PERMEATION OF NITROGEN IN SOLID NICKEL AND DEFORMATION PHENOMENA ACCOMPANYING INTERNAL NITRIDATION

摘要

Internal nitridation of Ni alloys containing 5 at. Cr or 1.5 at. Ti was studied over the range of 600-700℃ in a flowing NH_3 ＋ 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 in- ternal 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~Θ·DN = 3.1 × l0~(-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 dif fusion-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 precipi- tation of the semi-coherent nitride particles.