The internal nitridation of alloys which contained 5at%Cr or 1.5at%Ti was studied, at 600 to 700C, in a flowing NH3 plus H2 gas mixture. The growth rate of the nitrided zone was found to obey a parabolic law, and N uptake by the sample was controlled by N diffusion in the metal. It was demonstrated that the classical Wagner treatment of internal precipitation could be used to describe the nitridation kinetics and to deduce the permeability constants. The permeability of Ni at 1bar could be described by:

P (m2/s) = 3.1 x 10-11exp[-17500/T]

A volume change which was associated with the precipitation reaction resulted in a stress gradient between the alloy surface and the internal nitridation front. It was observed that stress relief occurred mainly via the transport of Ni to the gas/metal interface. Pipe-diffusion controlled creep was shown to be the predominant stress-accommodation mechanism. This mechanism was prevalent because of a large dislocation density which was generated in the matrix during the internal precipitation of semi-coherent nitride particles.

Permeation of Nitrogen in Solid Nickel and Deformation Phenomena Accompanying Internal Nitridation. A.A.Kodentsov, M.J.H.Van Dal, C.Cserháti, L.Daróczi, F.J.J.Van Loo: Acta Materialia, 1999, 47[11], 3169-80