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 p rocesses viz., omega formation, alpha-formation and hydride precipitat ion 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 fo rmation under these two conditions. The morphology of the or-precipita tes and their internal structures has been found to depend upon the ty pe of thermal treatment with step quenching from the beta-phase field leading to an allotriomorphic morphology and quenching and aging leadi ng to internally twinned Widmanstatten alpha. The different morphologi es obtained due to change in thermal treatment and composition of the Zr-Nb alloys has been rationalized. Hydride formation has been examine d in alpha-Zr, beta-Zr and in alpha + beta microstructures. A comparis on has been made between the mechanism of formation of hydride phase i n these three types of microstructures and their morphology and intern al structures have been explained.