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.

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.

This paper aims to describe the experimental framework of the Directional Solidification Insert, installed onboard the International Space Station, dedicated to the in situ and real-time characterization of the dynamic selection of the solid–liquid interface morphology in bulk samples of transparent materials under diffusive growth conditions. The in situ observation of the solid–liquid interface is an invaluable tool for gaining knowledge on the time evolution of the interface pattern because the initial morphological instability evolves nonlinearly and undergoes a reorganization process. The result of each experiment, characterized by the sample concentration, a thermal gradient, and a pulling rate, is a large number of images. The interpretation of these images necessitates a robust identification of each cell/dendrite’s position and size during the entire solidification. Several image analysis methods have been developed to reliably achieve this goal despite varying contrast and noise levels and are described in detail. Typical solidification experiments are presented, and the dynamics of the pattern formation are analyzed to illustrate the application of the image analysis methods.