Formation and Oxidation of Nitrided layers formed on Titanium alloys by Gas Nitriding = 가스질화에 의한 Ti합금의 질화층 형성과 산화

다국어 초록 (Multilingual Abstract)

As a result of the nitrogen interaction during gas nitriding, a titanium nitride layer was formed, followed by an interstitial solution of nitrogen in the hcp α-itanium. The phase composition at the outmost nitrided surface layer of Ti-6Al-4V alloy is TiN and Ti₂N. In the Ti-N compound layer, Ti₂N is the major phase and TiN is the minor one. Al exists as dissolved ions at the top surface of the Ti-N compound layer. Nanoindentation microhardness testing was conducted on the nitrided titanium alloys to analyze their hardness evolution in relation to the nitriding processing parameters and alloy composition. It was found that themicrohardness increases due to the strengthening effect of interstitial nitrogen and the formation of nitrides. When the nitrided Ti-6Al-4V titanium alloy was oxidized at 600℃ and 700℃ for 10 hrs in air, the nitrided alloys oxidized slightly. Above 800℃, they oxidized fast and microcracking developed on the nitride surface. The microhardness of the surface layers was very high, and it decreased through the diffusion zone to approach the base microhardness of the matrix. Nitriding and oxidation increased the surface microhardness due to the strengthening effect of interstitial nitrogen and oxygen as well as the formation of titanium-nitrides and -oxides. The microhardness of the nitrided alloys increased with the increase of the oxidizing temperature.