The effect of thermal aging on the strength and the thermoelectric power of the Ti-6Al-4V alloy

When the Ti-6Al-4V alloy is overaged at 500-600°C, nanometer-sized α₂ (Ti₃Al) particles can be homogeneously precipitated inside a phases, thereby leading to strength improvement. Widmanstätten and equiaxed microstructures containing fine α₂ (Ti₃Al) particles were obtained by overaging the Ti-6Al-4V alloy. Precipitation of α₂ (Ti₃Al) particles was monitored using thermoelectric power measurements for different aging conditions in the Ti-6Al-4V alloy. Overaging heat treatments were conducted at 515, 545 and 575°C for different aging times. In addition, overaging samples were examined by optical microscopy, scanning electron microscopy and hardness measurements. It was found that the thermoelectric power is very sensitive to the aging process in the two studied Ti-6Al-4V structures.     

Hydrogen peroxide treatment on Ti-6Al-4V alloy: A promising surface modification technique for orthopaedic application

Ti-6Al-4V alloy was treated with various concentrations (5 wt.%, 15 wt.% and 25 wt.%) of hydrogen peroxide (H₂O₂) and then heat treated to produce an anatase titania layer. The surface modified substrates were immersed in simulated body fluid (SBF) solution for the growth of an apatite layer on the surface and the formed apatite layer was characterized using various surface characterization techniques. The results revealed that titania layer with anatase nature was observed for all H₂O₂ treated Ti-6Al-4V alloy, irrespective of the H₂O₂ concentrations. Ti-6Al-4V alloy treated with 15 wt.% and 25 wt.% of H₂O₂ induced apatite formation, however 5 wt.% of H₂O₂ treated Ti-6Al-4V failed to form apatite layer on the surface. The electrochemical behaviour of H₂O₂ treated specimens in SBF solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy. Ti-6Al-4V alloy treated with 25 wt.% of H₂O₂ solution exhibited low current density and high charge transfer resistance values compared to specimens treated with other concentrations of H₂O₂ and untreated Ti-6Al-4V alloy.     

Grain refining of Ti-6Al-4V alloy fabricated by laser and wire additive manufacturing assisted with ultrasonic vibration

The formation of the coarse columnar crystal structure of Ti-6Al-4V alloy in the process of additive manufacturing greatly reduces the mechanical performance of the additive manufactured parts, which hinders the applications of additive manufacturing techniques in the engineering fields. In order to refine the microstructure of the materials using the high intensity ultrasonic via the acoustic cavitation and acoustic flow effect in the process of metal solidification, an ultrasonic vibration technique was developed to a synchronous couple in the process of Laser and Wire Additive Manufacturing (LWAM) in this work. It is found that the introduction of high-intensity ultrasound effectively interrupts the epitaxial growth tendency of prior-β crystal and weakens the texture strength of prior-β crystal. The microstructure of Ti-6Al-4V alloy converts to fine columnar crystals from typical coarse columnar crystals. The simulation results confirm that the acoustic cavitation effect applied to the molten pool created by the high-intensity ultrasound is the key factor that affects the crystal characteristics.     

Fabrication of Zr and Zr-N surface alloying layers and hardness improvement of Ti-6Al-4V alloy by plasma surface alloying technique

Titanium alloys are very attractive materials because they have high specific strength, excellent corrosion and erosion resistance in many active environments. However, their low hardness values and poor tribological properties require improvement of their surface properties. The present study is concerned with the fabrication of Zr and Zr-N alloying layers in the surfaces of Ti-6Al-4V substrates by plasma surface alloying technique. The microstructure, chemical composition and hardness of the surface alloying layers were analyzed to understand the mechanisms of surface alloying and hardness improvement. The Zr and Zr-N surface alloying layers formed were homogeneous and compact, in which the surface alloying elements all displayed gradient distributions. The Zr and Zr-N surface alloying layers all enhanced the surface hardness of Ti-6Al-4V alloy. Zr-N surface alloying resulted in greater improvement in hardness and the maximum microhardness of (1.37 ± 0.04) × 10³ HK was obtained at the subsurface, which was much higher than that of the untreated Ti-6Al-4V alloy. The Zr-N surface alloying layer consisted of an outer nitride layer and an inner diffusion zone of Zr and N, and its very high hardness owed to the formation of the nitride layer. The mechanism of hardness improvement of Zr surface alloyed Ti-6Al-4V alloy was solid solution strengthening.     

