The effects of different heat treatment processes on the anisotropy of TB6 titanium alloy fabricated by laser deposition manufacturing were investigated.The evolution of microstructure was analyzed by using optical microscope （OM）， scanning electron microscope （SEM）， and transmission electron microscope （TEM）. The variation trend and influence mechanism of anisotropy with heat treatment were investigated. The present research shows that the original β grains and the morphology of the primary α phase （α_p phase） are greatly affected by the thermal gradient. The original β grains in the microstructure of TB6 titanium alloy fabricated by laser deposition manufacturing are elongated along the deposition direction and are ellipsoidal. In addition， the relative slender α_p phase parallel to the deposition direction is found. These two factors jointly lead to the anisotropy of room temperature tensile property of the as-deposited samples. The tensile strength in the vertical deposition direction （X-direction ） is 7.3% higher， the yield strength is 5% higher， and the elongation is 32.4% lower than that in the deposition direction （Z-direction）. The low-temperature annealing treatment has little effect on microstructure， only the anisotropy of plasticity is decreased. After high-temperature annealing treatment， the difference in aspect ratio of α_p phase is significantly reduced， leading to the anisotropy of the room temperature tensile property decreases. The strength are still higher in the X-direction， and the elongation is higher in the Z-direction. The strengthening mechanism of the solution-aging treating sample is completely changed due to the precipitation of the secondary α phase （α_s phase）. In addition， there is no obvious preferential growth of α_s phase after heat treatment， so the anisotropy of the room temperature tensile property tends to be eliminated as the strength increases.

A large number of droplets and their products produced by titanium fire combustion in aeroengine compressor will cause burn through and non-inclusiveness failure of titanium alloy casing. This has shown great harm. In this study， a quantitative evaluation method for titanium fire inclusiveness of compressor was explored based on the mechanism of titanium alloy melt drop ablation and laser ignition technology. A test and evaluation method was established with the characteristic parameters of the melt drop penetration resistance of two configurations of TC4 titanium alloy casing， namely horizontal expansion and vertical drip. Meanwhile， the diffusion behavior of titanium fire and the critical failure conditions under simulated airflow environment were varified by experiments as well. Those results show that the mechanism of titanium alloy droplet burning through the casing lies in the local high heat concentration formed at the droplet contact interface. Under the action of heat transfer， the kinetic energy of the atoms in the base of the titanium alloy cartridge increases rapidly， forming a penetrating liquid phase， and finally causing burn-through， that is， titanium non-inclusiveness failure. When the droplet moves horizontally in the process of extended combustion， it will be affected by some mechanism such as reverse airflow， which will weaken the expansion effect. When the droplet is adhered to the surface of the casing simulation for a long time under the action of gravity or centrifugal force， the heat released is enough to burn through the titanium alloy casing. Its critical thickness is between 1.5-2 mm.

Due to the large temperature gradient in the laser melting deposition process， the coarse primary β columnar grains with preferred orientation are formed along the deposition direction， resulting in significant anisotropy of materials. This study aims to change the morphology of the primary β grains， refine the microstructure and weaken the texture of titanium alloy by adding Cu element in the materials during the process of laser melting deposition. The effects of Cu content on the microstructure and texture of TC4 titanium alloy manufactured by laser melting deposition are studied systematically. The results show that Cu element addition can refine the columnar primary β grains significantly and make the grain size distribution more uniform. The columnar grains are transformed to fully equiaxed grains when 4% Cu （mass fraction， the same as below） is added into the material， and the average size of primary β grains decreases to 385 μm from 1490 μm of TC4 titanium alloy. Basket-weave microstructure composed of α-Ti， β-Ti， and a small amount of Ti₂Cu is obtained inside primary β grains of the samples with Cu addition. The short rod-like Ti₂Cu distributes at the boundary of the α-Ti lath， and its proportion in the microstructure increases with the increase of Cu addition. The average width of α-Ti is 0.44 μm when 8% Cu is added， which is reduced by about 63% compared with 1.18 μm of the sample without Cu addition. When 4% Cu is added， the maximum multiples of uniform distribution（MUD） value of α-Ti pole figure is reduced by about 71% compared with TC4 titanium alloy，which demonstrates that the addition of Cu can significantly reduce the texture strength of titanium alloy manufactured by laser melting deposition.

The cooling rate after high-temperature heat treatment has a significant effect on the microstructure and properties of Ti65 alloy. The effect of cooling medium temperature on the cooling curve and microstructure of Ti65 alloy after high-temperature heat treatment has been systematically studied. The results show that the temperature of the oil medium has an opposite effect on the cooling curve to that of the air medium. The maximum cooling rate of oil is 73.2 ℃/s at room temperature， while the maximum cooling rate of air cooling is only 11.2 ℃/s. As the temperature increases， the cooling rate curve for oil quenching condition shifts to the right， and the maximum cooling rate and the minimum film boiling temperature increase. When the oil temperature is in the range of room temperature to 60 ℃， the cooling rate curve for oil quenching condition includes vapor， boiling， and convection three stages. When the oil temperature rises to 80 ℃， the vapor stage disappears. In addition， the microstructure shows a transition trend from α+β dual-phase microstructure to martensitic microstructure with the increase in oil temperature. Conversely， with the temperature increases， the cooling rate curve for air cooling condition shifts to the left， the maximum cooling rate decreases， and the temperature at the maximum cooling rate in the boiling stage gradually increases. Compared with oil quenching at different temperatures， the microstructure of air cooling under different temperatures shows typical bimodal microstructure with no obvious difference. The effect of oil temperature on the cooling curve is mainly attributed to changes in oil viscosity and fluidity， while the effect of air temperature on the cooling curve is mainly attributed to multiple complex factors such as air density and temperature gradient.

