METASTABILITY OF THE BETA-PHASE IN ZR-RICH ZR-NB ALLOYS

The decomposition of the beta-phase in Zr-rich Zr-Nb alloys by three processes viz., omega formation, alpha-formation and hydride precipitation has been examined. In the Zr-20Nb alloy, omega-formation has been examined after thermal treatment as well as after electron irradiation and a comparison has been made between the kinetics of omega-phase formation under these two conditions. The morphology of the or-precipitates and their internal structures has been found to depend upon the type of thermal treatment with step quenching from the beta-phase field leading to an allotriomorphic morphology and quenching and aging leading to internally twinned Widmanstatten alpha. The different morphologies obtained due to change in thermal treatment and composition of the Zr-Nb alloys has been rationalized. Hydride formation has been examined in alpha-Zr, beta-Zr and in alpha + beta microstructures. A comparison has been made between the mechanism of formation of hydride phase in these three types of microstructures and their morphology and internal structures have been explained.