Precipitation in zirconium-niobium martensites

Abstract

The structure of Zr-2.3%Nb and Zr-5.5%Nb alloy martensites on tempering at different temperatures in the range of 350 to 600°C was studied by optical and transmission electron microscopy. The equilibrium β-niobium phase (β₂) was found to be the precipitating phase on tempering the Zr-2.3%Nb martensites at temperatures up to 500°C and the Zr-5.5% Nb martensites at temperatures upto 450°C. Precipitation of the metastable β₁ phase of the monotectoid composition (Zr20%Nb) was observed to occur on tempering the Zr-2.3%Nb alloy at 550 and 600°C and the Zr-5.5%Nb alloy at 500 and 550°C. On tempering the latter alloy at 600°C, the martensite was found to revert back to the supersaturated β phase, which subsequently decomposed into a mixture of the α and the β₁ phases. These observations have been explained on the basis of hypothetical free energy versus composition diagrams. The_orientation relation of the β₁ precipitates with respect to the a phase was found to be as follows: (000l)_β || {011}_β1; (1120)_β || (111)_β1. It was also seen that a β₁ precipitate forming at a twin boundary maintains equivalent orientation relations with the two adjacent twin related portions.