Ti-6Al-4V合金中片层组织形成的相场模拟

Ti-6Al-4V是典型的α+β钛合金,不同热处理制度和热加工工艺下可得到形貌各异的微观组织,从而表现出不同的力学性能,深刻理解合金中微观组织的形成机制有助于合金的进一步优化和改造.采用相场方法模拟Ti-6Al-4V合金中片层组织的形成及演化,以热力学数据库和动力学数据库为输入,通过计算定量预测β晶界上已存在初生α相时合金组织随时间的演化.结果表明,在一定条件下,随着时间的延长晶界α向β晶内生长形成片层组织,片状α簇的形貌与界面能各向异性密切相关;晶界取向对片层生长有重要作用,垂直于晶界生长时产生最密集的片层,随倾斜角增大片层加厚且生长缓慢;此外,热处理温度显著改变片层组织形貌,温度越高,片层尖端生长速度越慢,片层间距越大. 关键词： Ti-6Al-4V 相场模拟 片层组织     

Evolution of the structural and phase states of a Ti-6Al-4V alloy in forming submicrocrystalline structure with the use of temporary hydrogenation

The special features of the evolution of the structural and phase states of a Ti-6Al-4V alloy in forming submicrocrystalline structure with the use of temporary hydrogen treatment are studied by electron microscopy and x-ray diffraction analysis. Plastic deformation in the α + β two-phase region at 1023 K is found to initiate a complete β → α transformation in a Ti-6Al-4V-H alloy to form an α phase with lattice parameters different from those of the equilibrium α phase. Isothermal annealing at a dehydrogenation temperature of 873 K gives rise to α + β two-phase submicrocrystalline structure with a grain size of ∼ 0.3 μm. The use of nonequilibrium hydrogen release in the deformed Ti-6Al-4V-H alloy exposed to electron beams is shown to result in one-phase submicrocrystalline structure and grain refining. Possible reasons underlying the phase transformations in the alloy under study are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 86–91, April, 2006.     

Investigation of Ti-6Al-4V alloy acoustic softening

High power ultrasonic vibration is widely used for improving manufacturing processes such as machining and metal forming. High frequency mechanical vibration affects material properties and friction forces in contacting surfaces. Flow stress reduction under superimposed ultrasonic vibration is called as acoustic softening. The amount of this parameter should be determined for ultrasonic assisted metal forming processes. For determination of this parameter for workhorse Ti-6Al-4V alloy, experimental setup was designed and fabricated. Then tensile test under longitudinal ultrasonic vibration was performed for different ultrasonic powers. Results show that ultrasonic vibration has considerable effect on plastic behavior of the alloy and decreases flow stress. Also, increasing ultrasonic power leads to higher acoustic softening. Yield stress reduction up to 9.52%, ultimate stress reduction up to 4.55% and elongation up to 13% were obtained at 340 W ultrasonic power. After applying ultrasonic vibrations and its termination, hardness of specimens were measured in which increase up to 9% was observed.     

Effect of Si addition on the stability of Al-10Ti-5Cu-xSi alloy/SiC interface

The effect of Si addition on the interfacial stability of Al-10Ti-5Cu-xSi (x = 0, 5, 10, 15) alloy/SiC is investigated. SiC and the Al-10Ti-5Cu-xSi alloys were compacted to obtain a stable interface with 10 wt% Si. Analysis of the processing conditions and the microstructures indicated that an excellent Ti₃SiC₂ phase had been formed and the deleterious Al₄C₃ phase had been eliminated successfully by the addition of 10 wt% Si to the Al-10Ti-5Cu alloy. Formation of Ti₃SiC₂ increased at first and then decreased, while the formation of Al₄C₃ was gradually inhibited with increasing Si content. Ti₃SiC₂ possesses good chemical stability, and flexibility. However, Al₄C₃ degrades within few days, in composites exposed to ambient conditions. The presence of Ti₃SiC₂ at the interface and the elimination of Al₄C₃ together ameliorate the bonding of Al-10Ti-5Cu-xSi alloy to SiC, thereby improving the interfacial stability of Al-10Ti-5Cu-xSi/SiC.     

