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.     

Dynamic behavior of HIPed Ti–6Al–4V

The overall dynamic properties of hot isostatically pressed (HIPed) Ti–6Al–4V were throughout investigated. MIL-T-9047G was chosen as a standard material for property comparison due to its best combination of strength and ductility. The elastic constants of HIPed Ti–6Al–4V were measured by resonant ultrasound spectroscopy (RUS) method. Three types of Hopkinson bar tests (constitutive relations tests, strain-controlled specimen tests and hat-shaped specimen tests) were conducted on HIPed material as well as on the standard material to evaluate the high-strain-rate properties of HIPed Ti–6Al–4V. Thick wall cylinder explosion test was conducted to compare the post-critical behavior of baseline material (MIL-T-9047G) and Ti–6Al–4V HIPed material. The texture effect in high-strain-rate properties was checked as the main difference between HIPed (texture free) and standard material (grains elongated along the rod axial direction). The results show that the nonmilled HIPed Ti–6Al–4V has similar yield strength and strain-hardening curve in comparison with the standard material in forging direction but has a lower ductility, which is similar to the ductility of standard material normal to forging direction. The same conclusion also generated from the strain controlled specimen tests. The results that obtained in hat-shaped specimen tests show that HIPed material starts shear localization relatively earlier than standard material. This result is somewhat contradicted with the ballistic testing results that show better ballistic performance for HIPed materials in all three types of penetration tests (flat-ended, conical and long-rod) [see Refs. Nesterenko VF, Indrakanti SS, Goldsmith W, Gu Y. In: Staudhammer KP, Murr LE, Meyers MA, editors. Fundamental issues and applications of shock-wave and high-strain-rate phenomena. Amsterdam: Elsevier Science; 2001. p. 593–600; Nesterenko VF, Indrakanti SS, Brar S, Gu Y. Shock compression of condensed matter. AIP Conf Proc 2000; 505(1): 419–22; Nesterenko VF, Indrakanti SS, Brar S, Gu Y. Key Eng. Mater. 2000; 177-180: 243–8; Nesterenko VF, Goldsmith W, Indrakanti SS, Gu Y. Int J Impact Eng 2003; 28(2): 137–60; Gu Y, Indrakanti SS, Nesterenko VF. Shock compression of condensed matter. AIP Conf Proc 2002; 620(1): 1294–7]. The explanation for this contradiction might be the influences coming from the texture, and other microstructure features such as phase content, grain size distribution and fast shear localization propagation path at the grain boundary and the phase interface.     

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.     

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.     

Fabrication and characterization of nanostructured Ti6Al4V powder from machining scraps

In recent years researches on properties of nanocrystalline materials in comparison with coarse-grained materials has attracted a great deal of attention. The present investigation has been based on production of nanocrystalline Ti6Al4V powder by means of high energy mechanical milling. In this regard, Ti6Al4V powder was produced by ball milling of machining scraps of Ti6Al4V. The structural and morphological changes of powders were investigated by X-ray diffraction, scanning electron microscopy, and microhardness measurements. The results revealed that ball milling process reduced the size of the coherent-scattering region of Ti6Al4V to approximately 20 nm. Also a remarkable change in morphology and particle size was occurred during ball milling. Moreover, phase evolution during milling was taken into consideration. The as-milled Ti6Al4V powder exhibited higher microhardness comparing to the original samples.     

The effect of quenching medium on the wear behaviour of a Ti–6Al–4V alloy

The aerospace alloy, Ti–6Al–4V is a difficult material to machine, and, in general, shows poor wear resistance due to the soft, ductile properties of the alloy. In this study, the Ti–6Al–4V alloy has been heat treated to a temperature above and below the β-transus temperature and then quenched using a medium of oil, water or liquid nitrogen to change the surface wear behaviour of the alloy. The results showed that no significant change in microstructure and surface properties was achieved when the alloy was heated to 750 °C and then quenched in liquid nitrogen. However, when the alloy was heated to 1,000 °C (above the β-transus), the hardness of the titanium alloy significantly increased from 400 VHN to about 800 VHN, but the wear resistance of the alloy did not improve. In fact, the wear resistance of the alloy decreased as the surface hardness increased, and this change in wear behaviour was attributed to a change in the mechanism of wear from plastic deformation to brittle-fracture of the surface.     

The effect of ion plating on the friction and wear of Ti6Al4V alloy

The effect of noble metal ion plating on the room temperature friction and wear of Ti6Al4V was investigated. Sliding wear experiments were conducted on a linear displacement machine which enabled the determination of coefficient of friction at regular intervals. The wear experiments were performed between two titanium alloy surfaces with various ion plating and dry lubrication conditions. Test results showed that the extent of wear damage of the titanium substrate was directly related to the rate of increase of the friction coefficient and not to its value. The best wear protection was provided either by gold ion plating on one surface combined with MoS₂ lubrication or by gold ion plating on both surfaces. The former case yielded a very low friction coefficient while the latter yielded a very high one. Modes of wear were determined by scanning electron microscopy of sectioned wear specimens.     

