Oxidation behaviour of Ti6Al4V titanium alloy in oxygen

Abstract

The isothermal oxidation behaviour of two phase (α + β) titanium base alloy Ti6Al4V (coupons) has been studied at 1050, 1150, 1250, and 1340 K in O₂ gas at atmospheric pressure for 2, 4, 6, 8, and 12 h. Investigations on kinetic behaviour followed by the metallographic examination of oxidised scale morphology was carried out. Thermogravimetric data (weight gain v time) exhibited parabolic behaviour. Below 1250 K, the rate of oxidation substantially decreased after 8 h exposure, however, at 1340 K the oxidation rate was markedly high over the whole 12 h period. Parabolic rate constants were 0.234×10^-7, 3.67×10^-7, 10.72×10 ^-7, and 31.17×10^-7 kg² m^-4 s^-1 at 1050, 1150, 1250, and 1340 K respectively. The effective activation energy of oxidation was 88 kJ mol^-1. The instantaneous rate constant k _i exhibited a marked deviation from parabolic behaviour at high temperatures e.g. 1150, 1250, and 1340 K, however, k _i at lower temperature (1050 K) remained broadly unchanged with time exhibiting no deviation from parabolic behaviour. Metallographic observation of the sample coupons treated at 1340 K revealed an identical oxide scale morphology with increased thickness over the time.     

Deformation of titanium alloy Ti–6Al–4V under dynamic compression

Deformation localisation is the main reason for material failure in cold forging of titanium alloys and is thus closely related to the production yield of cold forging. Recent research has revealed that the width of shear band of titanium alloys after dynamic compression is related with their static and dynamic mechanical properties and processing parameters. To explore the influences of these factors on titanium alloys in dynamic compression, the distributions of stress, strain, strain rate and temperature of the specimens over the macro and microscales have been systematically studied. This work can be beneficial to process parameter optimisation and material designing for cold forging. In the study of the influence of process parameters on dynamic compression, considering material constitutive behaviour, physical parameters and process parameters, a numerical dynamic compression model for titanium alloys has been constructed. The entire dynamic compression process is simulated and a good agreement with experiments is observed. By extracting and comparing the stress, strain and temperature distribution under prescribed conditions, the effects of friction and compression velocity on the macrostate and distribution of strain and stress of compression samples are studied. Friction and compression rate are important factors influencing the spread and the stress state of deformation localisation zone. When friction is reduced to a certain level, deformation localisation can be effectively alleviated. The increase of friction and compression rate can lead to early appearance of tension stress in the deformation localisation zone, which may explain the experimental finding that crack tendency increases with higher compression rate and poorer lubrication. By adjusting the process parameters, the severity of strain localisation and stress state in the localised zone can be controlled thus enhancing the compression performance of titanium alloys.     

Biocompatible and mechanical properties of low temperature deposited quaternary (Ti, Al, V) N coatings on Ti6Al4V titanium alloy substrates

A series of quaternary (Ti, Al, V) N coating layers were obtained by low temperature reactive plasma sputtering in differing deposition conditions to improve the wear resistance and the biocompatibility of a titanium surgical alloy, specifically Ti-6Al-4V. Characterization of the mechanical properties, structure and the chemical composition of the coating layer was explored by microhardness test, ball against flat wear test, scanning electron microscopy and X-ray diffraction. The biocompatibility of the optimum coating layer (as determined by mechanical performance) was examined by a modified MTT toxicity test and by monitoring cell growth assessed by quantitative stereological analysis. The experimental results are encouraging, indicating that this low temperature deposited, dense, quaternary (Ti, Al, V) N coating layer exhibits improved mechanical properties such as high hardness and excellent adhesion to a Ti alloy substrate and is highly biocompatible.     

Failure analysis of Ti6Al4V gas turbine compressor blades

The failure mechanism of Ti6Al4V compressor blades of an industrial gas turbine was analysed by means of both experimental characterisations and numerical simulation techniques. Several premature failures were occurred in the high pressure section of the compressor due to the fracture of the blade roots. Metallurgical and mechanical properties of the blade alloy were evaluated. A 2D finite element model of the blade root was constructed and used to provide accurate estimates of stress field in the dovetail blade root and to determine the crack initiation in the dovetail.

The results showed no metallurgical and mechanical deviations for the blade materials from standards. SEM fractography showed different aspects of fretting fatigue including multiple crack initiation sites, fatigue beach marks, debris particles, and a high surface roughness in the edge of contact (EOC). The numerical model clearly showed the region of highest stress concentration at the front EOC of the blade root in the dovetail region, correlated closely with the experimentally characterised fatigue crack region. It was concluded that this failure has occurred due to the tight contact between the blade root and the disk in the dovetail region as well as low wear resistance of the blade root.     

