Ti-6Al-4V alloys consisting of α-Ti grains and intergranular β-Ti islands were nitrided at 850°C for 1 to 12 h under a nitrogen pressure of 1 Pa. With increasing nitriding time, the Ti-N compound layer became thicker, and the α-Ti diffusion zone containing dissolved nitrogen became wider. In the Ti-N compound layer, the initially formed Ti2N became TiN as the nitriding progressed. The nitride layers were oxidized to rutile-TiO2 after oxidation at 700°C for 10 h in air.
Ti-6Al-4V alloys were nitrided through gas nitriding at 950°C for 3 h in deoxygenated, atmospheric nitrogen gas. During nitriding, nitrogen reacted and diffused into the alloys to form Ti2N and a meager amount of TiN in a Ti-N compound layer with a thickness of 20 µm to 25 µm. An α-Ti(N) diffusion layer with a thickness of 40 µm to 80 µm formed below this layer. A small amount of Al was dissolved at the top of the Ti-N compound layer because of the strong interaction of nitrogen with Ti and Al. Nitriding resulted in the dissolution of interstitial nitrogen and the formation of nitrides. Oxidation of the nitrided Ti-6Al-4V alloys initially resulted in the formation of a Ti-N-O layer, which later oxidized to TiO2. Above 800°C, the nitrided alloys oxidized rapidly, accompanied by microcracking of the TiO2 surface layer.
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