EFFECT OF COMPOSITION AND HEAT-TREATMENT ON THE MICROSTRUCTURE AND CORROSION BEHAVIOR OF ZR-NB ALLOYS

The influence of Nb content and heat treatment on the structure and isothermal corrosion behavior of Zr-Nb alloys at 300, 360 and 400-degrees-C in static heavy water and steam (1500 psi) are presented. The formation of the martensitic alpha'-phase by beta-quenching treatment results in high weight gains at all test temperatures. Up to 5 wt% Nb, the corrosion resistance of the quenched alloy decreases linearly with increasing Nb content. Through ageing of the beta-quenched specimens, Nb-rich beta-Nb particles are precipitated in the matrix, at twin and alpha'-needle boundaries. The corrosion resistance is greatly improved due to the reduction in Nb supersaturation. The agglomerated large beta-Nb precipitates have detrimental effects on the corrosion resistance as compared to fine precipitates. The corrosion rates of the specimens containing the beta-Zr phase are affected by the Nb content in the beta-Zr phase and seem to show similar weight gains irrespective of the amount of beta-Zr if they form a continuous beta-Zr network. Under the neutron irradiation the corrosion resistance of quenched specimens was sharply improved, but neutron flux had little or no effect on the aged ones.