【Citation Format】
HU Jinbao, MU Yiqiang, XU Qinsi, et al. Research Status and Application Progress of Ti-45Nb Alloy[J]. Special Casting & Nonferrous Alloys,2024,44(3):317-322.
Citation:HU J B，MOU Y Q，XU Q S，et al. Research status and application progress of Ti-45Nb alloy［J］. Special Casting & Nonferrous Alloys，2024，44（3）：317-322.
Ti-45Nb alloy (domestic grade TB14) is one of the commonly used titanium alloy engineering materials, which has good plasticity, anti-spontaneous combustion performance, anti-corrosion performance, extremely low elastic modulus and excellent biocompatibility. The solid solution of 45% Nb in Ti forms a solid solution with a single β-phase body-centered cubic structure (BCC), which has good cold working performance, and Nb element is non-toxic, and is gradually replacing pure titanium and some duplex (α+β) titanium alloys, such as replacing pure titanium rivets in aerospace fasteners and replacing duplex Ti-6Al-4V alloy in medical implant materials. The research results of Ti-45Nb alloy are reviewed, and the preparation process, strengthening method and application status of Ti-45Nb alloy are discussed.
The density of Ti-45Nb alloy is only 5.7 g/cm3, Nb is the metal with the lowest heat of oxidation per unit mass, its melting point is 2 468 °C, its boiling point is 4 742 °C, and its solubility in Ti is very high, so it can be considered as one of the most effective elements to reduce the spontaneous combustion of Ti. Titanium alloys with Nb as an alloying element have good flame retardant properties and corrosion resistance. Ti-45Nb alloy has excellent anti-spontaneous combustion performance and excellent cold working performance, with elongation of more than 20% and shrinkage of 60%~80%.
The unit cells of Nb element are body-centered cubic unit cells, and each unit cell contains 2 metal atoms. There are two allotropes in the metal Ti, the low-temperature (<882.5 °C) stable state is the close-packed hexagonal crystal system, and the high-temperature stable state is the body-centered cubic crystal system. Nb and Ti form a binary eutectic alloy system. The Ti-45Nb alloy has a single β-phase body-centered cubic structure (BCC) with a single equiaxed β phase, as shown in Figure 1. The Ti-45Nb grain has obvious orientation, which is (110) orientation, and its main phase is BCC structure. Because the crystal structures of elemental Nb and Ti at room temperature are both body-centered cubic structures, and the atomic radii of Ti and Nb are very close, they are mutually soluble to form a body-centered cubic TiNb solid solution, which is obviously characterized by high plasticity, but low strength and hardening rate, and excellent cold and hot processing performance.
Fig.1 Microstructure of Ti-45Nb alloy
Researchers from the School of Civil Aviation of Shenyang University of Aeronautics and Astronautics and the Institute of Metals of the Chinese Academy of Sciences published an article entitled "Research Status and Application Progress of Ti-45Nb Alloy" in the journal "Special Casting and Non-ferrous Alloys", Volume 44, Issue 3, 2024, the author pointed out that Ti-45Nb alloy has low density, high plasticity, high specific strength, low Young's modulus, good mechanical properties, cold working properties, spontaneous combustion resistance and excellent corrosion resistance. The physical properties, crystal structure, microstructure and microstructure of Ti-45Nb alloy and the common preparation process of Ti-45Nb alloy are reviewed, and the ways to improve the properties of Ti-45Nb alloy and the research status of Ti-45Nb alloy in recent years are introduced.
