Microstructure Evolution During Fabrication of Zr–Sn–Nb Alloy Tubes

Abstract

Zr–Sn–Nb alloy is an important field of Zirconium alloy for high burn-up Pressurized Water Reactors (PWRs). This article reports the microstructure evolution of Zr–0.5Sn–0.15Nb–0.75FeV alloy tube during fabrication. The morphologies of grain and second phase particles (SPPs), their shape, size and distribution have been studied in detail using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure evolution is as follows: lath martensite (β-quenching) → heterogeneous deformation structure and dynamically recrystallized grain (hot extrusion) → heterogeneous deformation structure (cold rolling) → heterogeneous recrystallized grain (intermediate annealing) → homogeneous deformation structure (finishing rolling) → homogeneous fully recrystallized structure (final annealing). SPPs in the hot extruded tubes tend to precipitate along the direction of deformation. With the processing and heat treatment of the alloy, the SPPs are distributed more dispersively. At last, homogeneous and fine SPPs are distributed on Zr matrix uniformly. It can be found that two kinds of SPPs exist in the products. One are ZrFeV SPPs without Nb and the other are ZrNbFeV SPPs containing a small amount of Nb. Fe/V in SPPs are similar to the Fe/V in addition of alloying element and the results show Fe and V can precipitate fully in the form of SPPs.