Pseudo-4D X-Ray Imaging Strategy Captures the Solidification of a Polyphase Pattern

Full-field synchrotron-based X-ray imaging has opened a paradigm shift in solidification science, allowing us to capture transient microstructural dynamics in optically opaque materials [1]. The traditional imaging strategy for <i>in-situ </i>4D (i.e., 3D plus time) computed tomography (CT) is to capture a series of forward transmitted X-ray projections whilst the sample is continuously rotated; a sequence of projections from a 180° rotation are used to reconstruct successive volumes to capture the microstructure formation in 4D [2]. In such experiments, there is a fundamental tradeoff between the rotation speed and minimum exposure time (and hence, a compromise between spatial and temporal resolutions). Until now, the solidification dynamics of a binary eutectic — a polyphase pattern composed of two interpenetrating crystals — was difficult to visualize with 4D CT due to the rapid growth velocity (on the order of micrometers per seconds) and vanishingly small crystal dimensions (on the order of micrometers).<br/><br/>Here, we present a new, pseudo-4D imaging strategy that maximizes the available temporal and spatial resolutions of synchrotron CT by digitally combining <i>in-situ</i> X-ray radiography and <i>ex-situ </i>X-ray tomography, respectively. As a proof-of-concept, we capture the solidification behavior of an Al-Al₂Cu eutectic alloy in a planar geometry. This strategy temporally resolves the advancing solid-liquid interface on the order of seconds and spatially resolves the solid-solid microstructure on the order of micrometers, thereby overcoming the space-time trade-off. The pseudo-4D reconstruction enables a real-time view of the solid eutectic pattern emerging from a liquid. We provide experimental evidence of a dynamic microstructural adjustment within crystallographically ‘locked’ eutectic colonies under an oscillatory growth velocity. Most notably, we validate and expand upon existing theories of microstructure selection in eutectic colonies. We find the transition from lamella to rods occurs through a Plateau-Rayleigh-like instability. These insights could only have been attained through a 4D characterization.<br/><br/>[1] Shahani, A. J. <i>et al.</i> <i>Materials Research Letters</i> <b>8</b>, 462 (2020).<br/>[2] Maire, E. & Withers, P. J. <i>International Materials Reviews</i> <b>59</b>, 1 (2014).<br/>[3] Akamatsu, S. & Plapp, M. <i>Current Opinion in Solid State and Materials Science</i> <b>20</b>, 46 (2016).<br/>[4] Bottin-Rousseau, S. <i>et al.</i> <i>Journal of Crystal Growth</i> <b>570</b>, 126203 (2021).