TEM investigations of active screen plasma nitrided Ti6Al4V and Ti6Al7Nb alloys

Direct current plasma nitriding (DC PN) helped to lower processing temperature as compared gas nitriding (GN) allowing to protect the surface with a layer of TiN compound and preserve the mechanical properties of the core material, which otherwise might lose its fine grain microstructure. However, samples of more complicated shapes are subjected to edge effects resulting in their overheating and uneven coverage. Introduction of active screen plasma nitriding (AS PN) should take care of both of these disadvantages, but the understanding on its effect on the compound and diffusive layers of processed parts is far from being clear. The present experiment was aimed at comparing the microstructure and the phase composition of DC PN and AS PN treated Ti6Al4V and Ti6Al7Nb alloys at temperatures of 680 degrees C and 740 degrees C. The microstructure investigations were performed with TEM/EDS methods, while phase analysis relied on electron diffraction indexing. It showed that switching from DC PN to AS PN resulted in covering it with a compact TiN layer backed with alpha ''-Ti(N) martensite and Ti3Al-type layers, i.e. same as in the former case (DC PN) except the missing delta'-Ti2N layer. Additionally, it changes nitriding mechanism from absorption-diffusion of nitrogen ions to adsorption-diffusion of these species, what is a reason of slowing down of the rate of nucleation and growth of both the compound and diffusive layers. The growth of beta-Ti(N) layer, which changes on cooling to alpha ''-Ti(N) is controlled by in-diffusion of nitrogen to its front.