Reaction path of the synthesis of α-Al2O3-TiC-TiB2 in an Al-TiO2-B4C system

Differential thermal analysis was undertaken to determine the reaction path of the synthesis of α-Al₂O₃-TiC-TiB₂ in an Al-TiO₂-B₄C system under argon. The Al content plays a significant role in controlling the reaction path and product. When the Al content is no more than 26.7 wt.%, TiO₂ first reacts with B₄C to yield TiB₂ with TiBO₃ being the intermediate phase, and then increasing temperature leads to the subsequent reactions between Al and TiO₂ or its sub-oxides to yield α-Al₂O₃ and Al₃Ti, and the resultant Al₃Ti then reacts with B₄C to produce TiC and TiB₂. When the Al content is high (e.g. ≥ 34 wt.%), the reaction between Al and TiO₂ for the formation of α-Al₂O₃ and Al₃Ti occurs initially, and then the Al₃Ti reacts with B₄C. With the increasing Al content, the onset of the exothermic reaction in the Al-TiO₂-B₄C system shifts to lower temperature and the degree of reaction conversion is enhanced.     

Cyclic oxidation behavior of TiAl composites incorporated with TiB2 dispersoids

TiAl alloys incorporated in (0,3,5,10) wt.% TiB₂ dispersoids were manufactured via mechanical alloyingspark plasma sintering (MA-SPS), and their cyclic oxidation characteristics were studied at 800, 900 and 1000°C in air. The cyclic oxidation resistance of the prepared TiAl-TiB₂ composites effectively increased with increases in TiB₂ content. The oxide scale formed consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (Al₂O₃+TiO₂) mixed layer. The scale adherence was relatively good, and much thinner oxide scales, when compared to TiB₂-free TiAl alloys, were formed on the prepared composites. The incorporated TiB₂ dispersoids oxidized to TiO₂ and B₂O₃ which evaporated during oxidation.     

Effect of ultrasonic vibration on microstructural evolution of the reinforcements and degassing of in situ TiB2p/Al-12Si-4Cu composites

Some issues such as clustering of TiB₂ particles, formation of long rod-like Al₃Ti particles, as well as high porosity level usually associate with the fabrication of in situ TiB_2p/Al-alloy composites via conventional stir casting technique using Ti and B as reactants. High-intensity ultrasonic vibration was introduced in our research to solve the above issues. The process involved that the original in situ TiB_2p/Al-12Si-4Cu sample with large clusters of TiB₂ particles, large long rod-like Al₃Ti particles and high porosity was remelted at 850 °C, and then ultrasonic vibration was applied to the melt with an ultrasonic probe. The microstructural evolution of the samples treated by ultrasonic vibration with different time was examined by using SEM. After treated by ultrasonic vibration for 12 min, large clusters of TiB₂ particles were broken up effectively and TiB₂ particles were dispersed uniformly in the matrix, and long rod-like Al₃Ti particles were turned into blocky ones with the size of 10 μm due to the effect of ultrasonic stirring. In the meantime, the porosity in the composites decreased from about 6.5% to 0.86% due to the effect of ultrasonic degassing. Microhardness test suggested that a homogeneous microstructure of the composite was achieved after ultrasonic treatment. An effective approach using high-intensity ultrasonic vibration to optimize the microstructures of the particulate reinforced Al-alloy composites was proposed, and the mechanism of the effect of high-intensity ultrasonic vibration on the microstructural evolution of the reinforcements and degassing of composites was also discussed in our research.     

Ti 元素对B4C/Al复合材料力学性能的改善作用研究

B₄C/Al复合材料是目前最理想的中子吸收材料,广泛用于乏燃料储存。本文利用液态搅拌法制备B₄C/Al复合材料,通过添加Ti元素,探讨了界面反应对材料的界面结构和力学性能的影响。研究发现,Ti元素通过参与界面反应,改变了界面结构,在B₄C颗粒表面形成了紧密结合的纳米TiB₂界面层;Ti的添加消除了界面微裂纹、微孔、分离等缺陷。随着界面反应程度的加强,材料强度提高,尤其反应脱落的纳米TiB₂颗粒作为原位第二强化相进一步增强基体。B₄C/Al复合材料断裂过程表现为韧窝延性断裂;TiB₂界面层增强了B₄C颗粒与基体的结合,断裂行为从B₄C-Al界面脱落转变为B₄C颗粒断裂;但过渡的界面反应会形成微韧窝,引起材料延伸率下降。     

