Effect of annealing treatment on the microstructure and mechanical properties of a duplex Zr-2.5Nb alloy

The microstructural evolution and mechanical properties of a duplex Zr-2.5Nb alloy annealed at different temperatures have been studied in detail. The results show that after annealing treatment, the residual stress in α phase is reduced effectively, and both the αs and αp phases become stable due to the reduced Nb content. The αs plates become thicker and the amount of αp phase becomes smaller after annealing treatment, especially at higher temperatures. The evolution of the ω phase is complicated due to the fact that there is a ωath phase in the initial duplex microstructure, which is gradually transformed into α and β phases during the annealing process, and at the same time, the ωiso phase is precipitated. The retained ω phase reinforces the specimens, with the specimen annealed at 300 °C exhibiting the highest hardness and strength. The decreased dislocation density and weakened solution strengthening effect of the Nb alloying element on the predominant α phase result in a lower yield strength and a higher ductility for the annealed specimens, especially for the specimen annealed at 600 °C.