Kinetic Behavior of Ordering in B2-type Alloys

Kinetics of ordering in B2-type FeCo alloy was investigated both experimentally and theoretically. Kinetic equation for B2-type ordering was generaily derived from the two physical conditions, namely the symmetry between two sublattices in the B2--type structure and the stability of the equilibrium state. The general kinetic equation with third order term of the degree of order has an advantage over the others in that it is simpler in form and more general, and it is applicable to the analysis of the kinetic behavior on ordering in B2-type alloys. The variation of lattice constant on isothermal ordering for the equiatomic FeCo alloy was observed by X-ray` diffraction, and analyzed using present kinetic equation. Isothermal change of lattice constant above 823K was reproduced well by the equation, but not satisfactorily below 773K, particularly about a two-step change of lattice constant observed at an early stage of annealing. This two--step change was interpreted very well by the extended kinetic equation containing the vacancy concentration. The values for the formation and migration energies of vacancies in the FeCo alloy were determined as 91 and 139 kJ/mol, respectively.