The Oxidation of TiB2 Particle-Reinforced TiAl Intermetallic Composites

The oxidation kinetics of TiAl alloys with and without (3, 5, 10 wt.%) TiB₂ dispersoids were studied between 1073 and 1273 K in atmospheric air. The inert TiB₂ dispersoids effectively increased the oxidation resistance of TiAl alloys. The higher the TiB₂ dispersoids content, the more pronounced the effect. The oxide scale formed on TiAl–TiB₂ composites was triple-layered, consisting mainly of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. No B₂O₃ was observed within the oxide scale because of its high vapor pressure. A thin Ti₃Al sublayer and discrete TiN particles were found at the oxide–substrate interface. During the oxidation of TiAl alloys with and without TiB₂ dispersoids, titanium ions diffused outwardly to form the outer TiO₂ layer, while oxygen ions transported inwardly to form the inner (TiO₂+Al₂O₃) mixed layer. The increased oxidation resistance by the addition of TiB₂ was attributed to the enhanced alumina-forming tendency and thin and dense scale formation.     

Pressureless sintering of internally synthesized SiC-TiB2 composites with improved fracture strength

SiC-TiB₂ particulate composites were fabricated by converting TiO₂ to TiB₂ through the reaction between TiO₂, B₄C and C. The presence of initially very fine, in-situ created, TiB₂ particles increased driving force for sintering and enabled fabrication of a dense composite utilizing pressureless sintering and the liquid phase created between Al₂O₃ and Y₂O₃ additives. The effect of volume fraction of the in-situ formed TiB₂ on density, microstructure and flexural strength was discussed. It was found that the presence of TiB₂ particles suppressed the growth of SiC grains and enhanced fracture strength. The fracture strength of samples containing 12 vol% TiB₂ was more than 30% higher than that of the monolithic SiC. The effect of SiC grain size on fracture strength was also analyzed.     

激光焊接颗粒增强铝基复合材料中TiB2颗粒的演变行为（英文）

研究大功率激光器焊接TiB2颗粒增强铝基复合材料时TiB2粒子的演变行为。采用X射线衍射(XRD)、扫描电镜(SEM)及能谱(EDS)分析焊缝内粒子的物相、热力学过程及形貌特征;同时对TiB2和铝基体的界面反应进行讨论。结果表明:当TiB2团簇尺寸大于激光光斑直径时,焊缝中部的TiB2粒子会熔融在一起,较大尺寸的TiB2会发生断裂;当与铝熔体接触后,熔化后的TiB2粒子会与Al发生反应生成Al3Ti和AlB12,并且焊缝中部的界面反应比焊缝边缘的剧烈。     

In-situ TiB2 and Al2O3 formation by DC plasma spraying

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si, B₂O₃ and TiO₂ powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). It has been observed that, during the coating process, TiB₂ and Al₂O₃ are in-situ formed through the reaction between starting powders and finely dispersed in hypereutectic Al-Si matrix alloy. Also, obtained results demonstrate that in-situ reaction intensity strongly depends on spray conditions.     

原位生成铝基复合材料的激光焊接

采用大功率激光器研究新型铝基复合材料TiB2/ZL101的焊接性能,TiB2粒子的存在增加了焊缝熔池粘度降低了熔池流动性,影响了焊缝成形,增加了气孔敏感性.焊缝中气孔主要来源于氢和复合材料中的残留盐.激光焊接过程中较大的冷却速度使得焊缝晶粒非常细小,TiB2粒子在焊缝中分布更均匀,没有出现粒子偏析,主要是因为TiB2粒子是属于纳米级,在凝固过程中被凝固界面前沿所捕获而没有被推移.TiB2粒子没有与铝基体发生界面反应生成脆性相Al3Ti及AlB2,TiB2粒子与Al基体界面结合较好.结果表明,激光焊接后没有破坏TiB2粒子的增强效果.     