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【Overview of Preparation Process】
Ti-45Nb alloy has excellent properties and application prospects. The United States has conducted a lot of basic research on Ti-45Nb alloy, and the technology is relatively mature, and it was included in the AMS4982 specification in 1974 and revised to AMS4982G in 2020. The specification proposes that the alloy should be melted multiple times, and the first melt should be melted by vacuum self-consumable electrode, non-self-consumable electrode, electron beam cold furnace or plasma arc cold furnace melting process. The subsequent melt should be subjected to a vacuum arc remelting (VAR) process, and no alloys should be added during the final melt cycle. MARTINS G V et al. used the blended elemental technique to prepare the Ti-45Nb alloy, and the Ti-Nb alloy powder was prepared by hydrogenation-dehydration (HDH) process in a resistance furnace, weighted and mixed proportionally, and then put into a closed container for cold uniaxial pressing, and cold isostatic pressing (Cold) at a pressure of 350 MPa in an isostatic press isostatically presseing, CIP) for 30 s. Ti-45Nb alloy was prepared by sintering at 1 700 °C under vacuum conditions and holding for 2 h, and then cooled to room temperature with the furnace. The Ti-45Nb alloy prepared by this method has the characteristics of single β phase, high density, high hardness and low elastic modulus. Wang Zelong et al. studied a vacuum self-consuming smelting method to prepare Ti-45Nb alloy, using grade 0 titanium sponge and Nb particles as raw materials, weighing, batching and mixing according to the composition ratio of Ti-45Nb titanium alloy, pressing the mixed raw materials into a single electrode block, and then welding multiple identical single electrode blocks into a self-consumable electrode for three times of vacuum self-consumption remelting, and finally obtaining Ti-45Nb titanium alloy ingots. The Nb distribution in the Ti-45Nb titanium alloy ingots prepared by this method is uniform, and the deviation is not more than 0.5%. Li Yonghua et al. prepared a porous Ti-45Nb alloy by powder sintering. The problem of mass production of Ti-45Nb alloy is one of the factors restricting its wide application. Hou Fengqi et al. proposed a preparation method for Ti-45Nb alloy coiled round wire, in which the forged coarse bar is cold-rolled with holes, rolled by roller die, vacuum annealed, cold-rolled and rounded with precision, so that the size is accurate to a certain accuracy. The manufacture of Ti-45Nb alloy disc round wire provides a good solution for mass production. Because of the low heat of oxidation of Nb and the high diffusion rate to O2, the content of Nb, as a stable element of Ti-45Nb alloy, directly affects the alloy properties. Li Zhengquan et al. proposed a method for determining the Nb content in Ti-45Nb titanium alloy by tetraphenylarsenic chloride hydrochloride mass method, and calculated the Nb content in Ti-45Nb titanium alloy according to the mass burned to constant weight. Being able to determine Nb content is essential to determine and improve the preparation protocol.
Ti-45Nb合金的性能强化
Ti-45Nb alloy has the characteristics of low density and elastic modulus, high specific strength, good cold and hot workability, non-magnetic and non-toxic, good mechanical properties and biocompatibility, etc., and has a wide range of application prospects in the fields of aviation and medicine, but low modulus β titanium alloy has shortcomings such as poor mechanical properties and wear resistance. Therefore, heat treatment, superplastic deformation, surface modification and other processes are often used to improve the performance of Ti-45Nb alloy.
Heat treatment is an important means to improve the properties of alloy materials. Wang Xinnan et al. and Fan Kailun et al. studied the effects of heat treatment at different temperatures on the microstructure and mechanical properties of Ti-45Nb alloy. According to the AMS4982G, it was heated to 788~871 °C in a vacuum for annealing, and the recrystallized structure that met the performance requirements was prepared for a sufficient period of time, and the corresponding cooling method was selected according to the need. Liang Shujin et al. conducted a heat treatment study on Ti-45Nb alloy wire with a cold deformation greater than 90%, and found that the wire will be completely recrystallized only when the annealing temperature is higher than 810 °C, which is consistent with the research results of Wang Xinnan et al. Ma Fanjiao et al. found that with the increase of heat treatment temperature, the degree of recrystallization of Ti-45Nb alloy gradually increased. As the amount of deformation increases, the temperature of complete recrystallization gradually decreases. In addition, with the increase of annealing temperature, the grain size grows significantly, and the tensile strength and yield strength of Ti-45Nb alloy gradually decrease, while the elongation and section shrinkage increase significantly. For the forged specimen, the recrystallization temperature can be selected as 850 °C, and the tensile strength of Ti-45Nb alloy is 501.5 MPa, the elongation is 19.75%, the section shrinkage is 79.5%, and the shear strength is 366 MPa after 850 °C ×1 h + AC (air cooling) heat treatment. Heat treatment time can also have an impact on alloy properties. Wang Liya et al. found that for the traditional coarse-grained Ti-45Nb alloy, the holding time was controlled at 0.5~1.6 h, which could obtain a uniform microstructure and improve the mechanical properties.
Fig. 2 Properties and SEM morphology of Ti-45Nb alloy at different annealing temperatures
Lin Yang et al. studied the effect of aging temperature on the microstructure and properties of Ti-45Nb alloy, and found that with the increase of aging temperature, Ti-45Nb alloy always showed an equiaxed β phase structure. Figure 3 shows the strength and plasticity of Ti-45Nb alloy wire after aging for 4 h at different temperatures. It can be seen that the tensile strength, yield strength and shear strength of Ti-45Nb alloy wire increase first and then decrease with the increase of applicable temperature, reaching a peak at 300 °C, which are 510, 477 and 372 MPa, respectively, with an elongation rate of 32% and a cross-sectional shrinkage rate of 79%, showing good mechanical properties.