Titanium and titanium alloys with surface-prepared coatings have excellent overall performance as bipolar plates for proton exchange membrane fuel cells， but the bare plates will form passivation films with poor electrical conductivity during service， reducing the efficiency of the whole machine. By studying the feasibility of α-corrosion resistant titanium alloy Ti35 （Ti5Ta） and its Al alloying， the electrical conductivity of bare plate was improved on the premise of ensuring corrosion resistance. Using the cluster-type composition design method developed by the group and combing with the highest solid solution degree on the phase diagram， three kinds of Ti-Ta alloys with compositions of Ti-7Ta， Ti-8.3Ta， and Ti-9.6Ta and three kinds of Ti-Al-Ta alloys with compositions of Ti-2.6Al-5.8Ta， Ti-3.8Al-8.6Ta， and Ti-5Al-11.3Ta were designed. The experimental results show that the cathodic current density of the developed alloy is lower than that of the reference alloy TC4 under the simulated cathodic service environment （0.5 mol/L H₂SO₄+2×10^-6 HF） at a constant potential polarization voltage of 0.6 V （vs. SCE） for 4 h. The cathodic current density of the designed alloy is lower than that of the reference alloy TC4， among which the Ti-8.3Ta alloy has the smallest value of 0.72 μA·cm^-2. Under 1.5 MPa loading pressure， with the increase of Ta content， the interfacial contact resistance （ICR） value gradually decreases and is better than that of the pure titanium and TC4. The ICR value of Ti-5Al-11.3Ta alloy is the smallest， which is 18.3 mΩ∙cm^-2， and its cathodic polarization current density is 0.91 μA·cm^-2. In summary， the appropriate addition of Ta and Al can effectively improve the service performance of titanium alloy bipolar plates， and it is expected to realize the preparation of titanium alloy bipolar plate materials without coating.

In urgent demand for high-performance cryogenic titanium alloy for the heavy-lift launch vehicle， a novel 1500 MPa Ti-Al-V-Zr-Mo-Nb cryogenic titanium alloy （CT1400） was designed. Alloy bars and powder metallurgy materials of CT1400 were fabricated， and the microstructure， tensile properties， and cryogenic tensile deformation mechanism were also observed and analyzed. The results indicate that the CT1400 cryogenic titanium alloy mainly consists of α phase and a small quantity of β phase， which shows a typical near-α type cryogenic titanium alloy. CT1400 alloy bars display the apparent equiaxed fine-grain microstructure characteristic， and the powder metallurgy materials show the dominating lamellar microstructure combing with the “network” structure characteristic. CT1400 titanium alloys display excellent room and cryogenic tensile properties， which can stably reach cryogenic stress of 1500 MPa resulting from dislocation strengthening and grain boundary strengthening mechanisms. Furthermore， the twinning deformation at the cryogenic temperature of 20 K could additionally improve the cryogenic plastic deformation capacity of CT1400 titanium alloy by coordinating crystal orientation， promoting strain hardening， making it represent excellent coupling of strength and ductility at cryogenic temperatures.

The defects on TB6 titanium alloy were repaired using pulsed TIG（tungsten inert gas） additive manufacturing technology， and the effects of process parameters （pulse current and pulse time） and heat treatment on the microstructure and mechanical properties of the repaired TB6 titanium alloy were studied to determine the optimal heat treatment process parameters. The results show that the mechanical properties are relatively better in the as-repaired state when the pulse current is 50 A and the pulse time is 40 ms， with a tensile strength of 1113 MPa and an elongation of 5.26%. These samples are sequentially subjected to solid solution and aging heat treatment. When the samples are solid solution treated for 2 h under different temperatures （740， 760， 780 ℃）， the primary α phase is increasingly dissolved， while the β phase gradually grows and evenly distributes in the matrix. After water quenching， the growth of the β phase is inhibited， and acicular rhombic martensite α'' phase is precipitated in β grains， resulting in a decrease in tensile strength and a remarkable increase in elongation. Under different aging temperatures （500， 520， 540 ℃） for 8 h， the α'' phase continuously grows and gradually transforms to equiaxial grain， and the mechanical properties are greatly improved. The optimal microstructure and mechanical properties are achieved under the conditions of 780 ℃/2 h WC+520 ℃/8 h AC， with a tensile strength of 1119 MPa and an elongation of 7.36%.

The microstructures and properties of two kinds of zirconium-enriched α alloys （Ti₆₀Zr₄₀）₉₇Al₃（mass fraction/%，the same below） and （Ti₅₀Zr₅₀）₉₇Al₃ via different heat treatments were investigated and analyzed based on optical microscope （OM） observation， differential scanning calorimetric （DSC） measurement， X-ray diffraction （XRD） measurement， scanning electron microscope （SEM） observation and tensile tests at room temperature.The results show that after 850 ℃/40 min annealing， microstructures consisting of basket-weave α phase and a small amount of reticular β phase are formed in the alloys. After 850 ℃/40 min quenching， the acicular α' martensite phase is formed. After 850 ℃/40 min quenching and then 600 ℃/4 h aging， a large part of α' martensite phase converts into α phase， and the microstructures of the alloys consist of α and α'_remain phases. The yield strength of the T40Z3A alloy can reach 1100 MPa， with a favourable tensile elongation of 7%. The T50Z3A alloy exhibits higher strength but lower ductility than those of the T50Z3A alloy. Due to the higher content of Zr element， the T50Z3A alloy has more reticular β phase after annealing and more remaining α' martensite phase after quenching and aging， which results in higher strength and lower ductility.

