Improved strength-ductility combination of pure Zr by multi-scale heterostructured effects via rotary swaging and annealing

In the present study, pure Zr with a heterostructure containing nano/ultrafine grains and recrystallized coarse grains was processed by rotary swaging at room temperature followed by partial recrystallization. The as-swaged Zr shows a significantly increased strength compared to its coarse-grained counterparts, due to abundant nano-grains and ultrafine grains refined by rotary swaging and activation of pyramidal slips. In addition, a strong fiber texture parallel to the SD is formed in the as-swaged Zr. A similar texture with weaker intensity is found in the as-annealed Zr. Moreover, a basal-type HCP-FCC phase transformation was detected during rotary swaging and maintained after annealing. In this case, the as-annealed Zr with the heterostructure exhibits a superior combination of high strength and enhanced ductility, which is ascribed to the multi-scale heterostructured effects: a) the nano/ultrafine grains and hetero-deformation induced strengthening contributed to the observed high strength, b) the work hardening from recrystallized coarse grains and hetero-deformation induced strain hardening improved the ductility, c) the dislocation motion could be effectively impeded at the FCC/HCP coherent interfaces to enhance the strength, d) the soft FCC phase can accommodate c-axis strain and provide more slip systems, which is believed to obtain good ductility. Furthermore, pyramidal slips are activated at the interface between coarse and ultrafine grains, which helps to improve the tensile properties of heterostructured Zr.