Single-phase interdiffusion in the B2 type intermetallic compounds NiAl, CoAl and FeAl

Interdiffusion coefficients, D̃, in the B2 type NiAl, CoAl and FeAl phases have been determined by single phase diffusion couples over a wide temperature range from 1073 to 1773 K. The value of D̃ in the NiAl and CoAl phases shows a minimum at about 47 at.% Al deviating slightly from the stoichiometric composition, while the value of D̃ in the FeAl phase has a weak minimum at about 41 at.% Al. The value of the activation energy, Q̃, for interdiffusion in the NiAl and CoAl phases shows a maximum at about 47 at.% Al, where the value of Q̃ in the CoAl phase is much higher than that in the NiAl phase, while the value of Q̃ in the FeAl phase is nearly constant and much lower than those in the other two phases. It is observed that the larger the lattice constant of the compounds becomes, the lower the activation energy for diffusion in the compounds becomes. Thus, the magnitude of activation energy for diffusion in the B2 type NiAl, CoAl and FeAl is probably related to the lattice constant of the compounds. Using the interdiffusion coefficient and the tracer diffusion coefficients of Ni and Fe with the help of Darken's relation, the diffusion coefficient of Al in the NiAl and FeAl phases has been estimated.