The in-situ synthesized particle reinforced TC4 matrix composites were prepared by powder metallurgy pressureless sintering using polycarbosilane （PCS） as precursor. The thermal compression simulation experiments were conducted on TC4-1PCS （mass fraction of PCS is 1%） composites at 850-1100 ℃ and 0.001-1 s^-1 to analyze the stress-strain curves of the composites under different parameters using the Gleeble-3500 thermal simulation testing machine. The effects of deformation parameters on the reinforced phase particles， matrix structure and densification were analyzed by OM， SEM and EBSD methods. The results indicate that the TiC reinforced phase particles with the size of 5-10 μm and large amount of residual pores are observed in the TC4-1PCS composites before hot deformation. The β transition temperature（T _β） of TC4-1PCS matrix is 1000-1050 ℃. When deformed above T _β， matrix of composite consists of lamellar quenched martensite， while the matrix turns into duplex microstructure， when deformed below T _β. The deformation temperature determines the relative density and microstructure types of the composites， while the strain rate affects the phase size in the matrix and residual porosity. The densification of TC4-1PCS composites can be promoted by the increase of deformation temperature and the decrease of strain rate， while the increase of strain rate has obvious effect on the microstructure refinement. The microstructure refinement and densification of TC4-1PCS composites can be achieved by the deformation at 1050 ℃ and 0.1 s^-1.

The low-cycle fatigue （LCF） behavior of Ti-10Mo-xFe （x=1， 2， 3，mass fraction/%） alloys with different plastic deformation modes were investigated by OM， SEM， EBSD and electro-hydraulic fatigue test machine. The effects of the strain amplitudes （Δε _t/2=0.5%， 1.0% and 1.5%） and Fe content on the mechanical response， microstructures and fatigue crack propagation behavior were analyzed. The results show that the low-cycle fatigue performance of the alloys decreases with the increase of strain amplitude and Fe content. The cyclic stress response behaviors generally exhibit an initial cyclic hardening and then tend towards a cyclic stability or slight cyclic softening until fracture. The plastic deformation mode of Ti-10Mo-1Fe alloy is dominated by ｛332｝〈113〉 twinning， and changes to dislocation slip with the increase of Fe content. A few twins are formed in the fatigue initiation region of Ti-10Mo-1Fe alloy under low strain amplitude， and the twin area fraction increases gradually along the crack propagation direction， while a large number of twins are activated near the fracture area under high strain amplitude. The activation and intersection of abundant twins in Ti-10Mo-1Fe alloy divide the grain into network microstructures， which effectively release the stress concentration and delay the initiation of fatigue crack due to the dynamic microstructure refinement effect. At the same time， the abundant twin boundaries significantly extend the fatigue crack propagation path because the micro-crack deflects along the twin boundaries.

Besides cast-rolling speed and casting temperature， the melt pressure in the cast-rolling zone is also an important factor affecting the process stability and interfacial bonding strength of horizontal two-roll cast Ti/Al composite plate. In this study， several Ti/Al composite plates were prepared under different melt pressures in the cast-rolling zone by adjusting the liquid level of the melt in the sluice during the cast-rolling composite process. The microstructure and interfacial bonding properties of the composite plates were characterized by metallography， scanning electron microscopy， microhardness， tensile test and T-type peeling test at room temperature. Results show that when the melt pressure is relatively high， composite plate with full filling， good plate shape and high bonding strength can be produced successfully. However， excessively high melt pressure will affect the stability of the cast-rolling composite process. When the melt pressure is too low， the transverse flow capacity of the melt decreases. As a result， the cast-rolling zone cannot be completely filled with the melt， and the defects such as misrun and side penetration occur. At the same time， some micropores and microcracks appear on the Ti/Al interface. Under relatively high melt pressure， the solid/liquid contact distance is long. In this condition， the wetting between the strip surface and the melt is more sufficient， the melt is more evenly distributed， and the diffusion between solid and liquid is more sufficient. As a result， the composite plate has high bonding strength， which reaches 20.1 N/mm.

High-quality thick Ti-6321 titanium alloy welded joints were obtained by tungsten inert gas（TIG）welding， the microstructure changes of fusion zone， heat affected zone and base metal zone of the welded joints before and after annealing were compared.The impact properties， fracture toughness and tensile properties were tested， and which were compared with the base metal. The results show that the microstructure of the fusion zone before annealing is composed of coarse β columnar grains with fully grown intragranular acicular martensite α′ phase， and heat affected zone consists of equiaxed structure with β matrix and primary α phase， martensite α′ phase precipitates in β matrix.After annealing， the martensite α′ phase in β matrix of the fusion zone and heat affected zone completely transforms into secondary α phase. The impact toughness， fracture toughness， tensile strength and elongation of Ti-6321 titanium alloy welded joints are 80.3 J/cm²， 113.00 MPa·m^1/2， 873 MPa and 9%， respectively， which are 104.7%， 84.1%， 100% and 67.7% of the base metal，respectively. Compared with the fracture of the base metal， the impact fracture of welded joints has a coarser stepped surface and smaller equiaxed dimples， while the fracture surface of toughness is more flat and the fatigue crack propagation zone is narrower.

The ignition behavior of Ti₃Al-based alloy in 220-380 m/s gas flow environment was studied by using friction ignition method. Combined with numerical calculations, the influence of airflow velocity on surface oxygen concentration and oxidation control step was analyzed, and then the influence of airflow velocity on ignition behavior was discussed. The results show that the Ti₃Al-based alloy begins to ignite when the airflow velocity reaches 240 m/s.When the airflow velocity reaches 360 m/s, the Ti₃Al-based alloy no longer ignites.Under low airflow velocity conditions, the surface oxygen concentration at high temperature is lower than the critical value, and the oxidation reaction control step changes from the chemical kinetics process at low temperature to the diffusion process of oxygen to the alloy surface at high temperature. As the airflow velocity increases, although the convective heat dissipation rate increases, the increase rate of the oxidation heat generation rate caused by the increase of the surface oxygen concentration is greater than that of convective heat dissipation rate, which increases the heating rate and promotes the ignition of Ti₃Al-based alloy.Under high airflow velocity conditions, the surface oxygen concentration at high temperature is still higher than the critical value, and the control step of the oxidation reaction is always the chemical kinetics process. At this time, with the increase of airflow velocity, the increase rate of oxidation heat generation rate at high temperature is smaller than that of convective heat dissipation rate, resulting in a decrease in heating rate, which is not conducive to the ignition of Ti₃Al-based alloy.

