TIME-RESOLVED X-RAY IMAGING IN STUDIES OF ADVANCED ALLOY SOLIDIFICATION PROCESSES Ragnvald H. Mathiesen 1 and Lars Arnberg 2 1 SINTEF Materials and Chemistry, N-7465 Trondheim, Norway. 2 Dept. of Materials Technology, NTNU, N-7491 Trondheim, Norway. INTRODUCTION During solidification of metallic alloys growth patterns form dynamically at the solid-liquid (s-l) interface as archetypes of self-organization in systems away from equilibrium and belong to non-linear physics. These self-assembly solidification structures evolve in complex processes determined by the transport of heat and mass to and from the different constituents. The transport processes are also influenced and complicated by interacting mechanisms such as convection, solid flow and melt segregation. Physics describing a full scale casting must account for phenomena occurring over lengths from the atomic (interface attachment kinetics, nucleation) to the macroscopic (macro segregation), over a typical time-scale from microseconds (diffusion-controlled growth) to minutes (coarsening), e.g. [1-7]. While numerical computer modeling for simulations covering the range from solidification fundamentals to full scale castings has advanced considerably over the last couple of decades, provision of new experimental data to guide theory and modeling has fallen behind. In the past in-situ studies of evolving solidification microstructures and process phenomena has been limited to work with optically transparent organic model systems [8-10], that display cellular and dendritic growth patterns analogous to those common to metals. Owing to a density shift across the s-l interface, model system growth can be monitored by video microscopy. Such studies have been extensive and contributed to promotion of consistent theory with provision of benchmark data for advancement in modeling [11-14]. However, despite their importance, the organic systems fail as full analogs to metals and alloys. Firstly, the variety in suitable systems is limited and far from complete as generic representatives for the multitude of growth patterns relevant to binary and multi component alloys. Also, since most physical properties decisive to the solidification processes are distinctively different between the models and their metal counterparts, e.g. heat conductivities and capacities, freezing temperatures, viscosities, solute/solvent mass ratios, etc, many fundamental aspects in solidification science are either inaccessible or impossible to scale from model observations to real systems of interest. Metal transparency and interaction with X-rays constitute candidate principles from which methods for in-situ monitoring of solidification processes could be constructed. However, source brightness and detection efficiency has limited the practical impact of X-rays as a diagnostics tool for studies at physically relevant time- and length scales (ms, μm). The first X-ray investigations were based on radiography with conventional sources and used for in-situ studies of solute redistribution and boundary layer propagation[15,16]. The geometrical resolutions obtained, r g > 50 μm, prevented studies with curved fronts, but the resolution by contrast was adequate to verify proximity to conditions where solute diffusion in the solid can be neglected (Scheil-conditions). Already in their early work Stephenson and Beech [16] demonstrated the influence of buoyancy convection on the solute boundary layer by comparative measurements varying the growth direction relative to gravity. In the following years, however, little progress was made in this field. In the mid 90’s, micro focus sources were introduced in solidification science by a series of successive studies of striations, droplet formations and engulfment in 37 Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002