Titanium alloys after surface gas nitriding

Experimental studies using differential scanning calorimetry (DSC) for nitriding of four titanium-alloys near α Ti–8Al–1Mo–1V, near α Ti–6Al–2Sn–4Zr–2Mo, α + β Ti–6Al–4V and near β Ti–10V–2Fe–3Al at different temperatures and for different periods of time are presented. The X-ray diffraction (XRD) technique was used in order to study the phase transformations that occur during gas nitriding. As a result of the nitrogen interaction, a nitrided layer was formed that consists of titanium nitrides, followed by an interstitial solution of nitrogen in the hcp α titanium phase. The microstructural changes of these alloys in relation to the alloy composition and processing parameters were studied. It was found that the microstructure of alloys nitrided at temperatures below their β transus temperatures for various periods of time is uniform and homogeneous. With the increase of the temperature above their β transus temperatures the microstructure changes to irregular. Microindentation hardness testing using a Knoop indenter was conducted on the nitrided titanium alloys to analyse their hardness evolution in relation to the nitriding processing parameters and alloy composition. It was found that the microhardness increases with the increase of the temperature and time of nitriding. The surface morphology of the Ti–6Al–2Sn–4Zr–2Mo alloy in relation to the nitriding processing parameters was analysed.