A study of morphological and compositional evolution of nanoprecipitates in the Zr-Nb system and their transformational behavior

Abstract

The metastable transformational behavior (both martensitic and omega) along with compositional and morphological evolution of bcc β precipitates, dispersed in the hcp a matrix of α Zr-1 wt% Nb alloy, were studied as a function of temperature and time. The evolution of the chemical composition of the β phase suggested preference towards metastable compositions having Nb content higher than the equilibrium value. Thermodynamic analysis showed that the metastable chemical compositions are the driving force for the nucleation of such β precipitates. The β to martensite transformation was observed to be possible only if β precipitate size exceeded a critical value of 160nm. Micromechanical modeling was performed to estimate the critical size of β precipitate required to induce martensite transformation and the model predictions were in close agreement with the experimental observations. The omega transformation, on the other hand, showed less size dependence.