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

Abstract

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.

Pseudo-4D view of the growth and form of locked eutectic colonies

“…With the aid of these results, we are able to determine completely the interfacial bi-crystallography). The habit planes of Al and Al 2 Cu are not the expected dense, low index planes [32]. Instead, we find the primary lamella habit plane is the (011) Al // (433) Al 2 Cu and the secondary lamella habit plane is the (223) Al // ( 451) Al 2 Cu (See Fig.…”

“…Based on rotating directional furnace experiments [20], large tilted domains can persist due to interfacial anisotropy. In many cases, the monocrystal's growth direction is a result of the initial seed crystal [32], not pictured in our experiments. Another consequence of a crystalline anisotropy is that the eutectic cells will appear to drift across the imaged FOV, as we see here in Fig.…”

“…The most common description of the atomic density of a lattice plane is ρ = n 2D d/Ω, where n 2D is the number of atoms per unit cell in the plane, d the interplane spacing, and Ω the volume of the 3D unit cell. Unfortunately, this definition forces the selection of a specific atomic layer which may have variable atomic density [23,32]. We bypass this limitation by following Kraft's description of a "puckered" interface to calculate the atomic planar density [49].…”

“…This model utilizes the competitive growth criterion, wherein selection of morphology is determined by which has the higher interface temperature under steady-state conditions. Extensions to this approach account also for the possibility of a coexistence between rods and lamellae [30,31,32] but do not predict where in the eutectic microstructure rods or lamellae may be found nor how the rods may transition into lamellae and vice versa. According to Chadwick [24], rods should be located preferentially at the edges of the eutectic colonies (Fig.…”

Gabriel Faivre: his legacy on eutectic solidification

“…Using first the low-melting In-In 2 Bi system, Gabriel Faivre helped us -with Oriane Senninger and Laurent Carroz-to define a new experimental protocol including the analysis of x-ray diffraction pole figures, and to establish a method for identifying dense coincidence planes, and ORs [55]. More recently, this methodology was applied to thin Al-Al 2 Cu samples [56].…”