The composition of high-damage-tolerance dual-phase TC21 (Ti-6Al-2Zr-2Sn-2Mo-2Nb-1.5Cr) were analyzed on the basis of the dual-cluster formula of Ti-6Al-4V, consisting of 13 α-Ti and 4 β-Ti unit. Its β-Ti unit is reduced from 5 to 4 compared with Ti-6Al-4V, while adding more β-stabilizing elements to enhance the strength and plasticity. Subsequently, the atoms of each β-stabilizing element within the β-Ti cluster formula of TC21 were proportioned equally to increase the mixing entropy, and more Zr content was increased to substantially enhance the β-phase stability, giving the cluster formula as α-{[Al-Ti₁₂](AlTi₂)}₁₃+β-{[(Al-(Ti₁₂Zr₂)]Sn_0.75Mo_0.75Nb_0.75Cr_0.75}₄(atom fraction), named as TC21Z2, with the corresponding mass fraction of Ti-5.9Al-5.4Zr-2.6Sn-2.1Mo-2.0Nb-1.1Cr. The samples were prepared by using vacuum copper mold pouring process and the as-cast microstructure and tensile mechanical properties of the alloy were studied.Results show that the as-cast microstructure of TC21Z2 is composed of α+α' martensite+ a small amount of β phases, and its ultimate tensile strength, yield strength and elongation are 1289, 1181 MPa and 1.4%, respectively. Its strength and plasticity are better than those of the TC21 at the same state.

Metastable β type Ti-34Nb-4Zr-0.3O(mass fraction/%)alloy (TNZO) was prepared by vacuum arc melting, single-phase hot forging and cold rolling. The cold-rolled TNZO alloys were aged at low-temperature of 250 ℃ and 300 ℃ in order to reveal the effect of aging temperature and time on the precipitation behavior of ω phase and the mechanical properties of the alloys. The results show that ω phase precipitates in nanometer size as a result of low temperature aging which leads to the increase of strength and elastic modulus of the alloys. ω phase is easy to coarsen and agglomerate when the alloy is aged at 300 ℃, which results in the rapid drop of the elongation and embrittlement of the alloy. Short time aging at 250 ℃ could make the ω phase precipitated dispersedly with small volume fraction, enabling the alloy to have excellent comprehensive properties of high strength, low modulus, ultra-high elasticity and good plasticity and show a broad application prospect in the field of aerospace elastic titanium alloys and medical implant titanium alloys.

TCGH(TC4+GH4169)composite material was prepared by selective laser melting(SLM). The optimum forming process parameters of TCGH composite material were investigated, and the microstructure and mechanical properties of as-deposited samples and heat-treated samples were studied. The results show that the optimum process parameters for fabrication of TCGH composite material are scanning speed of 900 mm/s with laser power of 150 W, and density higher than 99.5%. The addition of GH4169 powder changes the solid phase transformation behavior of TC4 titanium alloy material, and the as-deposited structure shows obvious high temperature solidification characteristics, which makes the forming characteristics of progressive scanning overlap and layer-by-layer scanning accumulation obvious. The original coarse columnar β grain size along the printing direction is significantly reduced, and the tensile strength of the composite is improved. Compared with the as-deposited sample, the microstructure of the heat-treated sample is transformed into a near-equiaxed structure. At the same time, with the increase of heat treatment temperature, the dissolution of the second phase leads to the dominant solid solution strengthening effect of the composite material, which improves the tensile strength and plasticity of the composite material.

Ti-13Nb-5Sn dental alloy was prepared by powder metallurgy method. The effects of ball milling time (3, 12, 24 h and 48 h) on powder performances, material microstructure, electrochemical corrosion and tribological behavior were investigated. The results show that with the increase of ball milling time from 3 h to 48 h, the powder morphology gradually changes from bulky to fine particles, and a part of Nb and Sn atoms diffuse into Ti lattice to form a certain volume of Ti(Nb) and Ti(NbSn) solid solutions. Moreover, equiaxed α-Ti decreases and shifts into columnar grain boundary α-Ti, and the basket structure changes to Widmandelsteiner structure. The potentiodynamic polarization curves show that the corrosion potential (E_corr) and polarization resistance (R_p) of the alloy display an upward trend, the corrosion current density (I_corr) reveals a downward trend in artificial saliva (AS) and simulated body fluid (SBF). The corrosion resistance of the alloy is improved because of reduction of α-Ti and increase of β-Ti. The hardness of the alloy increases, while the friction coefficient, wear depth and wear rate gradually decrease. More grain boundaries generate in sintering of the fine powder, resulting in the wear resistance of the alloy intensifying. The Ti-13Nb-5Sn alloy prepared by mechanical alloying combined with molding and sintering shows good corrosion and wear resistances, and has a great potential in the dental field.

