Effect of b-Zr decomposition on the solubility limits for H in Zr-2.5Nb

The zirconium alloy Zr-2.5Nb (Zr-2.5 wt.% Nb) is used to fabricate pressure tubes for CANDU nuclear reactors. The microstructure of the as-extruded tubes consists of elongated h.c.p. *a-Zr grains surrounded by a network of b.c.c. *b-Zr containing at least 20 wt.% Nb. The *b-Zr phase is meta-stable at temperatures below 900 K and gradually dissociates into its constituent elements *a-Zr and *b-Nb. The aim of this study was to determine the effect of *b-Zr decomposition on the terminal solid solubility for dissolution (TSSD) for H in Zr-2.5Nb. Specimens of Zr-2.5Nb were charged to different hydrogen concentrations and aged at temperatures ranging from 673 to 773 K for 30 to 6000 min, to decompose their *b-Zr component by different amounts. Hydride dissolution temperature and hydrogen concentration were determined using differential scanning calorimetry and hot vacuum extraction mass spectrometry, respectively. The results show that the as-extruded pressure tube structure, with the *b-Zr almost intact, has the highest TSSD and that the TSSD for structures with fully decomposed *b-phase is the lowest and approaches that of unalloyed Zr.