Isothermal and isochronal variations of η (so-called order-order reactions) in homogenised Ni76Al24-0.19at%B were measured by means of residual resistometry. The isotherms of η observed after having abruptly changed the temperature of the sample were composed of two relaxation processes exhibiting markedly different rates and equal activation energies (4.6eV). The experimental results suggested that this high value of the activation energy resulted predominantly from the vacancy formation term. A model was proposed in which the two distinct η-relaxation processes originated from a difference between the vacancy formation energies on particular sub-lattices. It was noted that, while the activation energy of Ni* tracer diffusion in Ni3Al oscillated around 3eV (considerably lower than the activation energy for order-order kinetics) Ti* tracer diffusion in Ni3Al, where Ti preferentially occupied the Al sub-lattice, exhibited an activation energy which exceeded 4.6eV. By analysing available data on the diffusion of various elements in Ni3Al, it was concluded that the migration of atoms occupying the minority sub-lattice (cube corners) was the predominant mechanism of the order-order reactions in Ni3Al-based systems. The selective insight into atomic diffusion offered by the analysis of order-order reaction kinetics was therefore complementary with respect to standard experimental methods.

"Order-Order" Reaction Kinetics in Ni3Al as a Selective Tool for the Studies of Auto-Diffusion Modes in L12 Superstructure. Kozubski, R., Pfeiler, W.: Defect and Diffusion Forum, 1997, 143-147, 279-84