Study On Microstructures And Mechanical Properties Of New Metastable BETA Titanium Alloys

Metastable β-Ti alloys are widely used in medical treatment,aerospace and other fields due to the advantages of low density,high specific strength,great plasticity and excellent working ability,and high strength and fracture toughness can be enhanced by solid solution aging treatment.The most of metastable β-Ti alloys that exhibit excellent strain hardening ability are benefited from the effect of transformation induced plasticity(TRIP)and twinninginduced plasticity(TWIP)during deformation.However,the low critical stress of stressintroduced martensitic(SIM)usually causes the low yield strength;on the other hands the single deformation mechanism of high strength β-Ti alloys deteriorates the plasticity.How to simultaneously achieve high strength and plasticity is the difficulty in the design and development of β-Ti alloys.In this study,the molecular orbit method,molybdenum equivalent and valence electron concentration methods are combined to design the composition of Ti alloys.Two new Ti-Mo based metastable β-Ti alloys Ti-11Mo-2V and Ti-7Mo-2V-2Cr-1Fe were developed.The microstructures and mechanical properties was studied,and the influence of deformation mechanisms on strain hardening was also revealed,so as to provide theoretical guidance for the development of new metastable β titanium alloys with high strength,plasticity and excellent workability.The study of Ti-11Mo-2V alloy indicates that the alloy has high yield strength(693 MPa)and excellent elongation(31.8 %),showing excellent comprehensive mechanical properties.The main deformation mechanism is stress-induced {332}<113> deformation twinning at early stage of deformation,the network structures formed by the multiplication of twins lead to the increase of strain hardening rate.Secondary SIM α’ is formed inside the twins with the increase of strain,and secondary {112}<111> deformation twinning is introduced at the twin-twin intersection interface,forming a unique hierarchical structure.The two secondary deformation structures realize the stress concentration of primary twinning boundaries and interfaces of twin-twin intersection,respectively,optimizing the local stress state and improve the plasticity and strain hardening ability of Ti-112 alloy.At later stage of deformation,the primary twinning boundaries are destroyed due to the strong interlacing between twins,and the twin density in the β grain tends to be saturated,the strain hardening rate thus decreases.The study of Ti-7Mo-2V-2Cr-1Fe alloy indicates that the alloy has high yield strength(649MPa)and elongation(30.5 %).The high strain hardening rate(≥ 1246 MPa)over a wide strain range(17.3 %)provides alloy with a strength increment exceeding 300 MPa and a uniform elongation ratio of more than 80 %.The density of twins is significantly different among individual grains due to the various grain sizes and crystal orientations,resulting in heterogeneous deformation.The heterogeneous deformation effectively improves the plasticity of alloy as a strain accommodation mechanism.The Hall-Petch effect introduced by the formation of dense deformation twinning networks increases strain hardening rate.With strain increases,the β-spinodal structures with heterogeneous mechanical stabilities facilitates the generation of secondary {112}<111> deformation twinning inside primary twinning to accommodate increase of local strain.The internal tertiary SIM α’’ is further formed in secondary {112} <111> deformation twinning under larger strain,which forming complex hierarchical structures.At later stage of deformation,decrease of strain hardening rate is caused by the saturated twin density in grains.The sequential activation of heterogeneous deformation,β-spinodal structures and hierarchical structures imparts excellent strain hardening ability to Ti-7221 alloy,and then exhibiting great strength-plastic match.