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.     

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.     

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.     

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。     

Foaming behavior of Ti6Al4V particle-added aluminum powder compacts

The foaming behavior of 5 wt.% Ti6Al4V (Ti64) particle (30–200 μm)-added Al powder compacts was investigated in order to assess the particle-addition effects on the foaming behavior. Al compacts without particle addition were also prepared with the same method and foamed. The expansions of Ti64 particle-added compacts were measured to be relatively low at small particle sizes and increased with increasing particle size. At highest particle size range (160–200 μm), particle-added compacts showed expansion behavior similar to that of Al compacts without particle addition, but with lower expansion values. Expansions studies on 30–45 μm size Ti64-added compacts with varying weight percentages showed that the expansion behavior of the compacts became very similar to that of Al compact when the particle content was lower than 2 wt.%. However, Ti64 addition reduced the extent of drainage. Ti64 particles and TiAl₃ particles formed during foaming increased the apparent viscosity of the liquid foam and hence reduced the flow of liquid metal from cell walls to plateau borders. The reduced foamability in the compacts with the smaller size Ti64 addition was attributed to the relatively high viscosities, due to the higher cumulative surface area of the particles and higher rate of TiAl₃ formation between liquid Al and Ti64 particles.     

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.     

In situ synthesis of TiN/Ti3Al intermetallic matrix composite coatings on Ti6Al4V alloy

The aim of this paper was to develop an in situ method to synthesize the TiN reinforced Ti₃Al intermetallic matrix composite (IMC) coatings on Ti6Al4V alloy. The method was divided into two steps, namely depositing pure Al coating on Ti6Al4V substrate by using plasma spraying, and laser nitriding of Al coating in nitrogen atmosphere. The microstructure and mechanical properties of TiN/Ti₃Al IMC coatings synthesized at different laser scanning speeds (LSSs) in laser nitriding were investigated. Results showed that the crack- and pore-free IMC coatings can be made through the proposed method. However, the morphologies of TiN dendrites and mechanical properties of coatings were strongly dependent on LSS used in nitriding. With decreasing the LSS, the amount and density of TiN phase in the coating increased, leading to the increment of microhardness and elastic modulus and the decrement of fracture toughness of coating. When the LSS was extremely high (i.e., 600 mm/min), only a thin TiN/Ti₃Al layer with thickness around of 100 μm was formed near the coating surface.     

Surface characteristics of Ti6Al4V alloy: effect of materials, passivation and autoclaving

The properties of passivated films for Ti6Al4V alloy prepared by various methods (as-polished, brazed at 970 °C for 2 h and brazed at 970 °C for 8 h) were investigated. Four passivations (non-treated, nitric acid passivation, 400 °C-treated in air, and aged in boiling water), with or without autoclaving treatments, were adopted for evaluating the changes of surface properties, including chemical composition, chemical structure, and oxide thickness. From X-ray photoelectron spectroscopic (XPS) analyses, surface elements of copper and nickel in brazed samples were undetected for non-treated, acid-passivated and boiling water-aged specimens, while they were found in the 400 °C-treated specimen. The relative contents of Ti2++Ti3+ to Ti4+ were determined by passivation treatments, but were not related to the experimental materials and autoclaving treatment. Passivation and autoclaving decreased the Ti to Ti4+ ratio by virtue of an increase in oxide thickness. Of the four types of passivation treatment, the 400 °C thermal treatment exhibited the lowest content of suboxides and metallic elements and the thickest oxide by XPS analysis; however, this treatment may cause a desorption of the basic OH group in the hydration layer on the surface of titanium alloy.     

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.     

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。     

氧化处理时间对Ti6Al4V微弧氧化陶瓷膜的影响

采用交流微弧氧化法于：Na：SiO3-KOH-(NaPO3)6溶液中在％6A14V表面形成了氧化物陶瓷膜．利用扫描电镜、电子探针及X射线衍射研究了陶瓷膜的组织形貌、元素的分布和相组成．研究表明：在恒定的微弧氧化电参数(U =500V，U-=100V和f=600Hz)下，随氧化时间延长，电流密度逐渐降低，膜层厚度不断增加；相对致密均匀的膜分为3层：过渡层、致密层与疏松层．膜层主要由TiO2(锐钛矿和金红石)相组成，延长处理时间，锐钛矿相及金红石相的相对含量发生变化，金红石相TiO2逐渐增多，而锐钛矿相TiO2减小．膜层相的形成过程可分为两个阶段。     

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.