Formation of duplex microstructure in Zr–2.3Nb alloy and its plastic behaviour at various strain rates

• Duplex microstructures (DM) obtained at 850–925 °C were investigated.
• The stable Dαp with increasing annealing time or temperature was discussed.
• With increasing strain rate, an increased plasticity parameter of RA was studied.
• Microvoids were more prone to be formed along the interfaces of αp/βp.
• A deformation sequence of β, αs and αp to coordinate the plasticity of DM was found.

The formation of duplex microstructure in Zr–2.3Nb alloys at a temperature range of 850–925 °C, its plastic behaviour at various strain rates and the accommodation of each phase in duplex microstructure during the tensile deformation process were investigated thoroughly in this paper. It is shown that with increasing temperature, the dimension of βp exponentially increased, while the dimension of αp remained nearly unchanged due to the competition of grain growth and phase transformation from α to β. The duplex microstructure obtained at 850 °C displayed a higher strength and a comparable plasticity compared with other duplex microstructures obtained at higher temperatures, due to the reinforcement of the ω phase and the uniform grain size. With increasing strain rate, the strength of the duplex microstructure increased, while the plasticity parameters – elongation (EL) and reduction of area (RA), showed opposite tendencies, a decreased tendency for EL and increased tendency for RA. Analysis indicated that the decreased EL was due to the fact that the higher dislocation annihilation rate at higher strain rate reduced the work hardening rate, resulting in an early onset of necking. The increased RA was consistent with the fracture surface observation: the increased mean size and mean depth of dimples, which revealed an increased post-necking EL and then slowed down the falling tendency of EL. Microvoids were prone to form along the interfaces of the αp and retained β phases. Finally, SEM and TEM observations revealed the deformation sequence of these phases as follows: β, αs, αp. And then a simple deformation model illustrating the deformation behaviour of each phase was proposed.