Fig.3. Strength and plasticity of Ti-45Nb alloy wire after aging for 4 h at different temperatures
For the nano-level ultra-fine Ti-45Nb alloy, VÖLKER B et al. annealed the ultrafine Ti-45Nb alloy at (100~500) °C for × 30 min, and found that the hardness of the alloy first increased and then decreased, reaching a peak at 300 °C. The annealed TEM image is shown in Figure 4. A clearer microstructure and sharper boundaries are shown, qualitatively confirming that the increase in hardness can be attributed to the decrease in defect density upon annealing. Annealed at 300 °C for 10 h yielded a tensile strength of up to about 1 200 MPa, which is almost three times the strength of the initial coarse-grained material. However, tensile tests have shown that the strength of the alloy increases after annealing, but the plasticity decreases.
Fig. 4 Brightfield image and high-angle annular darkfield (HAADF) microscope image after annealing at 300 °C × 10 h
Both nanoindentation technology and ultrasonic resonance spectroscopy showed the increase of elastic modulus during annealing. The reason for the increase may be an increase in the α-Ti content, which is detrimental to the material properties, therefore, future research should focus on finding the optimal value between the increase in strength and the change in plasticity. In the heat treatment process, the influence of the cooling process on the properties of the alloy is also crucial. Liang Shujin et al. found that after the annealing of Ti-45Nb alloy wire, there was martensite α" phase precipitation after rapid cooling with argon gas, and the yield strength was significantly lower than that after cooling with furnace. Fan Kailun et al. studied Ti-45Nb rivets and found that the shear strength of Ti-45Nb alloy rivets increased with the decrease of annealing cooling rate. As the cooling rate increases, the content of α" phase in titanium alloys increases, which leads to a significant decrease in the hardness and elastic modulus of titanium alloys. The slower the cooling rate and the longer the cooling time, the more microscopic precipitated phases, the stronger the obstruction to dislocation, and the greater the contribution to the improvement of matrix strength. This is the fundamental reason why the shear strength increases with the decreasing annealing cooling rate.
The superplastic deformation process is defined as the metal forming process for the manufacture of ultra-fine-grained materials, by introducing extremely high plastic strains into the bulk metal without significantly changing the overall dimensions. The metal passing through the SPD produces ultra-fine grains, which gives the material high strength. Characteristics of the material include high angular grain boundaries due to deformation and high dislocation density within the grain. High-pressure torsion (HPT), hydrostatic extrusion (HE), rolling and folding (R&F), etc., are effective SPD technologies, which usually require a fairly small total strain to achieve the desired submicron grain size.
Hydrostatic extrusion does not reduce the plasticity of high-strength materials due to the presence of some large grains, which are still capable of accommodating plastic deformation during further strain. PACHLA W ET AL. FOUND THAT THE REPETITIVE CYCLE OF HYDROSTATIC EXTRUSION LED TO CONSIDERABLE CHANGES IN THE MICROSTRUCTURE OF PURE TI, WHICH ALLOWED THE MICROSTRUCTURE TO BE REFINED TO THE NANOMETER SIZE. Equiaxed grains of about 50 nm in the cross-section and elongated grains in the longitudinal direction in layered microstructures are observed, both of which show grains with high angular grain boundaries and high dislocation densities.
A schematic diagram of the HPT is shown in Figure 5, where the disc located in a closed die is compressed by very high pressure and a plastic torsional strain is achieved by the rotation of one of the anvils.
Figure 5 Schematic diagram of HPT treatment methods
The ultimate goal of the above superplastic deformation process methods is to refine the grain, the minimum can reach the nanometer size, obtain the ultrafine grain microstructure, combined with the heat treatment process can significantly improve the performance of Ti-45Nb alloy, but at present, it only stays in the experimental stage of small samples (less than 30 mm disk), and how to convert it into large-size engineering applications needs to be further studied.
Titanium and its alloys do not have good wear resistance, such as when implants made of titanium alloys come into direct contact with the bone, wear fragments appear, which can cause some important problems in the body. Therefore, one of the best ways to improve friction and corrosion properties is to deposit a protective film on the surface of the alloy, and the commonly used methods are physical vapor deposition (PVD) technology, plasma nitriding process, laser irradiation, anodizing, etc.
The above methods are used to create wear-resistant films on the surface of alloys, showing better surface properties compared to some metal materials. However, due to the formation of a deposited layer on the surface, its thermal conductivity, thermal diffusion, and specific heat capacity decrease, and the thermal conductivity is relatively low compared to untreated alloys.
