Age hardening of cold-worked Zr-2.5wt% Nb pressure tube alloy

Abstract

Age-hardening of a cold-worked Zr-2.5wt% Nb pressure tube alloy at ageing temperature, 300–500°C was studied with crystallographic texture as a variable. Results showed that hardness, yield and ultimate tensile strengths increased with increasing ageing temperature up to about 400°C, while ductility remained practically unaffected by ageing. Further, the observed increase in both hardness and strength, was more pronounced in the direction parallel to the tranverse direction of tube than in the direction parallel to the longitudinal direction. X-ray diffraction indicated the formation of small volume fraction of ω-phase at all ageing temperatures. Transmission electron microscopic examination revealed partial decomposition of β-Zr phase and formation of fine spheroidal precipitate at an ageing temperature of 400°C, while ageing at 500°C, caused formation of cell structure and coarsening of the precipitate. Based on these observations, it was concluded that age-hardening of cold-worked 2r-2.5wt% Nb pressure tube alloy is a competition between precipitation-hardening and softening by recovery and the phenomenon is strongly influenced by texture.