Interfacial Dynamics in Liquid-Solid Mixtures: A Study of Solidification and Coarsening

The microstructural evolution of liquid-solid mixtures is examined using X-ray computed tomography to make in-situ, 4D (three spatial dimensions and time) measurements of the interfaces between the liquid and solid phases. Samples are a variety of hypo-eutectic Al-Cu alloys and the total characterized volume is approximately 1 mm3. The coarsening datasets span solid volume fractions of 30% to 80% and have durations of up to 15 hours, making them some of the largest, most comprehensive coarsening datasets.;This data is used show that the solid fraction does not have a significant effect on the coarsening rate, unlike in a system of spherical particles. The interfacial curvature and velocity data that is made possible by the 4D measurements is used to develop a relationship between interface shape and the average normal velocity for a piece of interface with that shape. This model accounts for both capillarity and shape-related diffusional effects. Neighborhood related diffusional effects lead to a distribution in velocities about the mean; these distributions are examined and shown to follow a Gaussian distribution.;A novel new data collection and processing algorithm for X-ray computed tomography, time-interlaced model-based iterative reconstruction, is used to achieve 4D data with micrometer level spatial resolution and 1.8 second temporal resolution. This is approximately an order of magnitude better than what has been achieved before and yields the first data with sufficient spatial and temporal resolutions to characterize the microstructure during solidification.;The resulting data of dendritic growth in an Al-24wt%Cu alloy that is being cooled at 2°C/minute shows the formation of split tip secondary dendrite arms that have not been seen before in transparent organic analogues. A single free-growing dendrite is isolated from this data and analyzed as a function of distance from the tip, resulting in relationships for the volume of solid, Vs proportional to Zˆ1.44 , the liquid solid interfacial area, Vl/s proportional to Zˆ1.32, and mean curvature, H proportional to Zˆ--0.21. The overall dendritic morphology is also characterized, proving the first quantitative 3D data that can be compared to phase field simulations.