【Overview of Application Status】
Relevant studies have shown that the low heat of oxidation of Nb element and the high diffusivity to O2 can significantly improve its flame retardant properties by adding Nb to ASTM2 grade titanium. This makes Ti-45Nb alloy have excellent flame retardant properties and has become the material of choice for many hydrometallurgical autoclave applications, such as oxygen tubes, vapor nozzles, etc. Shang Guoqiang et al. found that Ti-45Nb alloy wires of different specifications did not produce macroscopic cracks under cold shock load, indicating that Ti-45Nb alloy has good cold working forming properties. Liang Shujin et al. compared the mechanical properties of a certain type of TA1 rivet and Ti-45Nb alloy rivet for aviation, and found that the shear strength and tensile strength of Ti-45Nb alloy were higher than those of TA1, and the deformation resistance was lower than that of TA1. Ti-45Nb alloy and Ti-6Al-4V alloy are matched to make bimetallic rivets, which have been widely used in Airbus and Boeing aircraft, and the United States has replaced pure titanium fasteners with Ti-45Nb alloy fasteners in the aerospace field. Qi Zhenchao et al. analyzed the performance of Ti-45Nb rivet pulse current-assisted pressure riveting, and found that the hardness of the current-assisted riveting joint at the main shear zone decreased. At the secondary shear zone, the hardness of the current-assisted riveting group is higher than that of the room temperature riveting group. The hardness of the cap-bar transition zone is significantly increased by high-density current treatment. Ti-45Nb alloy is suitable for riveting composite joining structures and is widely used in the aerospace field.
Another important use of Ti-45Nb alloy is as a medical implant material. An important condition for medical implants to replace or interact with bone tissue is that their elastic modulus is as close as possible to the surrounding bone tissue. Pure titanium and Ti-6Al-4V alloy, as titanium alloys with the lowest elastic modulus (about 110 GPa) at the time, were widely used in medical implant materials, but their hardness was still very high compared to human cortical bone (about 20 GPa). Later, it was found that the elastic modulus of the β-Ti alloy of the body-centered cubic structure was the lowest among all titanium alloys, and the elastic modulus of the Ti-45Nb alloy was 60 GPa, which was a low elastic modulus, and had been used in various medical implants that required moderate strength and improved ductility.
From the point of view of applicability as orthopedic materials, HPT and HE are considered to be the most effective SPD pathways because they maintain considerable ductility and high strength. In the case of R&F and HPT treatment of the specimen, a high mechanical compatibility of about 4.0% (percentage of microhardness to Young's modulus) was obtained. Grain refinement through superplastic deformation can enhance mechanical properties, thereby extending the life of orthopedic implants without changing their chemical composition. Combined with the excellent biocompatibility of Ti-45Nb alloy, the above SPD pathways have high potential for orthopedic applications.
The high price of Ti-45Nb alloy and the difficulty of traditional machining limit the application of this alloy. FALLAH V et al. used the highly concentrated power and high scanning speed of the fiber laser to deposit a crack-free and non-porous coating with excellent hardness by presetting the laser cladding of Ti and Nb mixed powder on a mild steel substrate, and found that during the laser cladding process, the dilution of Fe to the cladding should be kept at a minimum level to avoid the formation of harmful intermediate precipitate phases, and promote the formation of a more ductile β solid solution phase with very high hardness. This method involves laser cladding parts to form a thin coating of Ti-45Nb on a less expensive substrate, which can realize the advantages of Ti-45Nb alloy while minimizing the cost, providing a new idea for expanding the application of Ti-45Nb alloy.
【Summary and Prospects】
The physical properties, preparation process, performance enhancement process and application status of Ti-45Nb alloy were reviewed. Ti-45Nb alloy is currently widely used in aerospace and biomedical fields; However, some issues remain to be resolved.
(1) Ti-45Nb alloy is a low-modulus β-Ti alloy with poor mechanical properties. The superplastic deformation process provides a new and effective way to solve the problems of low strength and poor mechanical properties of Ti-45Nb alloy by refining the grain to the nanometer level, but the current superplastic deformation process is only in the experimental stage and cannot realize mass industrial production and engineering application. Achieving grain refinement and combining with the corresponding heat treatment system to achieve better stable performance of alloys is the key research direction in the future.
(2) In previous studies, it was found that the changes in elastic modulus, hardness and strength during the annealing process were thought to be the cause of the precipitation of the second phase. Determining the dynamic relationship between the second phase and performance is one of the main research directions in the future.
(3) Nb-containing titanium alloy has flame retardant properties and corrosion resistance. However, the addition of alloying elements is often not sufficient to improve the tribological properties of alloys, especially as medical implant materials. As an attractive material for medical implantation, improving the wear resistance and biocompatibility of Ti-45Nb alloy is one of the future research directions.