Coherency Strain induced Ordering in Substitutional Alloys

다국어 초록 (Multilingual Abstract)

It has been well known that one of the driving forces for ordering is the relaxation of elastic strain energy due to difference in atomic size, a clear evidence being the change in lattice parameter between ordered and disordered states. Because of mathematical complexity, however, only a few strain models have been presented for substitutional alloys. This work reports coherency-induced ordering in a simple model with a two-dimensional square lattice. At an equi-atom composition, a square lattice displays two elementary, ordered structures, S1_0 and S1_1 similar to L1_0(CuAul) and L1_1(CuPt) in fcc. The stability of two superlattices at the ground state depends strongly on elastic anisotropy. For a homogeneous system, Zener's anisotropy ratio is sufficient to measure the stability of one structure against the other. If the elastic constants are different, however, the stability depends not only on the elastic moduli of both elements but also on the solvent-solute bond length. The order-to-disorder transition is studied via the Discrete Atom Method, which is predicated upon statistical mechanics and linear elasticity. When both elastic and chemical interactions join together for ordering, the transition temperature is raised to a value greater than the sum of the two individual casesan indication of a coupling between the two driving forces.