In Situ Synchrotron X-ray Diffraction Investigation Of The Mechanical Properties And Physical Mechanisms Of The β-type Ti Alloys

The β-type Ti alloys that exhibit the low density,high specific strength,excellent corrosion resistance,ultralow elastic modulus,shape memory property,superelasticity,and high biocompatibility,have been widely used in many fields.In this thesis,in situ synchrotron X-ray diffraction was used to investigate the mechanical behaviors and deformation mechanisms of β-type Ti based alloys,which provided novel perspective for the design of Ti alloys.The obtained main research results are summarized as follows:Ti-24Nb-4Zr-8Sn polycrystalline alloy with β phase exhibited anomalous change in full width at half maximum for ｛110}β peak during elastic deformation of the uniaxial compression cycles,i.e.peak narrowing upon loading and peak broadening during unloading.The anomalous change was attributed to the formation of irreversible stress-induced ω phase.The different moduli of ω and βphases led to the different changes in lattice strain,which caused the narrowing and broadening of(110)β peak that actually contained(1120)ω,peak under different applied stress.The warm-rolled Ti-30Zr-10Nb alloy with β phase exhibited the two-stage yielding behavior under uniaxial tension.The stress-induced martensite transformation and elastic interaction between matrix and martensite contributed to the obvious strain hardening of the first yield stage.The large tensile strain(~13.5%)and ultra-low strain hardening of the second yield stage were related to a reversible stress-induced reorientation of martensite variants,which was characterized by the appearance of the(110)α" diffraction peaks with rigid lattice rotation of～23° and the[110]α" axis toward tensile direction.The warm-rolled Ti-30Zr-10Nb alloy after 500℃/0.5 h treatment with a"+βphases exhibited shape memory property with-3.15%recoverable strain.The alloy exhibited the two-stage yielding behavior under uniaxial tension.The strain-hardening rate of the first yield stage was large and fluctuant corresponding to the interaction of the stress-induced martensite transformation,martensite reorientation and martensite elasticity.The gradual decrease of strain-hardening rate in the second yield stage was related to the martensite reorientation with the appearance of the(110)α" diffraction peaks due to the rigid lattice rotation and the martensite elasticity.The warm-rolled Ti-30Zr-9Nb alloy with β phase exhibited the two-stage yielding behavior under uniaxial tension.The elongations of the rolling direction,transverse direction and 45° direction samples were 15.4%,17.1%and 21.1%,respectively.In situ synchrotron X-ray diffraction results demonstrated that the stress-induced martensite transformation of the alloy was anisotropy.The stress-induced martensite and martensite reorientation occurred during the second yield stage in rolling direction and 45° direction samples.Interestingly,the appearance of(110)α",and(111)α",diffraction peaks upon loading for 45° direction and transverse direction samples,respectively,contributed to their larger enlongation.The three samples exhibited superelaticity due to the reversible martensite transformation.Meanwhile martensite reorientation occured during unloading in the 45°direction sample with the appearance of(111)α" diffraction peak and the intensity of the new diffraction peak increased.In this thesis,the mechanical properties and deformation mechanisms ofβ-type Ti alloys were investigated,which provided direct evidences of the effect of the martensitic transformation and reorientation on the mechanical properties.Theoretical and experimental basis were provided for the improvement of mechanical and machining properties of titanium alloys.