Kinetics of High-Temperature Nitridation of Zr-Nb Solid Solutions

We have demonstrated general kinetic aspects of the formation of nitrides of Zr-Nb alloys (containing 0.1, 2.5, and 5 wt % Nb) at a temperature of 1900(degrees)C. The nitridation process has been shown to have a two-stage character, with both stages having an exponential rate law. The reaction rate in the second stage is considerably lower than in the first stage. We have determined the composition of the resulting heterostructures of the form Zr1-& khcy;Nb & khcy;N-ZrN1-n /beta-solid solution of zirconium in niobium (Zr1-& khcy;Nb & khcy;N) and identified the nitridation sequence of the components of the starting alloy. The first stage of the process is the formation of an alpha-solid solution of nitrogen in Zr and its conversion into a nonstoichiometric nitride. The kinetic curve of the second stage describes nitridation of the beta-Nb phase resulting from the decomposition of the Zr(sic)Nb > solid solution. The duration of the second stage of the process has been shown to be determined by the amount of niobium in the starting solid solution. Experimental evidence is presented that single-stage nitridation of Zr-M alloys can be used for producing single-phase ceramics containing active additions and having the shape of the starting metallic workpiece.