Deformation kinking and highly localized nanocrystallization in metastable β-Ti alloys using cold forging

Abstract

• Microstructural refinement involving deformation kinking was investigated. • Formation of kink band involves dislocation gliding, pre-kinking and its ripening. • Nanograins ∼15 nm in diameter are produced but highly localized in kink bands. • Localized nanocrystallization results from the intrinsic soft nature of kink bands. • The softening of kink bands originates from the inner degraded dislocation density. Deformation kinking as an uncommon plastic deformation mechanism has been reported in several materials while the relevant microstructure evolution and grain refinement behavior at a large strain remain unclear so far. In this study, the issue was systematically investigated by utilizing cold forging to impose severe plastic deformation (SPD) on Ti-11 V metastable β -Ti alloys. It is found that the formation of kink bands experiences dislocation gliding, pre-kinking and the ripening of pre-kinks in sequences. The kink bands are subsequently thickened through the coalescence of multiple kink bands in a manner of high accommodation. Ordinary dislocation slip is developed as a dominant deformation mechanism when deformation kinking is exhausted. The resulting grain refinement involves transverse breakdown and longitudinal splitting of dislocation walls and cells, which fragment kink bands into small β -blocks. Further refinement of the β -blocks is still governed by dislocation activities, and finally nanograins with a diameter of ∼15 nm are produced at a large strain of 1.2. Alternatively, it is revealed that nanocrystallization is highly localized inside kink bands while the outer microstructure maintains original coarse structures. Such localized refinement characterization is ascribed to the intrinsic soft nature of kink bands, shown as low hardness in nanoindentation testing. The intrinsic softening of kink bands is uncovered to originate from the inner degraded dislocation density evidenced by both experimental measurement and theoretical calculation. These findings enrich fundamental understanding of deformation kinking, and shed some light on exploring the deformation accommodation mechanisms for metal materials at large strains.