Volume 2, Issue 5 (May 2005)
Study of Nb and Fe Precipitation in α-Phase Temperature Range (400 to 550°C) in Zr-Nb-(Fe-Sn) Alloys
This work is aimed at improving our knowledge of the Nb and Fe precipitation in the low α-temperature range as a function of Fe and Sn contents in ZrNb based alloys. Several experimental variants, with Nb contents ranging from 0.3–0.8 wt%, Fe from 0.01–0.5 wt% (and also Sn additions), have been studied, with special attention paid to the residual Nb content in the matrix, and to the SPP crystallographic nature and chemical composition as a function of the nominal alloy composition, annealing time/temperature, and initial metallurgical condition.
For ZrNb alloys, recent studies have pointed out the necessity of performing long term annealing treatments in order to avoid the persistence of metastable phases, and to reach a thermodynamic equilibrium state of precipitation. For that purpose, experimental alloys were annealed for up to 2 years at 400, 470, and 550°C. The evolution of SPP chemical compositions with temperature and annealing time was first quantified. For the longer annealing times, i.e., for conditions close to thermodynamic equilibrium, the results suggest an existence of a composition range for the SPP phases under equilibrium conditions. This specific point is currently taken into account to improve the thermodynamic description of the ZrNbFe phase diagram.
Also, the influence of preliminary cold work on the precipitation kinetics of equilibrium βNb phase has been investigated. This study was conducted on two Zr-1%Nb alloys containing initially metastable βZr precipitates. It was observed that cold work significantly accelerates the precipitation of equilibrium βNb phase, mainly for annealing treatments too short to allow significant recovery of the dislocation network, i.e., between 1 and 100 h of annealing.
Finally, all of these experimental results are discussed with regard to their consequences on the thermodynamic modeling of the equilibrium phase diagram and precipitation kinetics.