Study of microstructure and mechanical properties of dilute binary Zr-Nb model alloys for the applications of nuclear fuel cladding

In this study, the effect of Nb addition in Zr on the microstructural evolution and its related mechanical behaviors was systematically investigated. Electron backscatter diffraction, transmission electron microscopy and tensile tests were jointly utilized to characterize the microstructure and evaluate their mechanical properties in Zr-Nb binary model alloys. Results showed that the addition of Nb greatly restrained the recrystallization process in Zr-Nb model alloys, leading to the obvious grain size reduction in higher Nb-containing alloys. Three factors, consisting of β-phase stabilizing effect, dragging effect, and pinning effect, are believed to be responsible for the grain refinement in these Zr-Nb model alloys. The presence of β-Nb particles was firstly observed in Zr-0.5Nb specimen, and its number density increased greatly with an increase of Nb content, like in Zr-1Nb and Zr-2Nb alloys. Both yield strength and ultimate tensile strength increased with the increasing of Nb content, however the drop of ductility was also confirmed. The strengthening contributions from solute Nb atoms, grain boundaries, and β-Nb particles were quantitatively calculated based on the related strengthening theory. Results of calculation demonstrate that the solid solution strengthening contributed the most when Nb atoms were primarily solid dissolved in matrix, whereas the precipitation strengthening surpassed any other factors when the excessive Nb atoms were steadily precipitated.