Failure analysis of a Ti6Al4V cementless HIP prosthesis

This work investigates the causes of in vivo fracture of a cementless hip prosthesis model manufactured with Ti6A14V alloy. To this end an assessment of geometric characteristics, a fractographic analysis of the fracture surface, and a microstructural study of the stem material were carried out. The results indicate that the fracture was initiated at the junction radius of the neck with the collared device, with around 90% of the fracture surface being due to a fatigue mechanism. In order to assess the impact of the material’s microstructure on its mechanical properties, two heat treatments, yielding a fine and a coarse microstructure, respectively, were performed on a 14 mm diameter bar. The notch effect on fatigue strength was evaluated by comparison of the results of polished specimens and specimens having a notch whose geometry produced a similar stress concentration factor to that which caused the fracture in the implant. It is shown that irrespective of the material’s microstructure, a notch greatly reduces the fatigue strength of this material.     

Investigation of ultrasonic indications in Ti alloy (Ti6Al4V) hot formed hemisphere

Pressure vessels of titanium alloy (Ti6Al4V) are made through electron beam welding of hot formed hemispheres. During ultrasonic inspection of a batch of hot formed and machined hemispheres, ultrasonic indications beyond acceptable levels have been observed while inspecting as per AMS 2630B for class A1, whereas input material (hot rolled and annealed plate) has been qualified to the level of class A1. This paper presents the detailed investigation carried out to study the problem and to bring out the reasons behind the same. The authors attempted to correlate the microstructural features of this (α–β) alloy in hot formed and annealed condition with the recorded ultrasonic indications.     

Influence of wire-EDM textured conventional tungsten carbide inserts in machining of aerospace materials (Ti–6Al–4V alloy)

Importance of this present investigation is to identify the influence of modified tool (tool with texturing) on the process of orthogonal turning of Ti–6Al–4V work material. To achieve the enhanced turning conditions, four different types of textures (plain conventional, cross, perpendicularly textured and parallel textured tool to the chip flow direction) were fabricated on the rake face of the tool insert and the lubricant used during the machining process is molybdenum disulfide (MoS₂). Machining forces (the force of cutting and feed), angle of shear, chip morphology, temperature distribution between tool and chip were measured. Shear strain and strain rate were also computed and compared with all type of cutting tools. Experimental results revealed that the cross-textured cutting tool exhibit an effective reduction in cutting force, friction, shear strain and strain rate. The favorable metal removal condition of curling chip with low diameter was achieved through cross-textured tool.     

置氢Ti6Al4V合金的微观组织演变规律

为研究置氢Ti6Al4V合金的高温加工改性机理,从微观组织的角度对合金进行了对比分析.利用OM、SEM、XRD等研究了置氢对Ti6Al4V合金变形前后微观组织演变的影响.研究结果表明:氢的加入不仅使置氢Ti6Al4V合金中β相比例明显增大,而且改变了α相与β相之间的电势差,在氢含量为0.3%~0.5%两相颜色将发生互换,氢含量增加到0.50%以上时,合金中将出现面心立方结构的δ氢化物;随氢含量的增加,合金超塑拉伸变形后的组织由α+β两相等轴晶粒变为粗大的β晶粒,造成α与β界面的协调能力下降,并改变了合金的变形机制.     

Failure and formability studies in warm deep drawing of Ti–6Al–4V alloy

Deep drawing experiments have been performed in order to study formability of Ti–6Al–4V alloy sheet at temperature ranging from room temperature to 400 °C. It is found that below 150 °C, formability of the material is very poor and above 150 °C till 400 °C, limiting drawing ratio (LDR) is found to be 1.8 which is substantially lesser than other structural alloys. For better understanding of failures in failed cups, failure regions have been identified in neck and wall which are validated using finite element (FE) simulations. Fractured surface has been examined with scanning electron microscope (SEM) which reveals different types of shallow dimples indicating predominantly ductile failure. Additionally, in the properly drawn cups, thickness distribution has been studied over a temperature range of 150–400 °C and blank diameter 50–54 mm. In order to optimize blank diameter and temperature to obtain uniform thickness distribution of drawn cups, artificial neural network (ANN) and genetic algorithm (GA) have been employed. Thickness distribution for optimized parameters is validated using FE simulation.     

