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We investigated the directional-solidification dynamics of slightly hypoeutectic Al-Al 2 Cu alloys in thin samples. Our goal was to establish a link between the growth of locked, tilted-lamellar patterns and the crystal orientation relationship (OR) between the Al-rich solid solution α and the Al 2 Cu intermetallic θ, as well as to gain information on the OR-dependent anisotropy of the surface energy γ of the α-θ interphase boundaries. Thin Al-Al 2 Cu films of thickness of 13 ± 2 µm were prepared by plasma sputtering. During solidification at pulling velocities between 0.05 and 0.5 µms −1 , the coupled-growth front was observed in situ with a long-distance optics. The growth of millimeter-sized eutectic grains was thus followed in real time during transient and steady-state regimes. The orientation of α and θ crystals was measured ex situ by x-ray diffraction and electron backscattering diffraction. In several eutectic grains, a {123} α plane and a {100} θ plane were found to be closely parallel to each other. These coincident planes define a new family of (type-C) ORs in the Al-Al 2 Cu eutectic, which are distinct from the prevailing ORs that have been previously identified in bulk samples. Crucially, the inclination of the lamellae was systematically close to that of a {100} θ lattice plane, which therefore corresponds to a deep γ minimum in eutectic grains with a type-C OR, or a neighbor one. We initiated a discussion on the selection of the OR, the formation of "stray" eutectic grains, and the lamellar-growth dynamics at eutectic-grain boundaries.

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We studied the formation and growth of locked-lamellar microstructures in a thin sample of a slightly hypoeutectic Al-Al2Cu alloy. The coupled-growth dynamics, including early stages and steady-state regimes, was observed optically in real time during directional solidification. The orientation of the α (Al) and θ (Al2Cu) crystals was measured ex situ in a series of eutectic grains by X-ray Laue microdiffraction. A nucleation event of a θ crystal on a pre-existing α crystal, and the subsequent growth of a eutectic grain with a type-C orientation relationship, that is, with a coincidence of {123}-α and {100}-θ planes, were observed in situ. In type-C eutectic grains, lamellar locking occurred parallel to the low-energy coincidence plane. A regular (floating) coupled-growth dynamics was observed in misoriented eutectic grains.