TiB was planted into the matrix titanium alloy through the rapid solidification process, and the new ultrafine network reinforced titanium matrix composites (TMCs) powder was formed. Based on the laser additive manufacturing technology, a new titanium matrix composite with alternate distribution of equiaxed network and columnar network structure were creatively fabricated, the formation mechanism of the network structure was systematically discussed, and the mechanical properties of the super-solidified TMCs by additive manufacturing were tested. The results indicate that the network structure (about 9 μm) of the additively manufactured TiB/Ti composites is mainly composed of in-situ nano-TiB whiskers, presenting two crystal structures of B₂₇ and B_f. The direct introduction of B element is easy to form constitutional supercooling at the solidification interface. The equiaxed α phases are obtained by promoting the alternating formation of equiaxed/columnar network structure and refining the grain size. In addition, the formed nano-TiB network structure, not only inhibits the crack deflection and passivate cracks, but also confines the large number of slip lines inside the TiB network structure via in-situ observation, inducing high-density dislocations at the grain boundaries, which limites its plastic deformation, and greatly improves the strength of the composites. The additively manufactured TiB/Ti composites increases the tensile strength by 42%, and maintains the elongation of about 10%.

In order to optimize the hot working window of the novel TiZrAlHf Ti-based medium entropy alloy, the thermal deformation characteristics and microstructural evolution during thermal deformation were investigated by using hot compression simulation experiments and microstructural characterization methods. The results show that the microstructure of TiZrAlHf alloy ingot primarily consists of lamellar α phase and Widmanstatten structure at grain boundaries. The β transformation temperature (T_β) of TiZr-AlHf alloys is 895 ℃. Within the α/α+β phase region (700-850 ℃) during deformation, an instability zone is identified within the temperature range of 700-750 ℃. The thermal deformation activation energy in the α/α+β phase region is 827.514 kJ/mol, the deformation microstructure predominantly comprised globular α phase, with the softening mechanism involving the globularization of lamellar α phase. When the alloy is deformed in the β phase region (900-1100 ℃) test process range, there is no instability zone in hot processing map, all samples remain intact without any signs of cracking. Consequently, free forging can be employed for roughing and finish forging operations. The hot deformation activation energy is 113.909 kJ/mol, and the deformation microstructure is mainly elongated β grains and acicular martensite α'. The softening mechanism in the region is dynamic recovery. The underlying nature of both deformation softening mechanisms lies in the proliferation of dislocations, slip and the evolution of cellular structures.

Post-weld heat treatment(PWHT) could improve the microstructure and properties of linear friction welded joints. The linear friction welded joints of TC11 titanium alloy were double annealed at 950 ℃ and 530 ℃. The effect of double annealing on the microstructure of the joints was studied by microscopic analysis techniques such as optical microscopy(OM) and scanning electron microscopy(SEM). The results show that the dynamic recrystallization grains in the weld zone(WZ) completely disappear after PWHT, and are replaced by coarse needle, strip, and spherical α-phase grains; the degree of deformation of the thermos-mechanically affected zone(TMAZ) decreases. The grains grow, and some α structures are spheroidized. The secondary α phase in the base metal(BM) grows up in the shape of coarse needle and short rod. The grain orientation of the WZ and the TMAZ is more random after heat treatment, and the preferred orientation is significantly reduced. The mechanical properties tests indicate that the double annealing treatment after welding eliminates the high hardness zone in the center of the joint, and the center hardness is about 50HV lower than that of the as-welded joint. The hardness of TMAZ is about 30HV higher than that of the as-welded joint. No significant decrease in tensile strength presents after PWHT. The impact toughness of the joint reaches 61.3 J·cm^-2 after PWHT, about 80% higher than that of the as-welded joint and close to that of the BM.

Due to the concentration of heat input in the plasma arc direct deposition technology, the material was prone to large residual stress and uneven deformation, which greatly affects the quality of the formed parts. All of the birth-death cell technique, the transient thermal model and the thermo-elasto-plastic model were adopted for the thermal process and residual stress simulation during the additive manufacturing process. The calculation results were used to study the effects of different deposition paths on the thermal cycle characteristics and residual stress distribution of TC4 part in plasma arc additive manufacturing. Meanwhile, the validation experiment was carried out to check the effectiveness of the finite element model through thermal tests. The simulated thermal curves match the experimental results well. The results show that both paths generate higher residual stress in the area around the arc-extinguishing point than the rest, and the zigzag with contour-offset path has better heat dissipation than the full zigzag path, and the residual stress of the deposited layer of the contour-offset path is significantly lower than that of the full raster path. Previous layers are subjected to a complicated thermal cycles, when the new layers are deposited on old layers. The peak temperature is increased from the bottom layer to the middle layer. As new layer is deposited on top, the transient stress distribution of parts changes regularly. Larger stress is located near the middle of the top layers and the area where the bottom meets the substrate, which is then maintained and gradually converted into residual stress within the part.

Bimaterial printing of 0.8%(volume fraction)BNNSs/TC4 ink and TC4 ink were carried out by direct ink writing (DIW) to build an inter-layered "interlocked" interface. Dense inter-layered "interlocked" laminated TC4-TiB/TC4 composites reinforced by TiB nanowires were obtained by combining DIW with fast hot pressed sintering (FHPS), and their mechanical properties were investigated. The results show that the inter-layered "interlocked" structure is successfully retained after sintering; the inter-layered "interlocked" laminated TC4-TiB/TC4 composites prepared by DIW have an improved strength (by 9.1% to 1206 MPa) while preserving the high toughness compare to the unconfigured 0.4%TiB/TC4 composites with uniform distribution of the reinforcement. The analysis shows that the inter-layered "interlocked" interface deflects the cracks significantly, hinders the crack expansion, and improves the overall material toughness, which provides ideas for forming complex and tough composite components.