Microstructure of diffusional zirconia–titanium and zirconia–(Ti–6 wt% Al–4 wt% V) alloy joints

Zirconia–titanium and zirconia–titanium alloy joints were made by diffusion bonding under an inert atmosphere at temperatures in the 1162–1494°C range. To inhibit the strong oxygen uptake by the titanium member a platinum insert was alternatively used. The microstructures and elemental profiles across the joints were investigated by scanning electron microscopy imaging and energy-dispersive spectroscopy or wavelength-dispersive spectroscopy microanalysis. It was found that direct ZrO2–Ti joining produces oxygen saturation in the Ti member and the formation of (Ti,Zr)2O at the interface. ZrO2/Pt/Ti joints present a complex layer sequence which at lower temperatures can be described on the basis of the Pt–Ti binary, except near the ceramic where a (Pt,Zr)-rich layer forms; at higher temperatures these joints develop an oxide layer of composition Ti2O3, this oxide probably resulting from local decomposition of the ceramic and reaction of oxygen with the incoming titanium. When Ti is replaced by the Ti–6 wt% Al–4 wt% V alloy in joints where Pt is present, the main consequences are the presence of liquid at lower joining temperatures and the earlier development of the oxide layer, now of nominal composition TiO. In all Pt-containing joints a phase of nominal composition Ti3Pt2 forms; it is advanced that this may be an equilibrium phase not predicted by the Pt–Ti diagrams available. All joints are weak, the fracture path running through the metal in the case of direct ZrO2–Ti joints and through the interface between the ceramic and the (Pt,Zr)-rich layer in joints where Pt is present.     

Quasi-static Torsional Deformation Behavior of Porous Ti6Al4V alloy

Laser processed Ti6Al4V alloy samples with total porosities of 0%, 10% and 20% have been subjected to torsional loading to determine mechanical properties and to understand the deformation behavior. The torsional yield strength and modulus of porous Ti alloy samples was found to be in the range of 185-332 MPa and 5.7-11 GPa, respectively. With an increase in the porosity both the strength and the modulus decreased, and at 20% porosity the torsional modulus of Ti6Al4V alloy was found to be very close to that of human cortical bone. Further, the experiments revealed clear strain hardening and ductile deformation in all the samples, which suggests that the inherent brittleness associated solid-state sintered porous materials can be completely eliminated via laser processing for load bearing metal implant applications.     

Phenomenological and crystal plasticity approaches to describe the mechanical behaviour of Ti6Al4V titanium alloy

This paper presents a multiscale study of the quasi-static behaviour of a Ti6Al4V titanium alloy sheet. Tensile and compressive tests were carried out on specimens along several orientations from the rolling direction in order to characterise the material anisotropy. In parallel, X-Ray diffraction texture measurements were performed before and after deformation in tension. A phenomenological model (CPB06exn) and a multiscale crystal plasticity model (Multisite) were investigated to describe the mechanical behaviour of the tested material. The identification of the material parameters provides good predictions of the plastic anisotropy using both tensile and compressive data. The crystal plasticity model is in good agreement with the experiments in tension but it was observed that some improvements should be done to take into account the tension-compression asymmetry displayed by the material. Moreover both models lead to a good prediction of the Lankford’s coefficients and yield strength.     

Photocatalytic inhibition of microbial fouling by anodized Ti6Al4V alloy

Biofouling is one of the major concerns in the use of titanium, an excellent material with respect to corrosion resistance and mechanical properties, for seawater-cooled condensers of power plants. Earlier studies conducted in our laboratory have shown that anodized titanium with a thin film of anatase (TiO₂) inhibits attachment of Pseudomonas sp. of bacteria when illuminated with near-UV light (350–380 nm) from black light blue (BLB) florescent lamps. The following work compares the photocatalytic efficiencies of anodized commercially pure titanium (grade 2) and Ti6Al4V alloy, in order to understand the role of the alloying elements such as Al and V on the photocatalytic activity in relation to inhibition of microbial attachment. The study was carried out by employing both methylene blue (MB) dye degradation as well as microbial adhesion experiments under near-UV light illumination. The results have shown that the anodized Ti6Al4V surfaces showed an order of magnitude increase in photocatalytic activity, as shown by the decrease in microbial attachment compared to titanium grade-2. The oxide film on both the surfaces has been characterized using Glancing Incidence X-ray Diffraction (GIXRD) and Atomic Force Microscopy (AFM). The GIXRD and AFM results showed that the oxide formed on anodized Ti6Al4V surface has higher crystallinity and is composed of particles, which are smaller in size; both these attributes are reported to enhance the photocatalytic activity. Since, vanadium is reported to shift the photo-response of the photoactive anatase thin film into visible range, the photocatalytic activity of anodized Ti6Al4V was also studied under visible light and it was observed that the surfaces showed significant photocatalytic activity even under visible light.     

Ti6Al4V合金热氢处理组织演变研究

应用热压缩、固溶淬火和真空退火热处理研究了未置氢和置氢0.18wt.%Ti6Al4V合金不同的工艺条件下的微观组织演变,并且通过硬度测试分析了微观组织和机械性能之间的关系.结果表明:置氢降低了Ti6Al4V合金的热变形抗力,促进了固溶淬火过程中马氏体转变的发生,使得真空退火得到的双态组织更加细小.对于未置氢和置氢的Ti6Al4V合金来说,合理的工艺顺序均应该是:热变形,固溶淬火和真空退火,并且最终都能得到双态组织.     

