Assessment of precipitates of aged Ti-6Al-4V alloy by ultrasonic attenuation

Abstract

Ti-6Al-4V alloy with different microstructures was investigated by means of ultrasonic attenuation measurements. Widmanstätten and equiaxed microstructures were obtaining by heat treating a Ti-6Al-4V alloy. These two microstructures were over-aged at 545 °C at different ageing times. In order to find out the factors affecting the variation in the ultrasonic attenuation, the heat-treated samples were examined by optical microscopy and scanning electron microscopy. Based on the theory of ultrasonic attenuation in a solid media, the mechanisms of ultrasonic attenuation in the Ti-6Al-4V alloy with different microstructures were analysed. It was found that in both cases with Widmanstätten and equiaxed microstructures, the ultrasonic attenuation increased with frequency. After ageing, the ultrasonic attenuation was mainly attributed to the scattering loss which included the stochastic and the Rayleigh scattering due to the precipitation of Ti₃Al particles homogeneously distributed in the α phase. Data analysis presented in the study showed that ultrasonic attenuation yields more accurate area fractions of precipitates predictions when a polynomial fit is performed.     

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.     

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.     

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.     

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.     

Electrical discharge machining of titanium alloy (Ti-6Al-4V)

In this study, the electrical discharge machining (EDM) of titanium alloy (Ti-6Al-4V) with different electrode materials namely, graphite, electrolytic copper and aluminium and process parameters such as, pulse current and pulse duration were performed to explore the influence of EDM parameters on various aspects of the surface integrity of Ti6Al4V. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrograph (EDS) and hardness analysis were performed. The experimental results reveal that the value of material removal rate, surface roughness, electrode wear and average white layer thickness are tendency of increase with increasing current density and pulse duration. However, extremely long-pulse durations such as 200 μs led to decrease MRR and surface roughness. Furthermore, the surface hardness is increasing due to the Ti₂₄C₁₅ carbides formed on the surface and obvious cracks are always evident in re-solidified layer when machining copper electrode. The surface crack densities and critical crack lines were determined for the tested material. The graphite electrode is beneficial on material removal rate, electrode wear and surface crack density but relatively poorer surface finish.     

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.     

Melting and solidification characteristics of Zr-, Ni-, and Mn-containing 354-type Al-Si-Cu-Mg cast alloys

This study investigated the effects of adding Zr, as a base alloying element, besides Ni and Mn in different amounts and combinations on the melting and solidification characteristics of 354-type Al-Si-Cu-Mg alloys. Differential scanning calorimetry (DSC) was used to characterise the sequence of reactions occurring during the heating and/or cooling cycles; whereas scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques were used to observe and identify existing intermetallic phases. Nickel proved to have a retarding effect on the kinetics of precipitation of the α-Al network and the eutectic Al-Si structure. Also, the presence of Ni consumed a considerable amount of Cu to form Al-Cu-Ni particles instead of Al₂Cu particles. Results revealed that solution treatment at 495°C for 5?h was sufficient to dissolve a large amount of Al₂Cu particles in the α-Al matrix, which is mandatory for a successful aging treatment of the alloys studied. Additions of these transition elements produced new intermetallic phases such as (Al,Si)₃(Ti,Zr), (Al,Si)₃Zr, Al₉FeNi, Al₃Ni, Al₃CuNi, and Al₉FeSi₃Ni₄Zr, in addition to the other phases, namely α-Al, eutectic silicon, Al₂Cu, Mg₂Si, Q-phase (Al₅Cu₂Mg₈Si₆), commonly observed in 354-type alloys, and Fe-based intermetallic phases including β-Al₅FeSi, α-Al₁₅(Fe, Mn)₃Si₂, and π-Al₈FeMg₃Si_6. Superheating the melt at 800°C instead of 750°C had an advantageous effect in that Al₃Zr particles originating from the Al-15%Zr master alloy were dissolved and hence coarse Zr-containing particles were barely spotted in the microstructures examined.     

Significant influences of metal reactivity and oxide films at particle surfaces on coating microstructure in cold spraying

Based on large amount of experimental observations, the effects of metal reactivity and oxide films at particle surfaces on coating deposition behavior in cold spraying were presented and discussed. The oxygen contents in as-sprayed Ti, Ti-6Al-4V and Al coatings were higher than those in the corresponding starting powders. The obvious flashing jets outside nozzle exit during deposition of Ti and Ti-6Al-4V were caused by the reaction of the particles with oxygen in the entrained or the adopted air. For Ti and Ti-6Al-4V coatings, their porous structures are predominantly attributed to the surface reactivity (defined as reactivity with oxygen). This surface reaction could be helpful for formation of a metallurgical bonding between the deposited particles. For Al, even though it is more reactive than Ti, the oxide films at Al particle surfaces suppress the surface activity.     

