THE INTERNAL NITRIDING BEHAVIOR OF CO-3CR, CO-3AL, AND CO-3TI (A/O) OVER THE RANGE OF 700-1100-PERCENT-C

Internal nitridation of Co containing 3 a/o of either Cr, Al, or Ti was studied over the range of 700-1100-degrees-C in NH3/H2 (6:94 ratio). The kinetics of thickening of the reaction zone followed the parabolic rate law, suggesting that solid-state diffusion was rate controlling. Activation energies obtained were 51.2 Kcal/mol for Co-3Cr, 46.6 Kcal/mol for Co-3Ti, and 27.9 Kcal/mol for Co-3Al. XRD showed only CrN, AlN, and TiN. Deep etching revealed that AlN formed hexagonal plates near the surface when formed at high temperature, the precipitates becoming more massive (blocky morphology) near the reaction front. TiN formed elongated dendritic precipitates, whereas CrN tended to form spheroids. The precipitate size varied with temperature, decreasing with decreasing temperature. In some cases, nitriding formed a case but no visible precipitates even at very high magnifications in the SEM. The solubility of nitrogen in cobalt was determined by long-time equilibration and subsequent chemical analyses. The diffusivity of nitrogen in cobalt was determined from measured permeabilities and the experimentally determined solubilities. Mechanisms are discussed, and the behavior of internal nitridation is compared with internal carburization and oxidation in cobalt alloys.