Ti-6Al-4V合金熔体中各组元挥发趋势

在基于本课题组建立的熔体组元活度系数计算模型的基础上,计算了Ti-6Al-4V熔体中各组元的活度系数及其蒸气压,导出了判断熔体组元挥发趋势的相对挥发系数β,研究结果表明,在熔体温度低于2000K时,铝钛相对挥发系数β（Al：Ti）＞17,铝钒相对挥发系数β（Al：V）＞96,钛钒相对挥发系数β（Ti：V）＞15,即熔过程中组元挥发趋势从大到小依次为Al,Ti,V.这说明熔体成分变化主要是由于Al元素的挥发所致。     

Morphological and chemical characterisation of biomimetic bone like apatite formation on alkali treated Ti6Al4V titanium alloy

The present study is an attempt to enhance the apatite-forming ability of titanium metal induced by the alkaline (NaOH) treatment. A cell free culture medium, acellular DMEM solution was utilised to develop bone-like apatite on alkali-treated titanium alloy surface. The main advantage of this process is the development of bone like apatite with essential trace elements on the metallic substrate by using the DMEM culture medium as a soaking medium. The formed apatite deposits were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS). The obtained results suggest that the method utilized in this work can be successfully applied to obtain deposition of uniform coatings of crystalline hydroxyapatite on alkali treated titanium substrates.     

置氢对Ti6Al4V合金室温组织的影响

通过在钛合金中引入临时元素氢,可以改变钛合金的相组成,进而改变钛合金的力学性能和加工性能.利用OP、XRD和TEM研究了固态置氢后Ti6Al4V合金的微观组织变化.研究表明:随氢含量的增加,合金中的β相含量增加,在置氢0.302%及0.490%(质量分数)的试样中发现面心立方(fcc)的氢化物δ,及大量的斜方结构的马氏体α",未发现亚稳态的氢化物γ.提出了一种基于扩散的由βH共析转变生成α及fcc结构的片状氢化物δ的机制,并指出氢的引入可能诱发马氏体转变.     

Ti6Al4V表面纹理制备及其润湿性

采用激光加工技术在Ti6Al4V表面分别加工直线、网格和具有规则点阵状结构的表面纹理,采用自组装技术制备自组装分子膜。采用扫描电镜、形貌分析仪和接触角测量仪对成膜后的钛合金表面进行形貌和接触角的表征与测量。结果表明,通过激光加工和沉积自组装分子膜,可显著增大Ti6Al4V的水接触角。其中直线纹理的试样表面水接触角可达124.8°,网格纹理的试样表面接触角可达126.1°,点阵状纹理的试样表面接触角可达151.6°。表面接触角与表面粗糙度相关,随着表面粗糙度值的增大,接触角呈增大趋势,当表面粗糙度>4μm时,接触角均>150°,形成超疏水表面。     

Investigation on high temperature compression behaviour of hydrogenated Ti6Al4V alloy

Abstract

Isothermal compression of hydrogenated Ti6Al4V alloy was carried out on a Gleeble-1500D simulation tester at the strain rate 3×10^?3 s^?1 and high temperatures. Before the isothermal compression, a simplified thermohydrogen processing (THP) was used for Ti6Al4V. Attention was paid to the effect of THP on subsequent compression behaviour. The results show that hydrogen can effectively lower the flow stress and deformation temperature and enhance the strain rate sensitivity index (m value) for isothermal compression. The increasing amount of β phase and the ultrafine and equiaxial microstructure precipitated between the original α or β laths are the main reasons for the simplified THP to improve the formability of Ti6Al4V.     

Thermal oxidation of Ti6Al4V alloy: Microstructural and electrochemical characterization

Thermal oxidation (TO) of Ti6Al4V alloy was performed at 500, 650 and 800 °C for 8, 16, 24 and 48 h in air. The morphological features, structural characteristics, microhardness and corrosion resistance in Ringer's solution of TO Ti6Al4V alloy were evaluated and compared with those of the untreated one. The surface morphological features reveal that the oxide film formed on Ti6Al4V alloy is adherent to the substrate at 500 and 650 °C irrespective of the oxidation time whereas it spalls off when the alloy is oxidized at 800 °C for more than 8 h. X-ray diffraction (XRD) measurement reveals the presence of Ti(O) and α-Ti phases on alloy oxidized at 500 and 650 °C, with Ti(O) as the dominant phase at 650 °C whereas the alloy oxidized at 800 °C exhibits only the rutile phase. Almost a threefold increase in hardness is observed for the alloy oxidized at 650 °C for 48 h when compared to that of the untreated one. Thermally oxidized Ti6Al4V alloy offers excellent corrosion resistance in Ringer's solution when compared to that of the untreated alloy.