Research On Relationship Between Microstructure, Texture And Tensile Properties

High temperature titanium alloys have been used widely in aerospace because of the low density, specific strength and good performance at high temperature. Ti-55alloy which is designed by IMR, is a near-α high temperature titanium alloy, and can be used stably up to550℃. Ti-55is mainly used in aero engines compressor disk and blades, while its plate in spacecraft and aircraft’s skin. On these special application occasions of high temperature titanium alloy, it is very important to achieve a good match between anisotropy of mechanical properties and the demand of use. Crystallographic orientations of hcp structure α-Ti making plate mechanical properties changes along different direction. In order to control the processing planar anisotropy of titanium alloy sheets, to make it develop in the direction of product performance optimization, people seek quantitative relationship between texture and properties on the one hand, and explore the realistic texture control process on the other hand.Microstructure and texture type are the key factors that decide the performance of titanium alloy. Annealing heat treatment after deformation is one of the key processes in forming microstructure and texture, especially for the plate after severe deformation. The morphology and volume fraction of α and β phase varies a lot in different process and heat treatment, and result in diversity in microstructure and texture of titanium alloy. By means of metallomicroscope, SEM and X ray diffraction, this paper analyzes the effect of different annealing treatment temperature to the titanium alloy plate microstructure and texture. Study on tensile properties of Ti-55alloy plate after annealing treatment at different temperatures in RD, TD, and45°direction; and discusses the corresponding relationship between microstructure, texture and its tensile properties of Ti-55alloy plate.The results show that, plate forms bimodal structure after annealing treatment in (α+β) phase filed, which is composed of equiaxed primary α and lamellar structure. The volume fraction of Primary α goes down as annealing temperature goes up, the slip in unidirectional rolling plates stucture prefers partition in the direction of rolling stress. Plates forms widmanstaten structure after annealing treatment in β phase temperature, α phase colonies cross distributes in original β grain. The size of α phase colonies increase along with annealing temperature. Processing technology affects the texture type of titanium alloy. B/T texture formed in Ti-55plate after unidirectional rolling in (α+β) phase filed at low temperature, and two-step cross-rolling forms B texture. Annealing treatment in (α+β) phase filed temperature, B/T texture in Ti-55plate does not changes, and B texture differentiates to RD and TD direction as the annealing treatment temperature goes up.(0002) basal plane forms relatively hard texture in TD direction and45°direction with RD after annealing treatment in (α+β) phase filed temperature. Crystal orientation paralleled to ND disappears after annealing treatment at970℃. When annealing treatment temperature goes above (α+β)/β transformation point, the changing process of bcc β phase to hcp α phase forms new type texture in plate different from original deformation texture, basal plane mostly distributes30°in TD to RD direction and the texture of RD hardens in a large scale after annealing treatment temperature above (α+β)/β transformation point.Tensile test shows that, the type of fracture of Ti-55plate changes from plastic dimple fracture surface to plastic and brittle mixed fracture surface with the increase of annealing treatment temperature. The tensile strength of Ti-55plate of B/T texture is weakest in45°direction, while strongest in TD direction; B texture plate has little differentiation in RD and TD in room temperature, and weakest tensile strength in45°direction. The tensile properties of the titanium alloy plate is affected by the types of texture and microstructure. Tensile break extension percentage in45°direction is higher than RD and TD direction at the same annealing temperature. The tensile elongation rate in room temperature is high in duplex microstructure of a phase after rolling and bimodal structure contain about20%primary a phase. Distributing of ReO severely affects the tensile strength of plates. Large particles of ReO in grain boundary reduce the tensile strength of the titanium alloy sheet and plastic.