Permeation of nitrogen in solid nickel and deformation phenomena accompanying internal nitridation

Internal nitridation of Ni alloys containing 5 at.% Cr or 1.5 at.% Ti was studied over the range of 600-700 degrees C in a flowing NH3 + H-2-gas mixture. Growth rate of the nitrided zone was found to obey a parabolic law and the nitrogen uptake by the sample is controlled by the nitrogen diffusion in the metal. It was demonstrated that under the experimental conditions used, a classical Wagner's treatment of the internal precipitation can be applied to describe the nitridation kinetics and obtain permeability constants. Permeability of nickel for nitrogen at the standard pressure of 1 bar was determined to be: N-N(Theta) . D-N = 3.1 x 10(-11) exp(-17 500/T) [m(2)/s]. A volume change associated with the precipitation reaction resulted in a stress gradient between the alloy's surface and the internal nitridation front. It was observed that stress relief occurred mainly by transport of nickel (solvent metal) to the gas/metal interface. Pipe diffusion-controlled creep was shown to be the dominant stress accommodation mechanism. This mechanism is prevailing because of a large dislocation density generated in the Ni matrix upon the internal precipitation of the semi-coherent nitride particles. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.