In Situ Synthesis Aluminum Borate Whiskers Reinforced TiB2 Matrix Composites for Application in Aluminum Reduction Cells

The TiB₂ matrix ceramics reinforced by aluminum borate whiskers (Al₁₈B₄O₃₃ w) had been prepared by the pressureless sintering method. The mechanical properties and densification behavior of the TiB₂ matrix ceramics were investigated. The results showed that Al₁₈B₄O₃₃ w was in situ synthesized by the reaction of boehmite (AlOOH) and TiB₂ powders during the sintering process. Increasing the sintering temperature had benefited for densification of the TiB₂ matrix ceramics. Al₁₈B₄O₃₃ w could increase the flexural strength and Vicker’s hardness. It is obtained that the maximum value Vicker’s hardness with 1.81 GPa and flexural strength with 82 MPa for samples sintered at 1600°C.     

Combustion synthesis of FeAl−Al2O3 composites with TiB2 and TiC additions via metallothermic reduction of Fe2O3 and TiO2

Combustion synthesis involving metallothermic reduction of Fe₂O₃ and TiO₂ was conducted in the mode of self-propagating high-temperature synthesis (SHS) to fabricate FeAl-based composites with dual ceramic phases, TiB₂/Al₂O₃ and TiC/Al₂O₃. The reactant mixture included thermite reagents of 0.6Fe₂O₃+0.6TiO₂+2Al, and elemental Fe, Al, boron, and carbon powders. The formation of xFeAl−0.6TiB₂−Al₂O₃ composites with x=2.0−3.6 and yFeAl−0.6TiC−Al₂O₃ composites with y=1.8−2.75 was studied. The increase of FeAl causes a decrease in the reaction exothermicity, thus resulting in the existence of flammability limits of x=3.6 and y=2.75 for the SHS reactions. Based on combustion wave kinetics, the activation energies of E_a=97.1 and 101.1 kJ/mol are deduced for the metallothermic SHS reactions. XRD analyses confirm in situ formation of FeAl/TiB₂/Al₂O₃ and FeAl/TiC/Al₂O₃ composites. SEM micrographs exhibit that FeAl is formed with a dense polycrystalline structure, and the ceramic phases, TiB₂, TiC, and Al₂O₃, are micro-sized discrete particles. The synthesized FeAl−TiB₂−Al₂O₃ and FeAl−TiC−Al₂O₃ composites exhibit the hardness ranging from 12.8 to 16.6 GPa and fracture toughness from 7.93 to 9.84 MPa·m^1/2.     

激光熔覆B4C-TiO2-Al粉末制备原位TiB2+TiC/Fe复合涂层（英文）

采用B4C、TiO2、Al以及Fe基自熔合金粉末为前驱体,利用激光熔覆技术在钢基体上制备TiB2+TiC颗粒增强Fe基复合涂层。结果表明,激光熔覆过程通过B4C-TiO2-Al反应生成了均匀分布于基体的TiB2-TiC复合陶瓷相。TiB2颗粒呈长条块状,TiC以不规则形状分布于基体中。涂层具有比基材1045钢更好的耐磨性能,但涂层的摩擦因数小。     

Reaction synthesis of TiB2–TiC composites with enhanced toughness

In situ toughened TiB₂–TiC_x composites were fabricated using reaction synthesis of B₄C and Ti powders at high temperatures. The resulting materials possessed very high relative densities and well developed TiB₂ plate-like grains, leading to a rather high fracture toughness, up to 12.2 MPa⋅m^1/2. The microstructure was examined by means of XRD, SEM, TEM and EDAX. The reaction products mainly consisted of TiB₂ and TiC_x. No other phases, e.g. Ti₃B₄, TiB, Ti₂B₅ and free Ti, were observed regardless of whether the starting composition was Ti:B₄C=3:1 or 4.8:1, and whether the sintering temperature was 1700 or 1800°C. The microstructural morphology is characterised by TiB₂ plate-like grains distributed uniformly in the TiC_x matrix. Some inclusions and defects were found in TiB₂ grains. The very high reaction temperature was believed to be responsible for the formation of plate-like grains, which, in turn, is responsible for the much improved mechanical properties. The main toughening mechanisms were likely to be crack deflection, platelet pull-out and the micro-fracture of TiB₂ grains.     

