMSX-4 single crystal superalloy observed in-situ by means of synchrotron X-radiography at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The use of a high-temperature directional solidification furnace, an intense monochromatic beam and a high-speed X-ray sensitive camera allowed the direct observation of the development of solute flow to be performed during dendritic growth of a Ni-based alloy. The transient stage leading to the formation of a dendrite network and the interaction with the concomitantly developing convective pattern are studied. The impact of solute flow on the dendrite growth velocity is characterized for three cooling rates. Measurements of dendrite tip positions show that the convective flow above the solidification front induces a periodic oscillation of the dendrite growth velocity and that the oscillation amplitude decreases for higher cooling rates. The development of residual liquid areas is observed on the sides of the sample. Fragmentation of dendrite parts occurs during the solidification of the residual liquid areas and leads to the formation of spurious grains. The experimental results are analyzed by using a Rayleigh number-based model, highlighting the need for direct simulations in the future.