Microstructure evolution of laser solid forming of Ti-Al-V ternary system alloys from blended elemental powders

Morphology evolution of prior β grains of laser solid forming (LSF) Ti-xAl-yV (x 11,y 20) alloys from blended elemental powders is investigated. The formation mechanism of grain morphology is revealed by incorporating columnar to equiaxed transition (CET) mechanism during solidification. The morphology of prior β grains of LSF Ti-6Al-yV changes from columnar to equiaxed grains with increasing element V content from 4 to 20 wt.-%. This agrees well with CET theoretical prediction. Likewise, the grain morphology of LSF Ti-xAl-2V from blended elemental powders changes from large columnar to small equiaxed with increasing Al content from 2 to 11 wt.-%. The macro-morphologies of LSF Ti-8Al-2V and Ti-11Al-2V from blended elemental powders do not agree with CET predictions. This is caused by the increased disturbance effects of mixing enthalpy with increasing Al content, generated in the alloying process of Ti, Al, and V in the molten pool.     

A study on wear resistance and microcrack of the Ti3Al/TiAl + TiC ceramic layer deposited by laser cladding on Ti-6Al-4V alloy

Laser cladding of the Al + TiC alloy powder on Ti-6Al-4V alloy can form the Ti₃Al/TiAl + TiC ceramic layer. In this study, TiC particle-dispersed Ti₃Al/TiAl matrix ceramic layer on the Ti-6Al-4V alloy by laser cladding has been researched by means of X-ray diffraction, scanning electron microscope, electron probe micro-analyzer, energy dispersive spectrometer. The main difference from the earlier reports is that Ti₃Al/TiAl has been chosen as the matrix of the composite coating. The wear resistance of the Al + 30 wt.% TiC and the Al + 40 wt.% TiC cladding layer was approximately 2 times greater than that of the Ti-6Al-4V substrate due to the reinforcement of the Ti₃Al/TiAl + TiC hard phases. However, when the TiC mass percent was above 40 wt.%, the thermal stress value was greater than the materials yield strength limit in the ceramic layer, the microcrack was present and its wear resistance decreased.     

Features of the plasma saturation of nanocrystalline and coarse-crystalline titanium samples with hydrogen and deuterium

We investigate the laws governing the saturation of nano- and coarse-crystalline Ti-6Al-4V alloy samples with hydrogen from high-frequency and glow discharge plasmas with a view to finding materials suitable for making hydrogen accumulators. We conclude that nanocrystalline Ti-6Al-4V alloy holds promise for creating hydrogen accumulators, and hydrogen plasma of high-frequency discharge origin is a cleaner hydrogen saturation medium than that of non-self-sustained discharge origin.     

XPS study of porous dental implants fabricated by electro-discharge-sintering of spherical Ti-6Al-4V powders in a vacuum atmosphere

A single electro-discharge-sintering (EDS) pulse (0.7-2.0 kJ/0.7 g), from a 300 μF capacitor, was applied to atomized spherical Ti-6Al-4V powder in a vacuum to produce a porous-surfaced implant compact. A solid core surrounded by a porous layer was formed by a discharge in the middle of the compact. X-ray photoelectron spectroscopy was used to study the surface characteristics of the implant material. C, O, and Ti were the main constituents, with smaller amounts of Al, V, and N. The implant surface was lightly oxidized and was primarily in the form of TiO₂ with a small amount of metallic Ti. A lightly etched EDS implant sample showed the surface form of metallic Ti, indicating that EDS breaks down the oxide film of the as-received Ti-6Al-4V powder during the discharge process. The EDS Ti-6Al-4V implant surface also contained small amounts of aluminum oxide in addition to TiO₂. However, V detected in the EDS Ti-6Al-4V implant surface did not contribute to the formation of the oxide film. The small amount of N in the implant surface resulted from nitride material that was also found in the as-received Ti-6Al-4V powders.     

