Effect of reacted compounds in Al2O3+Ti investment mold on alpha-case formation for Ti casting

This paper proposes a newly developed alpha-case controlled mold material for Ti castings. An Al₂O₃ mold containing alpha-case reaction compounds, titanium oxide (TiO₂, Ti₂O and Ti₆O) and titanium silicide (Ti₅Si₃) was manufactured via a reaction between Al₂O₃ and Ti powder under different firing conditions in air and a vacuum. In comparison with the Al₂O₃ and Al₂O₃+Ti mold fired in a vacuum, the micro-Vickers hardness and nano indentation profiles of Al₂O₃+Ti indicated that the alpha-case thickness was significantly reduced from 350 μm to ～45 μm. The alpha-case formation in the Al₂O₃+Ti mold was reduced due to the presence of TiO₂, which formed the TiO intermediate phase that acted as a diffusion barrier. In addition, Ti₅Si₃ was effective in minimizing Si contamination at the casting surface due to the reaction between Ti and the colloidal SiO₂ binder. Therefore, alpha-case reaction compounds, such as TiO₂ and Ti₅Si₃ in Al₂O₃, can effectively reduce alpha-case formation at the casting surface.     

铸造钛合金α层的组织与形成机理

利用熔模铸造的方法制备了铸造Ti-6Al-4V合金试样,通过金相、电子探针、显微硬度测试等技术研究了铸造钛合金表面α层的组织和形成机理。结果表明,铸造钛合金试样表面均形成了α层,显微组织特征为呈"网篮"状排列的粗大片状α相组织。α层形成机理主要与型壳材料中间隙氧原子和碳原子向合金基体扩散有关。显微硬度测试得到的α层深度略大于金相结果。为完整去除α层,以硬度测试的结果更为可靠。     

Ti/TiSiNO Multilayers Fabricated by Co-sputtering

Multilayer coatings of titanium/ titanium silicon oxynitride (Ti/TiSiNO) were fabricated on stainless steel 434-L substrates by sputtering technique using Ti and TiSiO targets. The multilayers were formed by alternately and simultaneously introducing and suspending a 4-sccm flow of nitrogen together with an interchange of targets, during different time periods of 1.0, 2.0, 3.0 and 4.0 min; thus, we obtained intervals of different thickness by alternating the target (Ti) and using only argon, and later with both targets (Ti + TiSiO) and the two gases (Ar + N₂). The final thickness was within a range of 0.4-0.8 μm, depending on the number of multi layers obtained by means of glow discharge optical emission spectroscopy. The structure, topography, color, durability and nanofriction were analyzed in accordance with the growth parameters used. It was found that the values of hardness depend on the number of layers deposited. The greatest number of layers, 48 of them, was obtained with a time period of 1 min, thereby attaining the maximum value of hardness and the minimum value for both the friction coefficient and the RMS, with 31 GPa, 7.42 × 10^?3 and 0.17 nm, respectively. The phases found correspond to Ti₂N, Si₃N₄, rutile phase of TiO₂ and Ti. The TiO₂ phase was corroborated with Raman dispersion spectroscopy. The color of the coatings was affected by variation in the working pressure, which varied because of the internal process of reactive sputtering.     

Relation Between the Microstructure and Properties of Commercial Titanium Alloys and the Parameters of Gas Nitriding

Titanium alloys are unique materials with an excellent combination of properties. However, their applications are limited due to low surface hardness. In the present work gas nitriding is performed with the aim of improving the surface properties of commercial titanium alloys. Four widely used titanium alloys, namely, Ti – 6% Al – 4% V, Ti – 6% Al – 2% Sn – 4% Zr – 2% Mo, Ti – 8% Al – 1% Mo – 1% V, and Ti – 10% V – 3% Fe – 3% Al, are studied. The process is performed in a nitrogen atmosphere at 950 and 1050°C for 1, 3, and 5 h. The resulting surface hardness exceeds the normal value for titanium alloys by a factor of 3 – 5 due to the change in the phase composition of the surface layer, yielding a solid solution of nitrogen in an α-Ti phase, a TiN nitride, and TiO₂ dioxide. The influence of the parameters of the treatment process and the chemical composition of the alloys on the phase composition, microstructure, microhardness, and thickness of the surface layer is analyzed. It is shown that nitriding of alloys with α- and (α + β)-structures at 1050°C yields surface layers with inhomogeneous composition and irregular thickness, whereas after nitriding at 950°C the surface layers are homogeneous and have high properties. Practical recommendations are given for choosing nitriding parameters for different alloys and variants of application are discussed.     