The optical microscope, scanning electron microscope, electron backscattered diffraction and three-dimensional atom probe techniques were employed to investigate the morphology, distributions of alloy elements and orientation of blocky α phase in β forging TC17 titanium alloy. The results show that the blocky α phases are distributed nearby the grain boundary, inside the prior β grains and recrystallized grains, while abnormal microstructure such as long and coarse α lamellar can also be observed inside the grains. There is no significant difference in the distribution of main elements between all kinds of abnormal α and β matrix phase. Therefore, the segregation of alloy composition does not cause the blocky α phase. The acicular α phase is randomly distributed in the grain and is acicular or cotton-blocky under the microscope. In addition, the orientation of α phase is also a main factor determining lamellar morphology. Coarse lamellar and agglomerate blocky α phase can be observed inside the fine recrystallized grains and slightly deformed grains. The fine recrystallized grains represent the undeformed or slightly deformed grains, which grain boundary α and α platelets inside the grain are basically single in orientation. Therefore, the α phase with the same orientation is easy to agglomerate under slow cooling. In addition to the dispersed cotton-blocky α phase, it also appears as blocky α phase which run through the entire grain or agglomerate near the grain boundary.

The three-dimensional small crack growth behavior of selective laser melting (SLM) TC4 alloy was investigated by in-situ observation fatigue test method, and the long crack growth curve was measured by load reduction method under the same experimental conditions. The results show that at the early stage of small crack growth, the fatigue crack growth(FCG) rate fluctuates obviously under the influence of microstructure, and the FCG path is zigzag. With the increase of crack length, the influence of microstructure decreases, and the FCG path is straight, and the FCG rate increases steadily with the crack length. Internal defects can still reduce alloy fatigue life. Considering the small and long crack growth data, it is demonstrated that the small crack can still propagate under threshold value of long crack, and under the same stress intensity factor amplitude ΔK, small crack growth rate is higher than that of long crack. There is a typical "small crack effect", thus the small crack behavior should be considered when fatigue life prediction is carried out.

TC4-DT titanium alloy forgings with X-type groove were joined by laser additive joining technology. OM and SEM were used to characterize the macro and micro morphology of TC4-DT titanium alloy base metal, heat affected zone and joining zone. The microhardness of the three regions was measured by Vickers hardness tester. The tensile and impact tests of different samples were carried out at room temperature with universal testing machine and pendulum impactor. The results show that there is a dense metallurgical bond between the joining zone and the base metal. Distribution of microhardness from the base metal to the joining zone shows an increasing trend. The strength of TC4-DT alloy increases first and then decreases with the decrease of the proportion of the joining zone in tensile specimen, while the plasticity decreases. The impact toughness a_kU is above 55 J/cm². The position of U-shaped notch opening has a great effect on the impact toughness of the joining zone but not the bond zone.

Titanium and its alloys are promising biomedical metallic materials due to their high specific strength, low Young's modulus, nonmagnetic, excellent biocompatibility and corrosion resistance. A new generation of metastable β-type Ti alloys with non-toxic Nb, Mo, Ta, Zr and Sn alloying elements and low Young's modulus has become the key research direction of Ti alloys for biomedical applications. The basic characteristics and development history of biomedical titanium alloys were reviewed. Taking Ti-Nb based biomedical titanium alloys as an example, the composition design method, alloying principle, research status and preparation technology of new metastable β-type biomedical titanium alloys were introduced. Finally, it was pointed out that the further reduction of elastic modulus and improving the comprehensive properties including strength, fatigue performance, and functional properties are the key development directions of β-type Ti alloys for biomedical applications. In the future, in-depth research should be placed on the interaction mechanism of alloying elements, chemical composition design approach, microstructure and mechanical properties regulation methods, as well as micromechanical mechanisms.

Ti₂AlNb based alloys is considered to be the most potential material material to replace the traditional Ni-based superalloys, because of its excellent high-temperature specific strength, creep resistance and high fracture toughness. Ti-22Al-25Nb alloy was fabricated by selective electron beam melting(SEBM), and the density of as-built samples reached 5.42-5.43 g/cm³through process optimization. The microstructure, phase evolution and mechanical property of the as-built and HIPed Ti₂AlNb alloy samples were investigated. The results show that the microstructure of the as-built and HIPed samples both show the columnar crystalline structures along the deposition direction, which are all composed of B2, O and α₂ phases, and the amount of O/α₂ phase gradually increases from top to bottom. After HIP, the width and amount of the O/α₂ phase are reduced and relatively uniform when compared with that of the as-built samples. In the bottom area, the microhardness of the as-built sample exhibits higher value of about (345.87±5.09)HV, while the hardness increases to 388.91-390.48HV after HIP. The ultimate tensile strength and elongation of the as-built sample at room temperature are (1061±23.71) MPa and (3.67±1.15)% respectively, and the ultimate tensile strength increases to (1101±23.07) MPa and the elongation reduces to 3.5% after HIP.

The study status on the joining of titanium alloy/steel dissimilar joint technology was summarized and reviewed, the microstructure characteristics of titanium alloy/steel direct joining and interlayer joining interface were analyzed.The formation and evolution process of titanium alloy/steel joint interface products with different interlayers (copper, copper-based alloy and others) were emphatically described, and the mechanical properties of titanium alloy/steel with different interlayers and preparation processes were also summarized.Finally, the methods and design ideas that can be adopted for the preparation of good titanium alloy/steel joints were summarized, and it was pointed out that the future development of this field can be combined with simulation in addition to using traditional methods to further the existing research, so as to achieve deeper understanding and experimental prediction.

The TiC-reinforced Ti-based coating was prepared in-situ on the surface of the titanium alloy TA15 by laser cladding technology. The forming quality, microstructure, phase composition, hardness, and tribological properties were investigated by optical microscope, X-ray diffractometer, scanning electron microscope, energy spectrum analyzer, microhardness tester and friction and wear apparatus. The results show that the coating mainly composes of β-Ti, Co₃Ti, CrTi₄ and TiC, and the good metallurgical bond is formed between coating and the substrate. The microstructure of the coating bond zone is planar crystal and columnar crystal, the middle is dendritic, and the top is equiaxed. Significant differences in the morphology of TiC are observed in each micro-area of the coating. TiC of the top and middle areas is thick dendritic and petal-like, while TiC of the bonding area is needle-like and spherical. The maximum microhardness of the coating is 715HV, which is about 2.1 times than that of TA15 (330HV). Under the same conditions, the wear loss of coating is 30.14 mg, which is about 30.7% of TA15(98.11 mg). The wear mechanism of the cladding coating and substrate is a composite wear mode of adhesive wear and abrasive wear, but the wear degree of the coating is lighter.

