Internal-nitriding behavior of Ni-V and Ni-3Nb alloys

Ni-2V, Ni-5V, Ni-12V, and Ni-3Nb alloys (w/o) were nitrided in 10 v/o NH3 ( bal. H-2) over the range of 700-1000 degrees C. The growth rates of the rea ction zones followed parabolic behavior for all of the alloys from 700 to 9 00 degrees C. At 1000 degrees C, Ni-2V and Ni-3Nb formed nitride scales, wh ereas Ni-5V and Ni-12V formed internal-nitride zones. Nitridation rates dec reased with increasing vanadium content for the Ni-V alloys. VN precipitate d in the Ni-V alloys and NbN precipitated in Ni-3Nb for all exposure condit ions in which internal nitridation occurred. The precipitate morphology cha nged with temperature and distance from the gas-metal surface. The VN and N bN precipitates were generally small and spheroidal near the surface, incre asing in size with distance and temperature. The NbN precipitates became Wi dmanstatten at higher temperatures and/or increasing distance within the ni trided zone. The solubility of nitrogen in pure Ni was determined and found to decrease with increasing temperature from 700 to 1000 degrees C. Expres sions for the diffusion coefficient of nitrogen in nickel were determined f rom the measured permeabilities of each alloy and the nitrogen solubilities in nickel.