Nitriding of dilute Mo-Ti alloys at a low temperature of 1373 K

For Mo-0.5 mass% Ti and pure Mo alloy nitrided in a NH₃ gas at a relatively low temperature of 1373 K, microstructural observations through optical and transmission electron microscopes, X-ray diffraction analysis and hardness measurements were carried out. A surface nitriding layer with very high hardness of approximately Hv ∼ 1800 consisted of two Mo-nitride regions: an outer one of γ-Mo₂N and an inner one of β-Mo₂N. The inward diffusion of nitrogen is a rate-controlling process in the growth of the surface nitriding layer. In Mo-Ti alloy additionally an internal nitriding layer with relatively high hardness of Hv ∼ 800 was formed beneath the surface nitriding layer. Such high hardness in the internal nitriding layer was found to result from the uniform dispersion of extremely fine plate-like particles of titanium nitride. The particles are approximately 0.4 nm thick and have coherent strain field in the matrix.