激光熔覆TiC陶瓷涂层的组织和摩擦磨损性能研究

采用激光熔覆技术在TC4合金表面上制备了TiC陶瓷涂层,分析了熔覆层的微观组织,测试了熔覆层的硬度和摩擦磨损性能。结果表明:TiC激光熔覆层分为熔覆区和稀释区两个区域,熔覆区未受到基底的稀释,由TiC颗粒和TiC树枝晶组成;稀释区受到了基底的稀释,由TiC树枝晶和钛合金组成;TiC激光熔覆层的显微硬度在HV700~1500之间,明显地改善了TC4合金表面的摩擦和磨损性能。     

Accelerated formation of an ultrafine-grained structure in a two-phase Ti alloy during compression with decreasing temperatures

ABSTRACT

Ultrafine-grained (UFG) structure is beneficial for overcoming the strength-ductility trade-off and enhancing the superplasticity of two-phase Ti alloys. Recently, it has been demonstrated that compression with decreasing temperatures is effective for producing UFG two-phase Ti alloys initially with lamellar microstructures. However, the effect of lamellar thickness on the microstructural evolution during this process has not yet been fully elucidated. In this study, Ti-6Al-4V alloys with different lamellar thicknesses were compressed while the processing temperature was decreased from 800°C to 600°C. The thinner lamellar microstructure was preferable for preventing void/crack formation, while accelerating the continuous dynamic recrystallisation, thus providing a fully UFG structure at a relatively low strain of 1.4. In addition, the origin of different plastic flows in each sample was analysed in detail by analysing the microstructural evolution. These findings demonstrate that the processing method is effective for reducing the grain size of a two-phase Ti alloy without severe plastic deformation techniques, which require large strain (≥4). A reduction in the strain required to achieve the UFG structure would be beneficial because conventional metal-forming processes, i.e. rolling, extrusion, or forging, which are suitable for mass production, could be used.     

Superplastic Deformation of Titanium Alloy with Different Structure and Phase Composition

Russian Physics Journal - The α+β titanium alloy Ti-6Al-4V having different structure and phase composition is investigated in this paper under the tensile deformation in the temperature...     

Growth characteristics of micro-plasma oxidation ceramic coatings on Ti alloy by inductively coupled plasma-atomic emission spectrometer technique

The aim of this work was to study the growth characteristics of micro-plasma oxidation ceramic coatings on Ti-6Al-4V alloy. Compound ceramic coatings were prepared on Ti-6Al-4V alloy by pulsed micro-plasma oxidation (MPO) in NaAlO₂ solution. The phase composition and surface morphology of the coating were investigated by X-ray diffractometry and scanning electron microscopy. The solution of Ti from the substrate and the content of Al in the electrolyte were studied by inductively coupled plasma-atomic emission spectrometer (ICP-AES) technique. Ti from the substrate dissolved and came into the coating and the electrolyte during MPO process. The content of Ti in the electrolyte under the pulsed bi-polar mode was more than that of the pulsed single-polar mode. The phase composition and structure of the coating was attributable to the space steric hindrance of Al congregated on the electrode surface due to the effect of the electric field and the electrolyte characters. For the pulsed single-polar mode, the coating was mainly composed of a large amount of α-Al₂O₃ and a small amount of γ-Al₂O₃. And the coating was mainly structured by Al from the electrolyte. However, the coating was composed of a large amount of Al₂TiO₅ and a little α-Al₂O₃ and rutile TiO₂ for the pulsed bi-polar mode. And the coating was structured both by Ti from the substrate and Al from the electrolyte.     

