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 b eam and concentration of 80% nitrogen, was undertaken. The vertical section s, perpendicular to the melt track were examined by optical microscopy and scanning electron microscopy (SEM), while specimens for X-ray diffractometr y (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 de pth from the laser treated surface, was studied. This supplemented XRD and XPS investigations undertaken previously. Two zones were identified. Zone 1 , within 50 mu m of the surface, contained well defined dendrites of fcc Ti N0.8, plus hcp TiN0.3 and hcp alpha'Ti. Zone 2, below 50 mu m, consisted of needles of hcp alpha'Ti. From a consideration of the hardness profiles in Zone 2, it is suggested that at the top of the zone, the alpha' phase is, i n fact, a solid solution containing 3-4% N, which decreased to < 1% N at th e bottom of the zone. The TEM/SAED study permitted the three phases fcc TiN 0.8, hcp TiN0.3 and hcp alpha'Ti to be identified through a combination of morphology and SAED patterns. This also showed that the fccTiN(0.8) contain ed fringes, which were considered to be stacking fault fringes and allowed this phase to be readily recognized in the TEM. The presence of stacking fa ults may be associated with the high nitrogen concentration of 80% used for the laser nitriding in this work. (C) 2000 Kluwer Academic Publishers.