Point-defect properties and diffusion mechanism in intermetallic compounds of L1_2 type

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We have studied the properties of point defects and diffusion behavior in A_3B itermetallic compounds of L1_2 ordered structure, Ni_3Al, Ni_3Ga and Ni_3Ge.1.The formation energies and migration energies of vacncies and substitutional defects in Ni_3Al, as well as other quantities characterzing the defect properties, have been calculated by computer simulation, using an improved set of inter-atomic potntials for Ni_3Al.2.Positron annihilation experiments have been carried out to determine the formation energy of vacancies in Ni_3Ga. It is in the range between 1.7 and 1.8eV.3.The tracer-diffusion coefficients of the constituent elements have been measured for Ni_3Ga and Ni_3Ge over a wide temperature range. The diffusivity of Ni is close to the self-diffusivity in pure Ni. The diffusion coefficient of Ga in Ni_3Ga is about 1/2 times that of Ni, whereas the diffusion coefficient of Ge in Ni_3Ge is less more than two orders of magnitude smaller than that of Ni.4.Chemical diffusion coefficient has been measured for the three compounds by single-phase interdiffusion experiments. The chemical diffusion coefficient increases with increasing the concentration of the minor element (B in A_3B), the dependency being the strongest in Ni_3Ge.5.The thermodynamic factor in chemical diffusion has been formulated in the framework of the pairwise interaction model for the thermodynamics of ordered alloys. By analysing experimental data of thermodynamic activity, the thermodynamic factor for Ni_3Al has been evaluated, and using this value the diffusivity of Al in Ni_3Al has been estimated to be about 1/3 times the diffusivity of Ni.6.A model for the atomic diffusion processes in these materials has been proposed : the diffusion of both species of A atoms, A and B,occur in the sublattice of atoms via ordinary vacancy mechanism. The model can account for the oberved magnitudes of the diffusion coefficients and their variations with composition.