Microstructure and composition of plasma-nitrided Ti-6Al-4V layers

Abstract

Nitriding of titanium alloys was performed in an r.f. plasma of nitrogen, hydrogen and argon at a substrate temperature of about 500 °C. The microstructure and the composition of the nitrided layers obtained were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). Three phases were identified: (a) a solid solution of nitrogen in titanium, α-(Ti, N), (b) tetragonal ϵ-Ti 2N and (c) cubic δ-TiN (NaCl-type structure). Two distinct layers were formed on top of the plasma-nitrided Ti-6Al-4V alloy, followed by a solid solution of nitrogen in the titanium alloy. The outer layer was identified as δ-TiN while the inner layer was identified as a mixture of δ-TiN and ϵ-Ti 2N. The ϵ-Ti 2N was found to be highly oriented (002) while the δ-TiN was obtained as a randomly oriented polycrystalline layer. The outer δ-TiN was found to have a fine structure, while the inner δ-TiN plus ϵ-Ti 2N comprised large and oriented grains. TEM studies showed that the crystallite size in the upper layer was tens to hundreds of ångstroms, while in the inner layer it was one tenth to about half a micrometre. Energy-dispersive analysis performed with the TEM system and AES shows a variation in the bulk composition along the nitride layer. The microstructures and the relative phase content are presented and discussed in relation to the gas feed composition in the plasma.