Oxidation Behavior of Ti50Ni40Cu10 Shape-Memory Alloy in 700–1,000 °C Air

The isothermal-oxidation behavior of Ti₅₀Ni₄₀Cu₁₀ shape memory alloy (SMA) in 700–1,000 °C air was investigated by TGA, XRD, SEM and EPMA. Experimental results indicate that a multi-layered oxide scale formed, consisting of an outermost Cu₂O(Ni,Ti) layer, a layer of the mixture of TiO₂, TiNiO₃ and irregular small pores, a layer of the mixture of Ni(Ti,Cu), TiO₂ and irregular large pores, a Ti(Ni,Cu)₃ layer and an innermost Ti₃₀Ni_43–47Cu_27–23 layer. The apparent activation energy for the oxidation reaction of Ti₅₀Ni₄₀Cu₁₀ SMA is determined to be 180 kJ/mol, and the oxidation rate follows a parabolic law. A schematic oxidation mechanism of Ti₅₀Ni₄₀Cu₁₀ SMA is proposed to explain the observed results.     

钛合金氧碳共渗层性能

将经磨平、抛光、超声波乙醇清洗、吹干的Ti6Al4V合金样品放入专用设备中进行氧碳共渗。分别采用X射线衍射仪（XRD）、扫描电子显微镜（SEM）、能谱仪（EDS）、HV硬度仪、万能材料试验机对渗层的物相、显微组织形貌、成分、硬度、摩擦磨损、力学性能进行分析。XRD结果表明,渗层中出现TiC和TiO_x相。氧碳共渗改变了原Ti6Al4V合金的组织,渗层的显微组织明显区别于Ti6Al4V的渗碳、渗氧以及在CO₂气氛下处理的组织。EDS结果表明,C、O元素含量呈现梯度变化。相比基体硬度,渗层表面硬度提高了3.8倍,渗层硬度呈梯度变化。氧碳共渗改变了原始样的粘着磨损和摩擦状态,渗层样表面只有轻微的摩擦痕迹、无磨损。氧碳共渗后磨损量是原始样的3.5%,摩擦系数约为原始样的30%。渗层样在拉断过程中,外表面有一定的剥落,表面布满裂纹,样品的强度略有下降,断面延伸率和断面收缩率与原始样相当。Ti6Al4V合金经过氧碳共渗后,提高了表面硬度,降低了磨损率和摩擦系数,基本保持了基体的力学性能。     

废弃SCR脱硝催化剂与废NaCl盐焙烧回收催化剂中的钛、钒、钨（英文）

通过废弃选择性催化还原(SCR)脱硝催化剂与废NaCl盐焙烧,可以将催化剂中的钨和钒与钛分离。在最佳浸出条件下(焙烧温度900℃,焙烧时间3 h,废盐与废催化剂的质量比为0.5,浸出温度80℃,反应时间60 min),钨和钒的浸出率分别达到84.63%和66.42%,同时钛的损失率仅为1.3%。废NaCl盐和焙烧温度可以促进锐钛矿型TiO₂转化为金红石型TiO₂,反应后得到了金红石型TiO₂。金红石型TiO₂中的钛的价态为四价,晶格氧和化学吸附氧分别占57.26%和42.74%。该方法可以同时解决2种废弃物的处置问题。     

Investigation of interface reaction between TiAl alloys and mold materials

This paper describes the investment casting of TiAl alloys. The effects of mold mateiral and mold preheating temperature for the investment casting of TiAl on metal-mold interfacial reaction were investigated by means of optical micrography, hardness profiles and an electron probe microanalyzer. The mold materials examined were colloidal silica bonded ZrO₂, ZrSiO₄, Al₂O₃ and CaO stabilized ZrO₂. When compared with conventional titanium alloy, the high aluminum concentration of TiAl alloys helps to lower their reactivity in the molten state. The Al₂O₃ mold is a promising mold material for the investment casting of TiAl in terms of the thermal stability, formability and cost. Special attention need to be paid to thermal stability and mold preheating when developing the investment casting of TiAl alloys.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Electrolytic Production of Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript>/TiC Composites by Solid Oxide Membrane Technology

