Formation of locked-lamellar grains in a slightly hypoeutectic Al-Al2Cu alloy during thin-sample directional solidification

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.