The microstructure evolution, tensile properties and fracture behavior of five-element Ti₂AlNb alloy Ti-22Al-23Nb-1Mo-1Zr (atom fraction/%) ring forging at different solution temperatures of 850, 880, 900 ℃ and 750 ℃ aging treatment (AT) were studied by scanning electron microscope (SEM), transmission electron microscopy (TEM) and mechanical testing machines. The results show that with the increase of solution temperature, the fine lamellar O phase is more solid-dissolved into the B2 phase matrix, the coarse lamellar O phase gradually becomes coarser and the volume fraction of O phase decreases after solution treatment (ST). After ST+AT, a very small amount of fine lamellar O phase precipitates from the B2 phase matrix at a higher solution temperature, coarse lamellar O phase is coarsened, the volume fraction of O phase tends to be the same. The tensile strength of the alloy decreases, while the ductility increases with the increase of solution temperature. The tensile fracture morphology is a quasi cleavage characteristic of typical cleavage and dimple mixed fracture. There are microcracks, slip characteristics and the bending O phase elongated along tensile direction in the longitudinal fracture. Dislocations distribute along the B2/O phase boundary. The small size of the lamellar O phase can effectively reduce the dislocation slip distance, resulting in strong strengthening effect.

In order to improve the surface friction, wear and high temperature oxidation resistance of TC4 titanium alloy, NiCrCoAlY+20%(mass fraction) Cr₃C₂ mixed powder was selected as the cladding powder to prepare NiCrCoAlY-Cr₃C₂ composite coating on the surface of TC4 titanium alloy by using laser cladding technology. The microstructure and phase composition of the coating were analyzed by OM, SEM, XRD, EDS, etc.The microhardness of the coating was measured by HXD-1000TB tester. MMG-500 three-body wear tester and WS-G150 smart muffle furnace were used to test the friction, wear and high temperature oxidation resistance of the coating and substrate. The results show that the laser cladding technology can be used to prepare the good composite coating on the surface of TC4 titanium alloy without cracks and pores. The microstructure of the cladding zone is dense, mostly needle-like crystals and dendrites.The microstructure of the bonding zone is mainly composed of planar crystals, cellular crystals and dendrites, which generates a variety of products including the carbides, oxides and intermetallic compounds that can improve wear resistance and high temperature oxidation resistance. The maximum microhardness of the composite coating is 1344HV, which is about 3.8 times of the 350HV of the titanium alloy substrate.The friction factor of the composite coating is 0.2-0.3, which is significantly lower than the friction factor of the titanium alloy substrate of 0.6-0.7. Under the same conditions, the wear mass loss of the composite coating is 0.00060 g, which is 0.9% of that of 0.06508 g of titanium alloy substrate. After oxidation at 850℃ for 100 h, the oxidation mass gain of the composite coating is 6.01 mg·cm^-2, which is about 24% of that of 25.10 mg·cm^-2 of titanium alloy substrate. Laser cladding technology effectively improves the friction and wear performance and high temperature oxidation resistance of the TC4 titanium alloy surface.

Based on grain refinement and secondary phase strengthening, minor boron (B) was added to near β-Ti alloy to strengthen the alloys. Ti₈₅Fe₆Cu₅Sn₂Nb₂ alloys with various B contents were designed, prepared by using a non-consumable vacuum arc melting furnace, and hot rolled at 800℃followed by quenching. The effects of minor B addition on the microstructure and mechanical properties of Ti₈₅Fe₆Cu₅Sn₂Nb₂ alloy were investigated through microstructure observation, tensile mechanical test, fracture observation and transmit electron microscopy. The results reveal that minor B addition can refine the grains, improve the strength whereas the plasticity of the alloy is decreased. The alloy containing 0.15% (mass fraction)B possesses the better comprehensive mechanical properties(σ_0.2=1105 MPa, δ_b=4.5%).With the increase of B content, the strength of the alloy is increased and reaches up to 1156 MPa. Orthorhombic TiB compounds are formed in the alloy, distributed in the β-Tialloy matrix. Upon deformation, the fracture of TiB phases, cutting and debonding of TiB phases to the alloy matrix, formed the fracture source, resulted in the decrease of the alloy plasticity.

TiAl/Ti₂AlNb dissimilar alloys were successfully bonded together by spark plasma diffusion bonding. The joints were subjected to post-weld heat treatment at different temperatures. The microstructure of the welded joint was analyzed and the tensile strength and microhardness of the joint were tested. The results indicate that after heat treatment, the microstructure of Ti₂AlNb base metal, TiAl base metal and interface has no obvious change. In the Ti₂AlNb heat affected zone(HAZ), most B2 phase gradually turns into O phase, due to the precipitation of acicular O phase, the microhardness increases significantly compared to as welded condition. With the increase of heat treatment temperature, the microhardness of the Ti₂AlNb HAZ gradually decreases, meanwhile, the tensile strength of the joint at room temperature shows an evident increase compared to as welded. The maximum tensile strength of the joint reaches 376 MPa at heat treatment temperature of 900℃. After heat treatment, the fracture mode of the joint is brittle fracture.

