High-temperature nitridation of Nb-Ti alloys in nitrogen

Microstructure evolution, phase composition, weight gain and layer growth kinetics of Nb-Ti alloys (10, 47, 63 and 90 at.% Ti) annealed in high-purity nitrogen atmosphere (0.3, 3 and 30 bar) were studied in the temperature range 1300-1600°C. After nitridation, the formation of an external compact nitride layer as well as extensive internal nitride precipitation was observed. The overall nitridation kinetics (weight gain) is invariably parabolic; a deviation from the initial rate law is observed at 1450 and 1600°C for the longer reaction times when the alloy core approaches nitrogen saturation and internal precipitation slows down. The parabolic rate constant is strongly affected by the Nb content in the alloy. The phases detected in the reacted samples are isostructural with those of the Nb-Ti, Nb-N and Ti-N systems. The surface nitride was [delta]-(Ti,Nb)N in any case. The morphology of the internal nitridation zone corresponds to the growth of large, oriented, nitride needles for the three alloys richer in titanium. The needles are composed of [alfa]-(Ti,Nb)(N) in the case of Ti90Nb10 alloy and of [delta]-(Ti,Nb)N1-x in the case of Ti63Nb37 and Ti47Nb53. Such a microstructure is evidence for nucleation difficulty; coarsening of the existing particles is favoured in comparison to the formation of new precipitates. Homogeneous nucleation is hindered by the small chemical Gibbs free energy available and the elastic strain energy related to volume misfit. After initial reaction, microstructure evolution is mainly determined by the fast inward diffusion of nitrogen and the slow Nb-Ti interdiffusion in the [beta]-(Ti,Nb) alloy. Internal nitridation of Ti10Nb90 at 1450 and 1600°C leads to the formation of fine and numerous precipitates of [beta]-(Nb,Ti)2N. In this case heterogeneous nucleation along grain boundaries and dislocation lines is the prevailing mechanism.