The bulk and interfacial structures of the η (Al₂Au) precipitate phase

The atomic structure of the Al₂Au η precipitate phase in an Al-0.2 at. %Au alloy is investigated using high-resolution scanning transmission electron microscopy together with first principles and elasticity theory calculations. We find experimentally that precipitates above a certain thickness exhibit the cubic crystal structure of the equilibrium phase η. However below that thickness precipitates adopt a tetragonal distortion of the cubic structure of η, which can be regarded as an intermediate η′ phase and allows full two-dimensional coherence for very thin precipitates (up to two unit cells' thick). Despite the structural similarities of the bulk structures of η and θ′ (Al₂Cu) precipitate phases, the former displays interfacial structures that differ markedly from those recently observed in θ′. This can be attributed to the much lower defect energy in aluminium of solute Au compared with solute Cu. Both first-principles and elasticity theory calculations are in good agreement with our experimental observations.