High-temperature nitridation of Nb-Ti alloys in nitrogen

Microstructure evolution, phase composition, weight gain and layer growth k inetics 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 ra nge 1300-1600 degrees C. After nitridation, the formation of an external co mpact nitride layer as well as extensive internal nitride precipitation was observed. The overall nitridation kinetics (weight gain) is invariably par abolic; a deviation from the initial rate law is observed at 1450 and 1600 degrees C for the longer reaction times when the alloy core approaches nitr ogen saturation and internal precipitation slows down. The parabolic rate c onstant is strongly affected by the Nh content in the alloy. The phases det ected 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 l arge, oriented, nitride needles for the three alloys richer in titanium. Th e needles are composed of alpha-(Ti,Nb)(N) in the case of Ti90Nb10 alloy an d of delta-(Ti,Nb)N1-x in the case of Ti63Nb37 and Ti47Nb53. Such a microst ructure is evidence for nucleation difficulty; coarsening of the existing p articles is favoured in comparison to the formation of new precipitates. Ho mogeneous nucleation is hindered by the small chemical Gibbs free energy av ailable and the elastic strain energy related to volume misfit. After initi al reaction, microstructure evolution is mainly determined by the fast inwa rd diffusion of nitrogen and the slow Nb-Ti interdiffusion in the beta-(Ti, Nb) alloy. Internal nitridation of Ti10Nb90 at 1450 and 1600 degrees C lead s to the formation of fine and numerous precipitates of beta-(Nb,Ti)(2)N. I n this case heterogeneous nucleation along grain boundaries and dislocation lines is the prevailing mechanism. (C) 1999 Elsevier Science S.A. All righ ts reserved.