Microstructural assessment of laser nitrided Ti-6Al-4V alloy

A microstructural study of the phases developed during the laser nitriding of a Ti-6Al-4V alloy by, using a CL5 continuous CO₂ laser with a spinning beam and concentration of 80% nitrogen, was undertaken. The vertical sections, perpendicular to the melt track were examined by optical microscopy and scanning electron microscopy (SEM), while specimens for X-ray diffractometry (XRD), X-ray photospectroscopy (XPS) and transmission electron microscopy/selected area electron diffraction (TEM/SAED), were taken parallel to the melt track. In this way the variation in microstructure as a function of depth from the laser treated surface, was studied. This supplemented XRD and XPS investigations undertaken previously. Two zones were identified. Zone 1, within 50 m of the surface, contained well defined dendrites of fcc TiN_0.8, plus hcp TiN_0.3 and hcp Ti. Zone 2, below 50 m, consisted of needles of hcp Ti. From a consideration of the hardness profiles in Zone 2, it is suggested that at the top of the zone, the phase is, in fact, a solid solution containing 3–4% N, which decreased to <1% N at the bottom of the zone. The TEM/SAED study permitted the three phases fcc TiN_0.8, hcp TiN_0.3 and hcp Ti to be identified through a combination of morphology and SAED patterns. This also showed that the fccTiN_0.8 contained fringes, which were considered to be stacking fault fringes and allowed this phase to be readily recognized in the TEM. The presence of stacking faults may be associated with the high nitrogen concentration of 80% used for the laser nitriding in this work.