Locked-lamellar eutectic growth in thin Al-Al2Cu samples: In situ directional solidification and crystal orientation analysis

•First in situ experiments in directional solidification of thin Al-Al2Cu eutectic samples.•A new family of crystal orientation relationships with (123)α//(100)θ was discovered.•Link between locked-lamellar patterns and coincidence planes in the two-phase solid.•Locked lamellae grow along a low-energy (100)θ plane.•New insight was gained on initial stages (eutectic selection, mosaicity). We investigated the directional-solidification dynamics of slightly hypoeutectic Al-Al2Cu 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 Al2Cu 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-Al2Cu films of thickness of 13±2μm were prepared by plasma sputtering. During solidification at pulling velocities between 0.05 and 0.5μms-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-Al2Cu 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.