Slip behavior of Bi-modal structure in a metastable β titanium alloy during tensile deformation

3 titanium alloys with bi-modal structure which exhibit improved strength-ductility combination and fatigue property are widely used in aviation and aerospace industry. However, owing to the small size of primary a (alpha(P)) and nano-scaled multi variant distribution of secondary cc platelets (alpha(s)), investigating the deformation behavior is really a challenging work. In this work, by applying transmission electron microscopy (TEM), the slip behavior in alpha(P) and transformed beta matrix with different tensile strain was studied. After alpha/beta solution treatment, the initial dislocation slips on {110} plane with < 1 1 1 > direction in beta matrix. During further deformation, (110), (101) and (1 1 2) multi slip is generated which shows a long straight crossing configuration. Dislocation cell is exhibited in beta matrix at tensile strain above 20 %. Different from the solid solution treated sample, high density wavy dislocations are generated in transformed beta matrix. High fraction fine alpha(s) hinders dislocation motion in beta matrix effectively which in turn dominates the strength of the alloy. In primary a phase (alpha(P)), a core-shell structure is formed during deformation. Both pyramidal a + c slip and prismatic/basal a slip are generated in the shell layer. In core region, plastic deformation is governed by prismatic/basal a slip. Formation of the core-shell structure is the physical origin of the improved ductility. On one hand, the work hardening layer (shell) improves the strength of up, which could deform compatibly with the hard transformed beta matrix. Meanwhile, the center area (core) deforms homogeneously which will sustain plastic strain effectively and increase the ductility. This paper studies the slip behavior and reveals the origin of the improved strength-ductility combination in Bi-modal structure on a microscopic way, which will give theoretical advises for developing the next generation high strength beta titanium alloys. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.