Coarsening in Low Volume Fraction Solid-Liquid Mixtures: Bridging the Gap Between Theory and Experiment
Coarsening is a diffusion driven process that takes place in all multiphase materials, in which the total energy of a system is lowered through the reduction of interfacial energy. In a two phase material, large features grow at the expense of smaller features, while the total number of features decreases and the volume fraction remains constant throughout the process. The evolution of the microstructure as a result of this process has a significant effect on the macroscopic properties of a material. This is especially important in precipitate hardened alloys, where the size, number and distribution of precipitates determine the mechanical strength of the alloy, and high temperature alloys, such as those used in the blades of jet turbine engines. A thorough understanding of the coarsening process will provide a vital tool for the design of materials for a wide range of applications.
Despite the ubiquity of the process and the wide range of theories and models treating it, there remains a dearth of robust experimental characterizations of the process. Recent experiments performed on the International Space Station (ISS) have provided new insight into the coarsening process. Lead Tin (PbSn) alloys, chosen due to the ability to compare to theory with no adjustable parameters, of varying volume fraction were coarsened over a range of times. In order to avoid sedimentation, the alloys were processed aboard the ISS. The microstructure of the alloys is then characterized using mechanical serial sectioning via a custom modified micro-miller. Using a number of automated imaging processing techniques, the microstructures of the alloys can be reconstructed in three dimensions, and from these reconstructions, quantitative measurements can be made. Quantities of particular interest include the rate of particle size and density evolution, particle size distributions and spatial correlations.