Effect of temperature on gas oxynitriding of Ti-6Al-4V alloy

The oxynitriding of Ti-6Al-4V alloy was carried out by the adding of controlled oxygen-containing environment into the system at the final stage of nitriding (during cooling). It was determined that oxynitriding was more intensive at 950 degrees C than at 750 degrees C and led to formation of surface layer based on TiO2 phase in the modification of rutile, 8-TiN-type and delta'-Ti2N phases, with the thickness 2 pm. At the lower temperature the surface layer was twice thinner and contained alpha"-Ti instead of delta'-Ti2N. 8-TiN-type phase showed the presence of twinning and stacking faults on 111 planes that can be explained by the penetration of oxygen into the lattice (substitution of nitrogen atoms by oxygen ones and formation of TiNxOy phase). By increasing temperature during oxynitriding from 750 degrees C to 950 degrees C, the surface microhardness of Ti-6AI-4V alloy increases from 10 GPa to 17 GPa, and the thickness of the diffusion layer-from similar to 40 to 70 mu m.