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

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 beta-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 beta-blocks. Further refinement of the beta-blocks is still governed by dislocation activities, and finally nanograins with a diameter of similar to 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. (C) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

Deformation kinking, an uncommon plastic deformation mechanism, was investigated in Ti-11 V metastable beta-Ti alloys through severe plastic deformation using cold forging. The study revealed the microstructure evolution and grain refinement behavior during deformation kinking. It was found that the formation of kink bands goes through dislocation gliding, pre-kinking, and the ripening of pre-kinks. The resulting grain refinement involves transverse breakdown and longitudinal splitting of dislocation walls and cells. Nanograins with a diameter of approximately 15 nm were produced at a large strain of 1.2. The study also showed that nanocrystallization is highly localized inside kink bands while the outer microstructure maintains original coarse structures.