Aging response and characterization of precipitates in Zr alloy Excel pressure tube material

Abstract Precipitation hardening in the Zr-based alloy Excel (Zr–3.5 wt.% Sn–0.8 wt.% Mo–0.8 wt.% Nb) was studied using hardness testing and transmission electron microscopy (TEM). Solution treatment at 890 °C, in the α Zr  + β Zr region, and 980 °C, in the β Zr region, followed by water-quenching and aging resulted in an increase of hardness, of up to 47% compared to the annealed material. The optimum condition for aging heat treatment was found to be 450 °C for 1–2 h. The precipitates were observed only in the transformed β Zr martensitic phase. Energy dispersive X-ray spectroscopy (EDS) in TEM showed the precipitate composition to be Zr–30 wt.% Mo–25 wt.% Nb–2 wt.% Fe. The crystal structure of the precipitates was determined to be hexagonal with a  = 0.294 nm and c  = 0.448 nm, i.e. c / a  = 1.526.