Mechanism of atomic diffusion in nickel-based intermetallic compounds of L1_2-type

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The mechanism of atomic diffusion in Ni_3Al, Ni_3Ga and Ni_3Ge has been studied by examining the thermodynamic properties of these materials, most importantly the concentrations of intrinsic point defects, using various experimental techniques.The hypothetical order-disorder transformation temperature of Ni_3Al has been evaluated from the transformation temperatures of a series of alloys Ni_3 (Al, Fe). The transformation temperatures have been determined by in situ measurements of electrical resistivity, and extrapolation to zero Fe content gives 1721 ± 34 K as the transformation temperature of Ni_3Al. This value is in good agreement with the value 1680-1740 K deduced from an analysis of thermodynamic activity data using the results of Monte-Carlo simulation. We proposed earlier an atomistic model of diffusion, where both Ni and Al atoms are assumed to migrate in the crystal primarily by the vacancy mechanism in the sublattice for Ni atoms (the α sublattice vacancy mechanism) ; the mod … More el can reasonably explain the diffusivities of Ni and Al in Ni_3Al. In that discussion, which is based on a pair interaction model of thermodynamic properties, the value of the effective interaction energy between atoms is most critical. The transformation temperature is physically equivalent to the effective interaction energy, and thus the value obtained in this investigation substantiates the discussion made earlier.The thermodynamic activity of components has been measured for Ni_3Ga and Ni_3Ge by solidelectrolyte electromotive-force method. For Ni_3Ga, the effective interaction energy has been determined to be 0.101 ± 0.002 eV from the variation of the activity with composition. The concentrations of point defects and the thermodynamic factor for chemical diffusion have been estimated with the aid of a thermodynamic model, and on their basis the mechanism of diffusion has been examined. The data of tracer diffusion and chemical diffusion for this compound can also be accounted for by the α sublattice vacancy mechanism.The method of activity measurements employed has turned out inappropriate for Ni_3Ge. To resolve the problem, knowledge of the details of the Ni-Ge-O ternary phase diagram is required but is not available at the moment. This is the task left to the future. Thus, no reliable value of the effective interaction energy in this compound has been obtained. Nevertheless, it has been shown that the diffusion behavior in this compound can be understood in terms of the α sublattice vacancy mechanism, if the interaction energy is assumed to be as high as 1.5eV.The α sublattice vacancy mechanism is thus a most plausible mechanism of diffusion for this class of materials. Less