Gas phase alloyed functionally graded titanium nitride: Investigation of phase formation and mechanical properties

In situ nitride treatment during additive manufacturing (AM) allows for the N concentration to be spatially tailored through the bulk of a part, enabling functionally graded properties. To inform the design of Ti-N functionally graded materials, a quantitative understanding of the effect of processing parameters on N uptake, and of composition on phase formation and resulting mechanical properties is needed. The N concentration and corresponding phase fractions of Ti-6Al-4V samples fabricated with in situ gas nitriding during directed energy deposition AM were measured and investigated using thermodynamic simulations. N uptake was shown to be limited by the surface reaction at the liquid-gas interface, in turn controllable by N₂ partial pressure. Phase formation involved initial solidification of metastable TiN_1-x dendrites and precipitation of ε particles out of the α matrix. Microhardness most strongly correlated with the fraction of nitride phases.