The influence of interface reaction zone on interfacial fracture toughness of SiC fiber reinforced titanium matrix composites

A fiber-reaction zone-matrix three-phase model is developed to evaluate the interfacial fracture toughness of titanium alloys reinforced by SiC monofilaments. Based on fracture mechanics, theoretical equations of G_IIc are presented, and the effects of several key factors such as crack length and the interface reaction zone thickness on the critical applied stress necessary for crack growth and interfacial fracture toughness are discussed. Finally, the interfacial fracture toughness of typical composites including Sigma1240/Ti-6Al-4V, SCS-6/Ti-6Al-4V, SCS-6/Timetal 834, SCS-6/Timetal 21s, SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are predicted by the model. The results show that the model can reliably predict the interfacial fracture toughness of the titanium matrix composites.     

Hydrogen storage in sonicated carbon materials

The hydrogen storage in purified single-wall carbon nanotubes (SWNTs), graphite and diamond powder was investigated at room temperature and ambient pressure. The samples were sonicated in 5 M HNO₃ for various periods of time using an ultrasonic probe of the alloy Ti-6Al-4V. The goal of this treatment was to open the carbon nanotubes. The maximum value of overall hydrogen storage was found to be 1.5 wt %, as determined by thermal desorption spectroscopy. The storage capacity increases with sonication time. The sonication treatment introduces particles of the Ti alloy into the samples, as shown by X-ray diffraction, transmission electron microscopy, and chemical analysis. All of the hydrogen uptake can be explained by the assumption that the hydrogen is only stored in the Ti-alloy particles. The presence of Ti-alloy particles does not allow the determination of whether a small amount of hydrogen possibly is stored in the SWNTs themselves, and the fraction of nanotubes opened by the sonication treatment is unknown. Received: 18 December 2000 / Accepted: 18 December 2000 / Published online: 9 February 2001     

Combination of laser keyhole and conduction welding: Dissimilar laser welding of niobium and Ti-6Al-4V

Pulsed Nd:YAG laser welding of pure niobium plate to titanium alloy Ti-6Al-4V sheet in butt joint is studied regarding the laser/metal interaction modes. To obtain the optimized process parameters in dissimilar welding of Ti-6Al-4V/Nb, the melting ratio of laser beam energy for each weld counterpart is evaluated experimentally. Different laser welding modes of keyhole and conduction are predicted regarding the absorbed energy from the similar laser pulses on each weld counterpart. Laser keyhole and conduction welding were observed simultaneously through direct visualization of laser interaction with dissimilar metals using High Speed Imaging (HSI) system.     

Microstructure,creep, and tensile behaviour of a Ti–15Al–33Nb (at.%) beta+orthorhombic alloy

The microstructural evolution, creep and tensile deformation behaviour of a Ti–15Al–33Nb (at.%) alloy was studied. Monolithic sheet material was produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Electron microscopy studies showed that depending on the heat-treatment schedule, this alloy may contain three constituent phases including:?β?(disordered body-centred cubic), α₂ (ordered hexagonal close-packed based on Ti₃Al) and O (ordered orthorhombic based on Ti₂AlNb). Heat treatments at all temperatures above 990°C, followed by water quenching, resulted in fully-β microstructures. Below 990°C, Widmanstätten O-phase or α₂-phase precipitated within the?β?grains. The fine-grained as-processed microstructure, which exhibited 90?vol.% β-phase, exhibited excellent strength (UTS?=?916?MPa) and ductility (?_f>12%). After heat treatment, greater volume fractions of the orthorhombic phase precipitated and resulted in lower ? _f values with UTS values ranging between 836–920?MPa. However, RT elongations of more than 2% were recorded for microstructures containing up to 63?vol.% O-phase. Specimens subjected to 650°C tensile experiments tended to exhibit lower strength values while maintaining higher elongation-to-failure. Tensile creep tests were conducted in the temperature range 650–710°C and stress range 49–275?MPa. The measured creep exponents and activation energies suggested that grain boundary sliding operates at intermediate stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behaviour are discussed in comparison to a Ti–12Al–38Nb O?+?β alloy.