Growth Dynamics of Three-Dimensional Grain Boundaries in Polycrystalline Solidification using DECLIC-DSI Facility

lloy microstructures formed by directional solidification are often polycrystalline, made up of a several large grains of different crystallographic orientations. Grain boundaries (GBs) have a crucial influence on the mechanical behavior of materials, by their peculiar interactions with dislocations, or by providing solute segregated regions prone to the formation of secondary phases. In technological alloys, GBs typically evolve in the solid state through a series of complex thermomechanical post-processes. However, their initial shape and existence goes back to the initial stage of solidification from the liquid phase. The importance of GBs is well-acknowledged at the macroscopic scale, and grain growth competition is already used in the design of technological components (e.g. grain selectors in single-crystal turbine blades). However, our understanding of GB selection during polycrystalline solidification remains qualitative at best, and a number of unanswered questions remain.<br/>Until now, grains have been assumed to occupy distinct compact regions of 3D space separated by smooth borders on a scale larger than the cellular/dendritic array spacing. In this presentation, we report experimental observations revealing for the first time that cells from one grain can invade a nearby grain during polycrystalline growth. This unexpected invasion process causes grains to interpenetrate each other on the fine scale of the array spacing, and hence grain borders to become highly irregular. By computational modeling, we further reveal the ubiquitous nature of this invasion process by showing that it occurs for a wide range of grain misorientations beyond those studied experimentally. Those results fundamentally alter the traditional view that cellular/dendritic grains are distinct regions embedded in three-dimensional space.<br/>Those unique observations are made possible by <i>in situ</i> visualization of the spatiotemporal evolution of the solid-liquid interface during directional solidification of a transparent organic alloy (Succinonitrile-0.24wt%Camphor) using the DECLIC-DSI experimental device installed onboard the International Space Station. Transparent alloys have similar solid-liquid interface properties as metallic alloys and thus form similar microstructures. The microgravity environment of the space station eliminates buoyancy-driven fluid convection and the associated large scale inhomogeneities of alloy composition, thereby facilitating quantitative comparisons with computational modeling in a purely diffusive regime.