Nitridation of niobium-46 wt.% titanium alloy in nitrogen at 1300 °C

The nitridation of niobium-46 wt.% titanium alloy (?Ti-Nb) was studied at 1300 °C in 15 kPa nitrogen under high purity conditions for times ranging from 0.5 to 52 h. Three different reaction layers were observed: (i) an outer, thin continuous layer of superconducting (Nb,Ti)N with f.c.c. structure; (ii) an intermediate, duplex layer formed by hexagonal Nb2N containing platelets of TiN; (iii) an inner duplex layer formed by the growth of TiN needles into the titanium-depleted ?Ti-Nb alloy. The observed microstructure is dominated by the different thermodynamic stability of the nitrides existing in the present conditions; in particular Ti is preferentially nitrided at the interior of the alloy. The TiN needles have the tendency to grow with linear kinetics and to form ordered patterns in the alloy; as a consequence TiN segregation can be tentatively described as a cooperative steady state process. The maximum value of Tc, measured from the variation of a.c. susceptibility with temperature, was 17.9 K after 24 h reaction; at longer times a plateau is reached.