Deformation And Phase Transformation Behavior Of Near ? High Strength Titanium Alloys Investigated By Neutron And Synchrotron Radiation X-ray Diffraction

Near ? titanium alloys are widely used in the manufacture of the large loadbearing structural component such as landing gear in the aviation industry as their high specific strength,good fracture toughness,and excellent corrosion resistance.The microstructure is one of the important factors that affect the mechanical properties,and the understanding of the deformation and phase transformation behavior of titanium alloy is an important basis for improving the microstructure and mechanical properties.The deformation and phase transformation behavior of the material is determined by the crystal structure and multi-scale microstructural units,and the hot deformation process of titanium alloy often involves dynamic softening mechanisms,the interactions between deformation and phase transformation,which makes the microstructure evolution of near ? titanium alloys become a more complex process.Neutron and synchrotron X-ray diffraction techniques are widely used to characterize the multi-scale stress and structure evolution of the metallic materials during the processing and service,and to reveal the deformation and phase transformation mechanisms.In this thesis,the micromechanical behavior and phase transformation behaviors of typical near ?titanium alloys during the deformation at ambient temperature and high temperature are investigated by using in-situ neutron and synchrotron X-ray diffraction techniques,and the mechanism of macro/micro stresses on the microstructure evolution during the deformation is revealed in combination with microstructure characterization.The mechanical properties of near ? titanium alloys are closely related to the size and texture of the constituent phases,but at the micromechanical level,the affecting mechanism of the microstructure on the mechanical properties is not clear.The micromechanical behavior of ? and ? phases,the contribution of constituent phases to the mechanical properties,and the evolution of micro-stresses during room temperature tensile deformation in the typical microstructure(bimodal microstructure,lamellar microstructure)of the near-? titanium alloy Ti-55531 were studied by using in-situ synchrotron X-ray diffraction technique.The results show that in the bi-modal microstructure,the a phase bears about 200 MPa higher stress than the ? phase,while in the lamellar microstructure,the ? phase matrix bears about 150 MPa higher stress than a lamellae.In the bi-modal microstructure,it is found that the secondary a phase bears about 330 MPa higher stress than the primary ? phase by analyzing the asymmetry of diffraction peaks.The texture also significantly affects the microscopic stress evolution,the<200>texture of the ?phase increases the intergranular stress between the different oriented grains of the? phase.The microstructure of titanium alloys is very sensitive to hot deformation conditions,but the deeply physical mechanism is not clear.In this chapter,the dislocation density evolution of the near ? titanium alloy Ti-55531 during hightemperature tensile deformation at different hot deformation parameters was studied using in-situ neutron diffraction techniques,and the intrinsic connection between the dynamic softening mechanism and dislocation density evolution was analyzed in combination with microstructure characterization techniques.The results show that dynamic restoration is the dominant softening mechanism of ?phase during the deformation in the near ? region(810?,0.1/s)and in the ?+?region at a low strain rate(760?,0.001/s).The dislocation densities of the ? phase significantly increase during the work hardening stage,then it reaches a steady-state after reaching a balance between work hardening and dynamic restoration.The continuous dynamic recrystallization of ? phase tends to occur at a faster strain rate in the dual phase region(760?,0.1/s),and an increase in strain rate causes a significant increase in the dislocation density of ? phase,which promotes the continuous dynamic recrystallization of the ? phase.To further address the continuous dynamic recrystallization behavior of near ?titanium alloy under a faster strain rate,the micromechanical behavior of near ?titanium alloy TC18 during the deformation at the dual phase region was investigated by using in-situ synchrotron X-ray diffraction technique.The stress partitioning between constituent phase and microstructure evolution of near ?titanium alloy during the hot deformation are discussed in combination with microstructure characterization.The results show that the a lamellae bears higher stress than the ? phase during the hot deformation,resulting in a non-uniform strain distribution near the ? lamellae,which promotes the continuous dynamic recrystallization of the ? phase.The<200>p//LD oriented grains are more likely to undergo dynamic recrystallization than<111>?//LD oriented grains during the hot deformation.The continuous dynamic recrystallization behavior during the hot deformation and subsequent annealing is important for ? grain refinement.The refinement of ? phase grains could be achieved by combining hot deformation and subsequent annealing to obtain equiaxed ? phase grains with an average grain size of 12 ?m.The variant selection behavior of the a phase during the ??? phase transformation is an important factor that affects the microstructure and texture of the ? phase.The ? phase precipitation behavior of solid-solution treated near ? alloy Ti-55531 during heat treatment and hot deformation was investigated using the insitu synchrotron X-ray diffraction technique.The size and precipitation kinetics of a phase and variant selection mechanism during hot deformation were analyzed in combination with microstructure characterization.The results show that the hot deformation significantly accelerated a phase precipitation during the deformation at 600?.The kinetics of ? precipitation in the ? grains with different orientations are anisotropic during the hot deformation.Variant selection of ? precipitates was verified by synchrotron X-ray diffraction.Only the variant whose phase transformation strain accommodated with the applied strain field will nucleate and grow preferentially,resulting in specific variants or pairs of variants occurring in the microstructure.The variant selection behavior of the a precipitation during the hot deformation induces a strong crystallographic texture of the a phase.