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.