Microstructure and microhardness of gas nitrided surface layers in Ti–8Al–1Mo–1V and Ti–10V–2Fe–3Al alloys

Abstract

Two different types of titanium alloy, near α Ti–8Al–1Mo–1V and near β Ti–10V–2Fe–3Al, were studied in this work. They were thermochemically treated in a pure nitrogen atmosphere in order to improve their surface properties. The nitriding was performed at two different temperatures, 950°C and 1050°C, and for three different periods of time, 1, 3 and 5 h. Nitrided layers were formed during the process with a thickness from 100 to 350 μm, depending on the processing temperature and time, for both alloys. Their microstructural characteristics and phase composition were studied using optical microscopy and X-ray diffraction. X-ray analyses indicated that new phases α-Ti(N), TiN and TiO₂ were formed during the process of nitriding. The lattice parameters a and c of the α phase and residual surface stress were obtained. The surface hardness was measured for all the samples. Microhardness profiles were obtained on cross-sections of the samples by applying a Knoop indenter. The surface hardness increases with increases in the temperature and duration of nitriding. The maximum values of the microhardness (HK0·05) varied between 550 and 820 for Ti–8Al–1Mo–1V. For Ti–10V–2Fe–3Al, they varied between 520 and 870.