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Decoupled versus coupled growth dynamics of an irregular eutectic alloy

Abstract : We present an experimental study of irregular growth patterns observed in real time during thin-sample directional solidification of a faceted/nonfaceted eutectic alloy, namely, the transparent 2-amino-2-methyl-1,3-propanediol (AMPD)-succinonitrile (SCN) system. The body-centered cubic SCN crystals are nonfaceted, while monoclinic AMPD crystals grow as faceted needles. At low velocities (<0.3 µms-1), a decoupled growth regime is observed, during which the tip of the AMPD crystals grows ahead of the SCN-liquid interface. At intermediate velocities, an unsteady coupled-growth regime takes place, with intermittent pinning of triple SCN-AMPD-liquid junctions, and frequent noncrystallographic branching. At higher velocities (> 1 µms-1), two-phase fingers form. Many irregular eutectic alloys of industrial importance are such that one of the solid phases is faceted (e.g., silicon, germanium, graphite), while the other one (e.g., aluminum, austenite) is nonfaceted [1-3]. In spite of extensive research, the growth of such faceted/nonfaceted (or f/nf) eutectics is still poorly understood. The term irregular refers to the disordered microstructures left frozen in the solid behind the growth front, whereas regular eutectics grow with fully nonfaceted solid-liquid interfaces. The growth dynamics of regular eutectics is determined by solute diffusion in the liquid, and the local equilibrium at the solid-liquid interfaces, and at the triple-contact pinning lines (trijunctions) between the liquid and the two solids [4]. During directional solidification at velocity V in a fixed temperature gradient G, steady periodic two-phased patterns can then form in a quasi-planar geometry at a temperature close to the eutectic point. The diffusive coupling between the growing entities (e.g., lamellae or rods) extends over distances much larger than the interphase spacing -hence the coupled-growth appellation. In contrast, the unsteadiness of the growth dynamics of f/nf eutectic alloys is attributable to a nonlinear growth kinetics of the faceted interfaces. A facet can remain macroscopically immobile irrespective to the diffusion field over a finite range of undercoolings ("blocked" facet). The mobility of a facet
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Submitted on : Thursday, June 10, 2021 - 1:53:22 PM
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Samira Mohagheghi, Sabine Bottin-Rousseau, Silvère Akamatsu, Melis Şerefoğlu. Decoupled versus coupled growth dynamics of an irregular eutectic alloy. Scripta Materialia, Elsevier, 2020, 189, pp.11-15. ⟨10.1016/j.scriptamat.2020.07.053⟩. ⟨hal-02936627⟩

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