Near β titanium alloy is widely used in automotive and aerospace industries due to their high strength-to-weight ratio and high corrosion resistance. The near β titanium alloy can precipitate ω phase and α phase after solution and ageing treatment, the strength of which can be remarkably increased, usually at the expense of ductility. It is one of the most important structural components of load-bearing that usually as aircraft skin, shell plating, main frame, linker and special fastener. The alloy used in this paper is a self-developed Ti-Al-V-Mo-Cr-Zr-Fe-Nb ultra-high-strength β titanium alloy, which is a typical near β titanium alloy. The characteristic of isothermal phase transformation of near β titanium alloys is diversity and complexity, which is sensitive to temperature and directly affects the mechanical properties after ageing. In this paper, the microstructural evolution and mechanical properties of a Ti-Al-V-Mo-Cr-Zr-Fe-Nb ultra-high strength β titanium alloy after isothermal treatment were investigated by scanning electron microscopy(SEM), transmission electron microscopy(TEM) and micro-hardness tester. The results show that only the isothermal ω precipitates are formed during ageing at 300℃, and the size of isothermal ω phases increase with the ageing time. The isothermal ω precipitates are first precipitated during ageing at 400℃. With the extension of the ageing time, the α phase nucleation occurs near the ω/β interface. No α precipitates are obtained in the alloy aged at 500℃, and needle-like α precipitates are directly precipitated from the β matrix, which is evenly distributed in the β matrix in a "V" shape. Tensile test shows that the tensile strength of the alloy is 1716.1 MPa and the elongation is 2% after ageing at 400℃ for 12 h. The tensile strength of the alloy is 1439.8 MPa and the elongation is 9.84% after ageing at 500℃ for 12 h, and has a good combination of strength and toughness.

Owing to the excellent biocompatibility and corrosion resistance of titanium and its alloys in the biological environments, they are one of the best materials in the medical implant applications. Moreover, it has a lower elastic modulus (comparable to bone) than traditional metal implant materials which is an influential property due to the stress-shielding effect. There are some requirements for implant materials according to their clinical use and the periphery tissues. Hence, some factors should be considered, such as metal degradation, toxicity issues, surface characteristics, biocompatibility, and fusion with bone. Considering the above-mentioned information, titanium material design with superior performance to meet the essential clinical needs is an important challenge and attracts much attention from the academicians in the biomaterial field. This paper discusses the structural and performance characteristics of medical titanium alloys and the current research status in the direction of orthopedic applications. Furthermore, in future research, through changing the elemental composition, increasing the surface modification, and optimizing the production process, titanium alloy materials could have the excellent comprehensive performance to serve human beings better.

The high cycle fatigue behavior of TC11 titanium alloy with lamellar structure before and after surface nanocrystallization was studied by supersonic fine particle bombardment (SFPB).The microstructure of the high cycle fatigue fracture and its vicinity were compared and analyzed by means of optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD).The results show that there are 30-50 μm thick nanolayers on the surface of titanium alloy after SFPB treatment, and the size of nanocrystalline is about 5-15 nm.The fatigue performance is improved obviously and the fatigue life is increased about 8-10 times under the same stress level, the fatigue striation width becomes narrow, and the multiple of fatigue life increases gradually with the decrease of loading level.The fatigue fracture surface before and after SFPB treatment consists of the fatigue source zone, the crack propagation zone and the instantaneous fracture zone, but the fatigue source after SFPB treatment moves from the surface layer before treatment to the subsurface.After fatigue loading, the surface microstructure of SFPB treated specimens is still in nanometer scale, but there are a lot of deformation twins, dislocation tangles and a small amount of deformation-induced martensite in the subsurface microstructure.

The hot deformation behavior of TB8 titanium alloy with a lamellar α structure in the α+β dulex phase region was investigated. The results show that at the strain rate of 1 s^-1, a continuous flow softening phenomenon is observed in the curve of the samples deformed at 650 ℃, while a discontinuous yield phenomenon is visual in the curve of the samples when the deformation temperature is higher than 650 ℃. The discontinuous yield phenomenon is gradually disappeared with increasing deformation temperature and strain rate. When the strain rate is 0.001 s^-1 and the deformation temperature is 750 ℃ as well as 800 ℃, typical characteristics of dynamic recrystallization is presented in the curve of the samples. The relationship among peak stress σ_p temperature T and strain rate and is characterized by Arrhenius-type constitutive equation. The equation between the material constants (α, Q, n and lnA) and strain is constructed. The effect of strain on the material constants (α, Q, n and lnA) of the Arrhenius-type constitutive equation is analyzed. The value of α is increased with true strain, while the values of Q, n and lnA are gradually decreased. The correlation coefficient (R²) and the AARE value between the experimental and the predicted stress are 0.945 and 9.08%, respectively. This indicates that the strain-compensates Arrhenius type constitutive equation can better predict the flow stress value under different deformation conditions for the TB8 titanium alloy with a lamellar α structure deformed in the α+β dulex phase region.

The Ti₂AlNb alloy and the titanium matrix composites can be joined by direct solid phase diffusion, but the high diffusion temperature causes the base metal to undergo a phase transition, even the joint properties were therefore deteriorated. A method for optimizing the properties of solid phase diffusion joints of Ti₂AlNb alloy and Ti matrix composites was proposed by using Ti foil intermediate layer. The results show that after adding 30 μm Ti foil intermediate layer, the diffusion bonding temperature reduces from 950 ℃ to 850 ℃, the deformation rate reduces from 5% to 1.7%, and the decreasing of diffusion bonding temperature can effectively change the structure of the joint interface. The typical interface is Ti₂AlNb/B2 phase/α+β dual phase structure/Ti matrix composite, in which the strength of the joint increases by the formation of α+β dual phase structure at the joint interface.When the optimum diffusion bonding parameter is 850 ℃/60 min/5 MPa, the shear strength reaches a maximum value of 399 MPa. The diffusion bonding of Ti₂AlNb and Ti based composite at low temperature is realized.