This paper investigated the electrolytic production of Ti₅Si₃/TiC composites from TiO₂/SiO₂/C in molten CaCl₂. The solid-oxide oxygen-ion-conducting membrane tube filled with carbon-saturated liquid tin was served as the anode, and the pressed spherical TiO₂/SiO₂/C pellet was used as the cathode. The electrochemical reduction process was carried out at 1273 K and 3.8 V. The characteristics of the obtained cathode products and the reaction mechanism of the electroreduction process were studied by a series of time-dependent electroreduction experiments. It was found that the electroreduction process generally proceeds through the following steps: TiO₂/SiO₂/C → Ti₂O₃, CaTiO₃, Ca₂SiO₄, SiC → Ti₅Si₃, TiC. The morphology observation and the elemental distribution analysis indicate that the reaction routes for Ti₅Si₃ and TiC products are independent during the electroreduction process.     

Effect of Graphite Addition on Structure and Properties of Ti(CN) Coatings Deposited by Reactive Plasma Spraying

Ti(CN) coatings with graphite addition ranging from 0 to 50 wt.% were prepared using reactive plasma spraying technology and their microstructure, mechanical, and tribological properties were investigated using scanning and transmission electron microscopy, x-ray diffraction analysis, x-ray photoelectron spectroscopy, Vickers microhardness testing, and block-on-ring wear testing. The results showed that graphite addition resulted in crystallite size refinement and an increase in the amount of amorphous phase. The Ti(CN) coatings consisted of a mixture of Ti(CN), graphite, CN _x , and amorphous phases. The hardness first increased then decreased as the graphite content was increased, with a maximum of 1450 HV_0.2 for 30 wt.% graphite addition. The fracture toughness decreased from 4.38 MPa m^1/2 to 2.76 MPa m^1/2 with increasing graphite content. The friction coefficient decreased due to unreacted graphite embedded in the matrix. Also, the wear rate first decreased then increased, with a minimum value of 2.65 × 10^?6 mm³ N^?1 m^?1 for 30 wt.% graphite addition. The wear mechanisms of the Ti(CN) coatings included abrasive, adhesive, and oxidation wear.     

Calcination/acid-activation treatment of an anodic oxidation TiO<Subscript>2</Subscript>/Ti film catalyst

The aim of this work was to investigate the effects of calcination/acid-activation on the composition, structure, and photocatalytic (PC) reduction property of an anodic oxidation TiO₂/Ti film catalyst. The surface morphology and phase composition were examined by scanning electron microscopy and X-ray diffraction. The catalytic property of the film catalysts was evaluated through the removal rate of potassium chromate during the PC reduction process. The results showed that the film catalysts were composed of anatase and rutile TiO₂ with a micro-porous surface structure. The calcination treatment increased the content of TiO₂ in the film, changed the relative ratio of anatase and rutile TiO₂, and decreased the size of the micro pores of the film catalysts. The removal rate of potassium chromate was related to the technique parameters of calcination/acid-activation treatment. When the anodic oxidation TiO₂/Ti film catalyst was calcined at 873 K for 30 min and then acid-activated in the concentrated H₂SO₄ for 60 min, it presented the highest catalytic property, with the removal rate of potassium chromate of 96.3% during the PC reduction process under the experimental conditions.     

Origin of Surface Irregularities on Ti–10V–2Fe–3Al Beta Titanium Alloy

We studied the origin of different characteristics and properties of a Ti–10V–2Fe–3Al beta (β) titanium alloy with surface height irregularities that occurred during machining. The height differences were observed in two different regions, labeled as “soft region” and “hard region.” The present study showed a higher Fe and a lower Al content in the hard region, which resulted in higher β-phase stability to resist primary alpha (α_p) phase precipitation caused by a failure of the solution treatment process. In contrast, the soft region contained a higher volume fraction of α_p phase and a lower volume fraction of the matrix, which consisted of a combination of β and secondary alpha (α_s) phase. A high number of α_s/β interface in the matrix with a predicted hardness of 520 HV generated an improvement of hardness in the hard region. Therefore, the hard and the soft regions had different abilities to resist wear during machining process, resulting in surface height irregularities.     

