On Secondary β-Nb Phase Precipitation within Primary α-Zr Phase in Zr-Nb Alloys During Tensile Deformation

摘要

Two-phase alloys of zirconium, Zr-1wt% Nb and Zr-2.5wt% Nb, are widely used in nuclear reactors as fuel-clad and structural material, respectively. Post-irradiation studies on Zr-Nb alloys have shown Nb rich β (containing more than 50wt% Nb) precipitation within the α-Zr phase. In the present study, slow deformation of the two alloys at elevated temperature was carried out on unirradiated material at different temperature up to 873 K. Secondary phase (β-Nb), identified by TEM, micro diffraction and Energy Dispersive Spectroscopy, was observed to precipitate within the primary phase (α-Zr), which is similar to the observations in the irradiated material. The precipitation was dependent on temperature and imposed strain. Measurements of the lattice parameter of the α-Zr phase using X-ray diffraction show a sharp increase in samples deformed above 550 K, where the precipitation was also observed. This change is due to Nb drawn out of the supersaturated α-Zr phase during precipitation. There is a progressive increase in the dislocation density, measured by X-ray diffraction, due to strain on elevated temperature tensile deformation (up to 723 K). No such change in lattice parameter was observed in the case of samples aged at test temperatures without deformation. Dislocations generated during deformation provide enhanced Nb diffusivity, increased preferential nucleation sites, and reduced the strain energy required for nucleation of the β-Nb phase within the existing α-Zr phase. The similarity in the secondary phase precipitation observed between slow deformation at elevated temperature and irradiation is explained on the basis of dislocation-enhanced precipitation. The change in the texture due to the uniaxial tensile deformation at elevated temperature gives information on the acting deformation mechanisms. By measurement of texture at each test temperature, the deformation mechanisms were determined, which is related to the observed precipitation behavior.