Comparison of Nitriding Behavior of Mo-Ti Alloys in NH₃ and N₂ Gases

For dilute Mo-Ti alloys and pure Mo nitrided at 1100°C for 16h in NH₃ and N₂ gases, microstructural observations through optical and transmission electron microscopes, X-ray diffraction analysis and hardness measurements were carried out and the nitriding behavior in N₂ and NH₃ gases was compared. A surface layer of γ-Mo₂N with Hv∼1700 was formed for the nitriding in NH₃ gas of 1 atm. In contrast, the surface layer was absent for nitriding in N₂ gas at pressures below 10 atm, whereas the β-Mo₂N layer was formed at a pressure of 15 atm. In an internal nitriding layer formed beneath the surface layer of Mo-Ti alloys nitrided in NH₃ gas, extremely small and plate-like particles of 2∼4nm in width and approximately 0.45nm in thickness were clearly found to precipitate on {100} planes of the matrix. Furthermore, a large coherent strain field was observed in the matrix around each Ti-nitride precipitate. Such a dispersion of the small precipitates resulted in remarkable hardening in the nitrided alloy. The hardness of the internal nitriding layer of the alloys nitrided in NH₃ gas was much higher than that in N₂ gas. The difference in hardness can be explained in terms of the differences not only in size of the Ti-nitride precipitates but also in their distribution density.