Research On The Deformation Mechanisms And Mechanical Properties Of ?-Ti Alloys

?-Ti alloys have been widely used in the fields of aviation,automobile and biology,due to their advantages such as the low density,high specific strength,excellent corrosion resistance,good biocompatibility and so on.With the application fields of Ti alloys expanding continuously,it puts forward higher requirements for mechanical properties.Thus,it is necessary to develop new ?-Ti alloys with high performances.The deformation behaviors of ?-Ti alloys strongly depend on their deformation mechanisms.Alloying and heat treatment are two major methods to adjust the deformation mechanisms and improve the mechanical properties of ?-Ti alloys.The widely-used “d-electron method” and “solution treatment and aging(STA)process” need to be improved to meet the increasing requirements during application.Accordingly,this paper establishes a novel theoretical model based on compositional average electron-to-atom ratio((?))and atomic radius difference((?)),and a step-quenching treatment to refine the design and process methods of ?-Ti alloys by taking the case of Ti-Mo-based alloys.The microstructures and deformation mechanisms of Ti-Mo-based alloys were investigated by X-ray diffraction(XRD),optical microscope(OM)and transmission electron microscope(TEM).The main contents of this thesis are as follows:Ti-(5,8,10,15,20 wt.%)Mo alloys were prepared by solution treatment and water-quenching.The microstructural observation indicated that a Mo content higher than 10 wt.% was needed to retain most ? phase in the room temperature.The tensile results showed that the mechanical properties were sensitive to Mo content and ? phase stability.The Ti-10 Mo and Ti-15 Mo alloys exhibited large ductilities of 30 and 37% respectively with tensile strengthes of 756 and 739 MPa.TEM observations revealed the extensive appearance of deformation twins in Ti-10 Mo and Ti-15 Mo alloys after tension.The Ti-20 Mo alloy displayed much smaller ductility of 13% because of the higher Mo content and ? phase stability.The main deformation products in tensioned Ti-20 Mo alloy were dislocations.It suggests the dominant deformation mechanism of ? Ti-Mo alloys changes from twinnings to dislocation slip with the increase of Mo content.The cooling methods after solution treatment affect the formation of ?? martensite and ? phase in ? Ti-Mo alloys,and thereby influence the dominant deformation mechanisms during tensile test.Both the re-orientation of ?? martensite and stress induced ?? martensitic transformation(SIM)occurred in the water-quenched(WQ)Ti-(10,11,12 wt.%)Mo alloys,resulting in 0.4 ~ 0.7% superelastic recovery strains after 3% pre-strain and 0.1 ~ 0.3% shape memory recovery strains during subsequent heating.A total recovery strain of about 1% and a total recovery ratio around 33% were obtained in the WQ Ti-11 Mo and 12 Mo alloys.The increase of Mo content suppressed the formation of ?? martensite and ? phase,and raised the amount of metastable ? phase,accordingly promoting the SIM and improving the superelasticity of WQ Ti-12 Mo alloy.In comparison,the air-cooled(AC)alloys containing relatively more ? phase showed limited shape memory properties due to the detrimental effect of ? phase.Appropriate step-quenching treatment generated a multiphase microstructure comprising ??,?,?,and ? phases by balancing the competitive martensitic ?? and diffusional ? transformations,thereby improving the mechanical properties of metastable ?-Ti alloy.Direct WQ or AC Ti-10Mo(wt.%)alloy produced a considerable amount of soft ?? martensite or hard ? phase,which inducing poor mechanical properties.After short aging at 650 ?,the ductility was decreased to 5% because of the formation of large-sized ? precipitates.A step-quenching treatment containing 650 ? holding for 0.5 h was designed based on the time-temperature-transformation(TTT)curve of ? transformation.Microstructural observation revealed that step-quenching favored the formation of ? precipitates thinner than 20 nm,and effectively moderated the subsequent ?? martensitic transformation,resulting in an excellent combination of tensile strength(790 MPa)and ductility(23%).Two ?-Ti alloys,Ti-8Mo-4.5Co and Ti-11Mo-7Zr(wt.%),were designed by the “d-electron method”.The microstructural investigation and tensile tests revealed that SIM and twinning occurred during the deformation of Ti-11Mo-7Zr alloy,which showed a low yield strength of 475 MPa and a high ductility of 28%.In contrast,the Ti-8Mo-4.5Co alloy displayed a high yield strength of 980 MPa and a low ductility of 8%,implying a slip-dominant deformation behavior.The results suggested that the “d-electron method” failed to predict the deformation mechanism of Ti-8Mo-4.5Co alloy and needed to be improved and refined.The inherent characteristics of alloying elements relate to the ability of lattice shear,thereby affecting the activition of Twinning/SIM.A theoretical model based on ((?)) and ((?)) was proposed to refine the “d-electron method”.Slip and Twinning/SIM regions were separated in the ((?))-((?)) diagram according to the compositions and deformation behaviors of ?-Ti alloys.It was found that a low ((?)) favored Twinning/SIM by reducing the resistance of lattice shear.The decrease of ((?)) represented a decreased volume of unit cell,indicating a stronger binding force among atoms.The increase of |(?)| generated stronger local lattice distortion,resulting in an increased resistance against lattice shear and Twinning/SIM.Ti-11Mo-7Nb-3.5Zr(wt.%)alloys were designed by the ((?))-((?)) diagram and tensile tested.The alloy showed a siginificant strain-hardening behavior and achieved excellent mechanical properties with good tensile strength of 780 MPa and high ductility of 53%.This study proposed a novel theoretical model for the composition design and a step-quenching method for the processing of ?-Ti alloys based on the experimental results of Ti-Mo-based alloys.The above investigations can not only promote the understanding of the deformation mechanisms of ?-Ti alloys,but also offer a theoretical and experimental basis for the improvement of mechanical properties.Furthermore,this paper provides reference and theoretical guidance for the development of advanced ?-Ti alloys.