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

Abstract

β 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 α (αp) and nano-scaled multi variant distribution of secondary α platelets (αs), investigating the deformation behavior is really a challenging work. In this work, by applying transmission electron microscopy (TEM), the slip behavior in αp and transformed β matrix with different tensile strain was studied. After α/β solution treatment, the initial dislocation slips on {110} plane with <1 1¯ 1> direction in β 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 β matrix at tensile strain above 20 %. Different from the solid solution treated sample, high density wavy dislocations are generated in transformed β matrix. High fraction fine αs hinders dislocation motion in β matrix effectively which in turn dominates the strength of the alloy. In primary α phase (αp), 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 αp, which could deform compatibly with the hard transformed β 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 β titanium alloys.