Semi-coherent interfaces without length misfit accommodation in a lamellar Al–Al2Cu eutectic

Abstract

The interfacial structure of a lamellar Al(α)-Al2Cu(θ) eutectic obtained by directional solidification is investigated using conventional transmission electron microscopy (TEM) and high-resolution TEM. The average lamellar habit plane is close to (2 3 3)α and lie 10° from the atomically densest planes (1 1 1)α//(2 –1 1)θ. Networks of linear contrasted features are observed along the interfaces, the lines being separated each other’s in a wide range of spacings, typically 6–500 nm. These features are identified as interfacial dislocations with 1/2<1 1 0>α Burgers vectors from image contrast analysis. According to previous works, they are associated with ledge-like defects, the heights of which can reach 3 nm. The high-resolution TEM images do not confirm the presence of atomic terraces parallel to the atomically dense plane (1 1 1)α or the habit plane (2 3 3)α. The interface ensures the quasi-continuity of atomically dense planes, which is a configuration corresponding to the plane-matching model. It is suggested that α/θ interfaces can be considered as semi-coherent but in a particular sense since, according to our analysis, the theoretical length misfits between the fcc and bct lattices are too large to explain the presence of some loose dislocation networks. Their general irregular geometry suggests that these dislocation networks behave like non-equilibrated low-angle grain boundaries superposed on the α–θ interfaces.