First observation of a hexagonal close packed metastable intermetallic phase between Cu and Al bilayer films

Abstract

Abstract In this paper we describe the structure and formation of a new intermetallic phase in the Cu – Al system, which has not been reported before. The phase was found in Cu/Al bilayer films, which were deposited at room temperature on (0001) sapphire substrates using molecular beam epitaxy. The interfacial intermetallic phase is 8 nm thick, and possesses a hexagonal close-packed structure. The lattice parameters of the phase gradually increase from the near-Cu-side to the near-Al-side. In parallel with the described lattice expansion, the chemical composition of the interlayer also varies from 27 to 58 Al at.% from the near-Cu-side to the near-Al-side. The formation and microstructural characteristics of this new phase are explained by Hume-Rothery laws and Shockley partial dislocations. In addition, in-situ heating experiments were performed in a transmission electron microscope at ∼600 °C to investigate the thermodynamic stability of this new Cu – Al intermetallic phase. During annealing the intermetallic layer disappears and other known equilibrium intermetallic phases develop. This indicates that this new hexagonal close-packed Cu – Al intermetallic phase is metastable.