Effect of Ti (Macro-) Alloying on the High-Temperature Oxidation Behavior of Ternary Mo–Si–B Alloys at 820–1,300 °C

The aim of the present investigation was to gain an initial understanding of the effect of (macro-) alloying with Ti on the oxidation behavior of Mo–Si–B alloys in the ternary phase region of Mo_ss–Mo₃Si–Mo₅SiB₂ at 820–1,300 °C. Motivated by recent studies and thermodynamic calculations, the alloy compositions Mo–9Si–8B–29Ti (at.%) and Mo–12.5Si–8.5B–27.5Ti (at.%) were selected and synthesized by arc-melting. Compared to the reference alloy Mo–9Si–8B, superior initial oxidation rates at 1,100–1,300 °C as well as a significant density reduction by nearly 18 % were observed. Due to enhanced initial evaporation of MoO₃ and mainly inward diffusion of oxygen, a borosilicate-rutile duplex scale with a continuous TiO₂ phase had formed. Detailed investigations of the oxidation mechanism by SEM, EDX, XRD and confocal micro-Raman spectroscopy indicated that Ti alloying is promising with regard to further improvement of the oxidation resistance as well as the strength-to-weight ratio of Mo–Si–B alloys.     

Sol-gel derived HA/TiO2 double coatings on Ti scaffolds for orthopaedic applications

Hydroxyapatite/titania （HA/TiO2） double layers were coated onto Ti scaffolds throughout for orthopaedic applications by sol-gel method. Differential scanning calorimetry （DSC）, thermogravimetric analysis （TG） and X-ray diffractometry （XRD） were used for the characterisation of the phase transformations of the dried gels and coated surface structures. Scanning electron microscope （SEM） equipped with energy dispersive spectrometry （EDS） was used for the observation and evaluation of the morphology and phases of the surface layers and for the assessment of the in vitro tests. The in vitro assessments were performed by soaking the HA/TiO2 double coated samples into the simulated body fluid （SBF） for various periods. The TiO2 layer was coated by a dipping-coating method at a speed of 12 cm/min, followed by a heat treatment at 600 ℃ for 20 min. The HA layer was subsequently dipping-coated on the outer surface at the same speed and then heat-treated at difference temperatures. The results indicat that the HA phase begins to crystallize after a heat treatment at 560 ℃. The crystallinity increases obviously at 760 ℃. SEM observations find no delamination or crack at the interfaces of HA/TiO2 and TiO2/Ti. The HA/TiO2 coated Ti scaffolds displays excellent bone-like apatite forming ability when it is soaked into SBF. Ti scaffolds after HA/TiO2 double coatings can be anticipated as promising implant materials for orthopaedic applications     

Phase Diagram Investigations of the Bi-Ti System

The liquidus and invariant reaction temperatures were measured by thermal analyses for Bi-Ti alloys containing 0-65 at.% of Ti. The liquidus temperatures were clearly much lower than the literature values. Equilibrium experiments showed the existence of a new phase, Bi₂Ti₃. However, the reported Bi₃Ti₂ phase was not observed in the present study. The temperatures of the peritectic reaction involving Bi₂Ti, Bi₉Ti₈, and Bi₂Ti₃ were also clarified to be 425, 930, and 970 °C, respectively. Thus, a new phase diagram of the Bi-Ti system was constructed.     