Surface characteristics of self-assembled microporous Ti-6Al-4V compacts fabricated by electro-discharge-sintering in air

A single electro-discharge-sintering (EDS) pulse (1.0 kJ/0.7 g), from a 300 (F capacitor, was applied to atomized spherical Ti-6Al-4V powder in air to produce microporous compact. A solid core surrounded by a porous layer was self-assembled by a discharge in the middle of the compact. X-ray photoelectron spectroscopy was used to study the surface characteristics of the compact material. C, N, O and Ti were the main constituents, with smaller amounts of Al and V. The surface was lightly oxidized and was primarily in the form of TiO₂. A lightly etched EDS 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 compact surface also contained small amounts of TiN in addition to TiO₂, resulting in the reaction between nitrogen in air and the Ti substrate in times as short as 125 μs.     

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.     

Laser gas-assisted processing of carbon coated and TiC embedded Ti-6Al-4V alloy surface

Laser gas-assisted treatment of Ti-6Al-4V alloy surface is carried out. The alloy surface is initially coated by a carbon layer, in which the TiC particles are embedded prior to laser processing of the surface. The carbon coating with the presence of TiC particles on the workpiece surface is expected to result in carbonitride compound in the surface vicinity after the laser treatment process. Optical and scanning electron microscopes are used to examine the morphological and the metallurgical changes in the laser treated layer. The residual stress formed in the surface region after the laser treatment process is critical for the practical applications of the resulting surface. Therefore, the residual stress formed in the laser treated region is predicted from the analytically equation. The X-ray diffraction technique is incorporated to obtain the residual stress formed in the surface region. It is found that the residual stress predicted agrees with the X-ray diffraction data. The dense structures consisting of TiC_xN_1−x, TiN_x, Ti₂N, and TiC compounds are formed in the surface region of the treated layer. This, in turn, significantly increases the microhardness at the surface.     

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.     

MicroRaman study of the effect of oxide layer on nitriding of Ti-6Al-4V

Ti-6Al-4V samples were subjected to nitrogen ion implantation and low pressure RF plasma nitriding in a PIII equipment after chemical etching in Kroll's reagent. The samples were characterized by optical microscopy, AFM, microRaman, XRD and micro hardness measurements. From microRaman, oxides of titanium were found in the inter-granular β region whereas the oxide on the α region was extremely thin. The oxide on the β region was found to be amorphous and from intensity dependent Raman spectra, it was found to have a layered structure. The top layer was rutile and the inner layer anastase. Presence of Ti₂O₃ was also found. After PIII treatment at 600 °C, microRaman showed the presence of nitride in both the regions and the oxide was absent in the β regions. Also, from intensity dependent Raman spectra, it was found that in-take of nitrogen by β regions was higher. The oxide layer remained unaffected after plasma nitriding. Nitride presence in the α was established by microRaman. Even though Raman spectra from β regions were nearly the same as that of oxide, presence of nitrogen was indicated by the spectra. XRD studies of implanted and nitrided samples showed the prsence of TiN and Ti₂N in implanted sample and presence of Ti₂N in the nitrided sample. The β regions were found to have higher microhardness values after PIII and nitriding treatments. This is attributed to the deeper diffusion of nitrogen in these regions.     

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...     

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。     

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     

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.     

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.     

Effects of duty ratio at low frequency on growth mechanism of micro-plasma oxidation ceramic coatings on Ti alloy

The aim of this work is to study the effects of duty ratio on the growth mechanism of the ceramic coatings on Ti-6Al-4V alloy prepared by pulsed single-polar MPO at 50 Hz in NaAlO₂ solution. The phase composition of the coatings was studied by X-ray diffraction, and the morphology and the element distribution in the coating were examined through scanning electron microscopy and energy dispersive spectroscopy. The thickness of the coatings was measured by eddy current coating thickness gauge. The corrosion resistance of the coated samples was examined by linear sweep voltammetry technique in 3.5% NaCl solution. The changes of the duty ratio (D) of the anode process led to the changes of the mode of the spark discharge during the pulsed single-polar MPO process, which further influenced the structure and the morphology of the ceramic coatings. The coatings prepared at D = 10% were composed of a large amount of Al₂TiO₅ and a little γ-Al₂O₃ while the coatings prepared at D = 45% were mainly composed of α-Al₂O₃ and γ-Al₂O₃. The coating thickness and the roughness were both increased with the increasing D due to the formation of Al₂O₃. The formation of Al₂TiO₅ resulted from the spark discharge due to the breakdown of the oxide film, while the formation of Al₂O₃ resulted from the spark discharge due to the breakdown of the vapor envelope. The ceramic coatings improved the corrosion resistance of Ti-6Al-4V alloy. And the surface morphology and the coating thickness determined the corrosion resistance of the coated samples prepared at D = 45% was better than that of the coated samples prepared at D = 10%.