Transmission electron microscopy and atomic force microscopy characterisation of titanium-base alloys nitrided under glow discharge

The microstructure of titanium alloys (Ti-1Al-1Mn, Ti-6Al-4V) nitrided under glow discharge was characterised using light microscopy, X-ray diffraction and analytical electron microscopy. The investigations revealed a complex multilayered microstructre produced by diffusion at 850 and 900 degrees C at the surface. The layers formed on the Ti-1Al-Mn alloy were identified as delta-TiN (face-centred cubic NaCI type), delta'-Ti2N (tetragonal body-centred), epsilon-Ti2N (tetragonal primitive) phases and alpha (N) solid solution containing up to 19 at.% nitrogen. The layers formed on the Ti-6Al-4V alloy also contained, besides the phases mentioned above, Ti2AlN and other minor Ti-Al-N ternary phases. The layer topography of both nitrided alloys, examined by atomic force microscopy, exhibited pronounced surface development. A relationship between the microstructure of the alloys and their micromechanical and tribological properties has been established. It shows that the improved properties are related to the presence of nanocrystalline delta-TiN in the outermost sublayer on the top of the layers composed of delta' + epsilon-Ti2N and Ti2AlN crystals. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.