TEM studies of low temperature cathode-plasma nitrided Ti6Al7Nb alloy

The Ti6Al7Nb alloy has been elaborated as a substitute for Ti6Al4V alloy especially for applications leading to its direct contact with human body fluids as the former totally eliminates the carcinogenic vanadium present in the latter. The Ti6Al7Nb presents so much needed high specific strength, but its aluminum addition is also harmful and will have to be taken care of. Nitriding could improve wear resistance of these alloys and form a diffusion barrier at least at first stage of their use. However, knowledge of phase composition of near surface area of Ti6Al7Nb after such treatment is still limited. Therefore, these experiments were planned to describe the effect of the plasma nitriding of Ti6Al7Nb alloy performed at 680 °C and 740 °C for 6 h. The transmission electron microscopy observations revealed that the plasma nitriding of this alloy performed at both of specified temperatures allows to produce a zone consisting of only three layers i.e. δ-TiN, α″-Ti martensite and (α-Ti + Ti3Al) phases. The only difference between both treatments was that plasma nitriding at 740 °C caused a porosity of upper part of the δ-TiN layer, while the one formed at 680 °C was only slightly thinner, but fully dense. Penetration of the nitrogen into sub-surface areas during plasma nitriding of Ti6Al7Nb alloy pushes down both the aluminum and niobium deep into the substrate freeing the near surface β-Ti from alloying additions. It allows to transform the latter into α″-Ti martensite during cooling down. The last layer in the nitrided zone was found to be formed by a mixture of α-Ti and Ti3Al phases. •Phase composition of plasma nitrided Ti6Al7Nb and Ti6Al4V alloys differs.•Plasma nitriding of Ti6Al7Nb at ~700 °C forms δ-TiN/α″-Ti/α-Ti + Ti3Al phases.•Lowering nitriding temperature to 680 °C eliminates the porosity of δ-TiN layer.•Diffusion of N into sub-surface areas pushes away the Al and Nb.•β-Ti, freed from alloying additions, turns on cooling into α″-Ti martensite.