Atomistic calculations of interfacial energies, nucleus shape and size of θ′ precipitates in Al–Cu alloys

θ′ (Al2Cu) is one of the primary strengthening precipitates in Al–Cu alloys. Although the precipitation sequence of various metastable phases in Al–Cu alloys is well known, fundamental information such as the shape and critical size of a homogeneous θ′ nucleus is not available. In this work, we developed modified embedded-atom method potentials for Al–Cu alloys. The interfacial energies between a θ′ precipitate and the Al matrix along experimentally observed orientations were calculated and compared with prior first-principles calculations. The critical sizes and nucleation barriers were obtained through both the classic theory for homogeneous nucleation and atomistic calculations. The results demonstrate that a plate-shaped nucleus with a semicoherent match between three face-centered cubic Al units and two θ′ units along the edge of the plate is energetically favored.