The Nature of the Vacancy-Wind Effect Occurring in Diffusion Via Six-Jump-Cycles in B2 Intermetallics~*

摘要

First discovered by the late Dr John Manning, the vacancy-wind effect is a subtle phenomenonthat occurs when two or more atomic species compete for vacancies in a net vacancy flux. Thevacancy-wind effect is incorporated in (for example) the vacancy-wind or Manning factor thatappears in the Darken-Manning Equation relating the interdiffusivity, the tracer diffusivities and thethermodynamic factor. The mechanism of the vacancy-wind phenomenon has long been verypoorly understood. Recently, a moving reference frame Monte Carlo method was used to illustrategraphically how the vacancy-wind effect operates in both ionic conductivity in an ionic solid with adilute solute and chemical interdiffusion in concentrated alloys and ionic compounds. That strategyis extended in this paper to show graphically how the vacancy-wind effect operates in interdiffusionin a stoichiometric intermetallic taking the B2 structure. A simple 4-frequency vacancy diffusionmodel is used. In previous work, it was shown that depending on composition and temperature, thismodel can exhibit the six-jump-cycle mechanism. It is shown that in the limit of perfect order thatthere is no vacancy-wind effect associated with this mechanism when both types of cycle operateequally (zero net vacancy flux). The non-unity value of the vacancy-wind factor found for thismechanism under zero vacancy flux conditions is purely a consequence of a particular geometricmix of tracer and collective atom displacements. The concept that a non-zero off-diagonalphenomenological coefficient provides the vacancy-wind effect is verified.