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.