Study On The Stress Induced And Thermal Induced Microstructures Of Metastable Beta-type Ti Alloys

Metastableβ-type Ti-Nb and Ti-Mo alloys are essential materials for biomedical applications due to their good biocompatibility,non-toxicity,low modulus and excellent mechanical properties.The main efforts of the previous studies are to improve the mechanical properties of those metastableβ-Ti alloys,however,the microstructures which affect the mechanical properties still need to be investigated in more details,especially in the aspect of the structural characterization at atomic resolution.In this study,the representative stress-induced and thermal-induced microstructures in Ti-Nb and Ti-Mo alloys have been studied by a combination of various techniques,especially by the aberration-corrected high-angle annular dark field scanning transmission imaging technique（HAADF-STEM）which allows us to investigate the structural characterization at atomic resolution level.The aims of this study are to depeen the understanding of these microstructures,to explain some relavent fundmental scientific questions and to provide valuable information for the design and development of metastableβ-Ti alloys.Some important results are summarized as follows:1.The boundary structure for the{112}<111>deformation twin at its initial growth stage in a tensile deformed Ti-50Nb（wt.%）alloy has been studied at atomic scale by aberration-corrected HAADF-STEM.The formation of step structures is found at the twin boundaries,which indicates that the growth of{112}<111>deformation twinning is via the double cross slip dislocation mechanism.It is the first time that this mechanism of the{112}<111>deformation twinning has been confirmed based on the experimental observations at atomic resolution level.Theωtransitional structure（ω_T）characterized by gradual collapse of{111}_βatom planes can be formed both in the twin growth frontier region of a{112}<111>deformation twin plate and along the longitudinal twin boundaries for the twin plate to accommodate stress associated with the{112}<111>twinning.These relusts offer a conclusive evidence to understand that the{112}<111>twinning shear is critical in assisting theβ-to-ωstructural transition.Additionally,on the basis of thest results,these formation mechanism of theω_T structure induced by the{112}<111>twinning shear has been discussed as well.2.The structures related to{112}<111>shear transformation in the tensile deformed Ti-50Nb（wt.%）alloy have been studied at atomic scale by means of aberration-corrected HAADF-STEM for the first time.The localized deformation structural features have been characterized and relevant formation mechanism discussed.The limitations concerning the application of the conventional electron diffraction and transmission electron microscopy（TEM）diffraction contrast imaging to distinguish the{112}<111>deformation twin and stress-inducedωphase during TEM characterization have also been pointed out and discussed.3.{112}<111>_βtwin-twin interaction in the tensile deformed metastableβ-type Ti-30Nb-3Pd（wt.%）alloy,which involves the reversibleα″martensitic transformation in its deformation process,has been investigated by means of TEM.A new type of the{112}<111>_βtwin-twin interaction which can give rise to{332}<113>_βtwinning products in the interaction region in theβphase has been observed for the first time.This characteristic twin-twin interaction is proved to be the result of the relaxation of the{110}<110>_α″twin-twin interaction combined with a{130}<310>_α″twin in the interaction region in theα″martensite phase when the stress is released.Based on the good crystallographic correspondence between the martensitic twinning andβtwinning,the feasible mechanism for understanding the{112}<111>_βtwin-twin interaction via stress-inducedα″martensitic transformation has been proposed.4.The formation mechanisms of the{332}<113>deformation twins in different metastableβ-Ti alloys with differentβphase stability have been systematically studied by HAADF-STEM at atomic resolution level.It is found that stress-inducedα″martensite structure remains in the{332}<113>_βtwin boundary region,exhibiting a gradual transition to theβstructure in the deformed metastableβ-type Ti-30Nb-3Pd（wt.%）alloy.This finding provides a direct experimental evidence to verify that the formation of a{332}<113>_βtwin in deformed metastableβ-type Ti-Nb-based alloys can be associated with a reversibleβ-to-α″martensitic transformation.In the deformed metastableβ-type Ti-15Mo（wt.%）alloy,the HAADF-STEM observations at atomic level clearly reveal that a new interfacial structure（B phase）can be formed in the{332}<113>_βtwin boundary region.Based on the structural transition features of the B phase,a new formation mechanism for the{332}<113>_βtwin,that is B phase transformation mechanism,has been proposed.It is for the first time that we chearly show that the formation mechanisms of{332}<113>deformation twins can be different deprnding on the alloy compositions and instability of theβphase.5.Theω/βinterface structures in the metastableβTi-15Mo（wt.%）alloy after isothermal ageing at 748K for 48h have been investigated at atomic scale by HAADF-STEM.The atomic resolution HAADF observations clearly reveal that,instead of the idealωstructures,a kind of gradually developingωtransitional structures,which can be regarded as poorly formedω-phases,are formed at theω/βinterface after a long time aging.It suggests that a pure atomic displacement mechanism is responsible for the formation and growth of the isothermalωphase in the Ti-Mo alloy.It is also found that theω/βinterface of isothermalωparticle can act as a nucleation site for theαprecipitation.The stress fields associated with the diffuseω/βinterface could play a more important role in enablingαnucleation at theω/βinterface,and this process in the Ti-15Mo binary alloy is actually different from theω-assistedαformation in multi-component alloys which Al-or O-rich regions provide potent nucleation sites for theαprecipitation.6.The features of bright and dark stripes exhibiting in a single isothermalωparticle in a metastableβTi-15Mo（wt.%）alloy after isothermal ageing at 748K for48h have been investigated by HAADF-STEM.The reasons why the bright and dark areas exist in a single isothermalωparticle could be qualitatively explained by taking the stress conditions between theβmatrix and theωparticle during theβ-to-ωphase transformation.7.The tensile testing for the aged Ti-15Mo alloy after isothermal ageing at 748K for 48h has been carried out at room temperature.The aged alloy shows brittle fracture,however,some pronounced plastic deformation bands can be formed at the fracture surface.The structural characteristics of the plastic deformation bands have been studied by aberration correction HAADF-STEM at atomic scale,through which the successive transformation mechanism has been proposed to explain the unusual plastic localization in the brittle aged Ti-15Mo alloy.It is clearly indicated that high local stress during the deformation process can be released by atom shuffles at different phase interfaces which could result in the formation of the plastic deformation bands.