We present an experimental investigation on the effects of the interphase energy anisotropy on the formation of three-phase growth microstructures during directional solidification (DS) of the β(In)–In2Bi–γ(Sn) ternary-eutectic system. Standard DS and rotating directional solidification (RDS) experiments were performed using thin alloy samples with real-time observation. We identified two main types of eutectic grains (EGs): (i) quasi-isotropic EGs within which the solidification dynamics do not exhibit any substantial anisotropy effect, and (ii) anisotropic EGs, within which RDS microstructures exhibit an alternation of locked and unlocked microstructures. EBSD analyses revealed (i) a strong tendency to an alignment of the In2Bi and γ(Sn) crystals (both hexagonal) with respect to the thin-sample walls, and (ii) the existence of special crystal orientation relationships (ORs) between the three solid phases in both quasi-isotropic and anisotropic EGs. We initiate a discussion on the dominating locking effect of the In2Bi–β(In) interphase boundary during quasi steady-state solidification, and the existence of strong crystal selection mechanisms during early nucleation and growth stages.
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][2][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 local equilibrium at the solid-liquid interfaces, and at the triple-contact lines (trijunctions) between the liquid and the two solids [4]. During directional solidification (DS) at velocity V in a 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 l -hence the coupledgrowth appellation. In contrast, the unsteadiness of the growth dynamics of f/nf eutectics 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 is tributary of active sources of
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