Reaction between Ti–6Al–4V and Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> based face shell for investment casting

The pH value and viscosity of Y₂O₃–SiO₂ (Y–Si) slurry made by Y₂O₃ powders and silica sol for the face coat of Ti–6Al–4V investment casting were measured. The thermal behavior of the shell made by the Y–Si face coat system was investigated by differential scanning calorimeter (DSC), thermal gravimetric (TG) analysis combined with mass spectrometry (MS), and the phase transformations were determined by X-ray diffraction (XRD). Hot strength, residual strength, linear expansion coefficient, and wearing resistance performance of the shell were also tested. The microstructure and elements distribution of the interaction layer were studied by scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS), respectively. The microhardness tester was applied for the microhardness. The results showed that the slurry was stable for at least 60 h. A very small amount of YZrO₃ was formed below 1050 °C and Y₂SiO₅ was formed around 1450 °C. The shell made by Y–Si system had good mechanical property which could reduce cracks during the procedure of dewaxing and inclusions during pouring. Some Al volatilized from the melt, permeated the surface of the face coat shell, and formed the black reaction layer, which blocked the permeation of O so that O penetration was limited to 5 μm. The depth of Si penetration was about 60 μm. The hard layer was also around 60 μm.     

Study on Reactive Atmospheric Plasma-Sprayed In situ Titanium Compound Composite Coating

A titanium composite coating containing in situ synthesized oxides or nitrides was deposited on mild steel by reactive atmospheric plasma spraying. The relationships between the porosity, hardness, the phase composition of the sprayed coatings, and the spraying parameters were investigated. Titanium powders were used as starting powder materials. XRD analysis and microhardness test revealed that titanium oxides and nitrides were synthesized during the spraying process. The longer the spraying distance, the more the Ti nitrides’ content in the coating. It is shown that the Ti nitrides’ content has a significant influence on coating hardness. An in situ titanium composite coating with a hardness value of 1534HV_0.1 and an adhesive strength of 55.4 MPa was made with appropriate spraying parameters.     

Effects of Micro and Macroalloying on the Formation and Thermal Stability of Nanocrystalline L12-Al3V

Structural modification of nanocrystalline trialuminde Al₃V by microalloying with Ti and macroalloying with Zr was studied. For micro and macroalloying, the Al₇₅(V _x Ti_1?x ) x = 0-25 and Al₅ZrV₂ were synthesized by mechanical alloying of elemental blends of the nominal composition. The structural changes of powder particles during mechanical alloying were investigated by x-ray diffraction patterns. Microstructure of powders was characterized by scanning electron microscopy and the hardness of powder particles was determined by Vickers microhardness measurement. It was found that microalloying of trialuminides by Ti led to the formation of Al₃(V,Ti) with stable tetragonal structure (DO₂₂) in different amounts of Ti after 40 h milling. Thermodynamic calculations, which were based on Miedema model, showed that the higher negative formation enthalpy of intermetallics in Al-V-Ti system induced refinement of trialuminides. In addition, it was found that the hardness of compounds was increased by enhancement of Ti in trialuminide composition. For Al₅V₂Zr compound, the metastable L1₂ phase was formed after 20 h milling. Thermal analysis of the synthesized compound showed an exothermic peak around 550 °C, which was related to the partial transition of L1₂ structure to DO₂₂. Moreover, ordering of L1₂ phase took place due to increasing temperature.     

Microstructure and Room-Temperature Mechanical Properties of FeCrMoVTi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys

FeCrMoVTi _x (x values represent the molar ratio, where x = 0, 0.5, 1.0, 1.5, and 2.0) high-entropy alloys were prepared by a vacuum arc melting method. The effects of Ti element on the microstructure and room-temperature mechanical properties of the as-cast FeCrMoVTi _x alloys were investigated. The results show that the prepared alloys exhibited typical dendritic microstructure and the size of the microstructure became fine with increasing Ti content. The FeCrMoV alloy exhibited a single body-centered cubic structure (BCC1) and the alloys prepared with Ti element exhibited BCC1 + BCC2 mixed structure. The new BCC2 phase is considered as (Fe, Ti)-rich phase and was distributed in the dendrite region. With the increase of Ti content, the volume fraction of the BCC2 phase increased and its shape changed from a long strip to a network. For the FeCrMoV alloy, the fracture strength, plastic strain, and hardness reached as high as 2231 MPa, 28.2%, and 720 HV, respectively. The maximum hardness of 887 HV was obtained in the FeCrMoVTi alloy. However, the fracture strength, yield stress, and plastic strain of the alloys decreased continuously as Ti content increased. In the room-temperature compressive test, the alloys showed typical brittle fracture characteristics.