Cyclic deformation behaviour and saturation bundle structure in Ti–5 at.% Al single crystals deforming by single prism slip

Abstract

Cyclic deformation behaviour and dislocation structure of Ti–5 at.% Al single crystals oriented for single prism slip have been studied. Ti–5 at.% Al tested under total strain control (Δεt/2) displays an initial slight cyclic hardening followed by a striking softening period, and then a saturation stage is reached in the range 0.2–1.0%, except at a very low strain amplitude (0.2%) at which the initial hardening is replaced by an immediate cyclic softening. The saturation stages occupy most of the lifetime. The cyclic stress–strain curve (CSSC) contains a clear stress plateau region within the tested plastic shear strain amplitudes (0.039–1.68%). The plateau shear stress was about 85 MPa. () single prism slip was observed on the surface of the fatigued specimen using an optical microscope, and became denser as the cyclic strain amplitude or cyclic number increased. The typical dislocation structure in the primary slip plane is the saturation bundle structure (SBS), which consists mainly of dense screw dislocations parallel to the direction and dislocation bundles nearly perpendicular to the direction, that is closely parallel to the [0001] direction. Some dislocation slabs were observed in the (0001) foils. The dislocation structure in Ti–5 at.% Al single crystals evolves from the elongated screw dislocation lines along [] during the stage of the first cyclic hardening to the parallel screw dislocation lines together with the dislocation bundles nearly along [0001] in the stage of cyclic softening, and a well-developed SBS is finally formed in the stage of saturation. The relationship between the cyclic plateau behaviour and the saturated bundle structure of Ti–5 at.% Al single crystals is discussed.