Effect of Superimposed Sinusoidal a.c. on the Characteristics of Electrodeposited Ni,Cu and Cu-Ni Alloy Composites with α-Al2O3 and TiO2

Nickel, copper and copper-nickel alloy composites were electrodeposited without and with inclusions of inert α-Al₂O₃ and TiO₂ particles from closely similar selected baths. It was found that during electrodeposition a superimposed sinusoidal a.c. exerted a decrease in the cathodic polarization for the individual metals and the alloy composites. The higher the superimposed a.c. density and the lower its frequency the greater was the depolarizing effect. The combined effect of superimposed a.c. and inclusion of the inert particles in the bath on the cathodic polarization proved to be additive. Superimposed a.c. induced negligible changes in the cathodic current efficiency of nickel and copper metal composites deposition, whereas it caused a marked decrease (about 20%) in that of the copper-nickel alloy composites. The inert (α-Al₂O₃ and TiO₂) particles content in the individual metal and alloy composites as well as the alloy composition were influenced by the superimposed a.c. and correlated with its depolarizing effect. The above-mentioned changes exerted by superimposed a.c. controlled the growth morphology, as revealed by SEM, and the microhardness of the as-deposited metal and alloy composites. A correlation could be detected between superimposed a.c. and dispersion of α-Al₂O₃ or TiO₂ particles in an individual metal or alloy matrix, the grain refinement, and an improvement of its microhardness could be detected.     

Combustion synthesis of TiC–TiB2 composites

An experimental study on formation of TiC–TiB₂ in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) using the reactant compacts from different combinations of Ti, B₄C, C, and B powders. Direct reaction of Ti with B₄C at stoichiometry of Ti:B₄C = 3:1 yields a TiB₂-rich composite with TiC:TiB₂ = 1:2. Formation of the products containing 20, 33.3, and 50 mol% of TiB₂ was achieved by the Ti–B₄C–C reactants. In addition, the test specimen composed of Ti, B₄C, and B was employed for the synthesis of a composite with 80 mol% TiB₂. Among three different types of the powder compacts, the boron-containing sample was characterized by the fastest combustion wave and the highest reaction temperature. The lowest combustion temperature and wave velocity were observed in the Ti–B₄C compact. When fine Ni particles were added to the Ti–B₄C reactant, it was found that the propagation rate of the reaction front was increased and the densification of the end product was enhanced significantly. This was attributed to formation of the Ti–Ni eutectic liquid during the reaction. As a result, the relative density of a TiC + 2TiB₂ composite increases from 30 to 86% with the Ni content from 0 to 20 mol%. Based upon the XRD analysis, small amounts of TiNi₃ and TiB were detected in the Ni-reinforced TiC–TiB₂ composites.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

Crystallography and refining mechanism of （Ti,B）-contained salts in pure aluminum

It is shown from experiment that the pure B contained salt exhibits little refining effect, while the pure Ti contained salt, especially the salt containing 5Ti/1B, shows obvious refining effect on the pure aluminum. Crystallographic study indicates that AI3Ti particle is a more suitable nucleation site for the aluminum matrix than （Ti, A1）B2 type particles （TiB2, A1B2 and （Ti,A1）B2）, because there exist more coherent planes with aluminum matrix in the former. Thermodynamics estimation, X-ray diffraction （XRD） and SEM detection show that the refining mechanism of （Ti, B）-contained refiners is mainly contributed to the heterogeneous nuclei of fine Al3Ti particles dispersed in the melting, which comes from the reaction between the Ti and aluminum. （Al, Ti）B2 type particle shows little or no direct refining effect, but it will reduce the size of Al3Ti since the Al3Ti nucleates and grows along the （A1, Ti）B2 type particle interface.     

Mechanochemical synthesis of Al2O3-TiB2 nanocomposite powder from Al-TiO2-H3BO3 mixture

Alumina-titanium diboride nanocomposite (Al₂O₃-TiB₂) was produced using mixtures of titanium dioxide, acid boric and pure aluminum as raw materials via mechanochemical process. The phase transformation and structural characterization during mechanochemical process were utilized by X-ray diffractometry (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and thermogravimetric analyses (TG-DTA) techniques. A thermodynamic appraisal showed that the reaction between TiO₂, B₂O₃ and Al is highly exothermic and should be self-sustaining. XRD analyses exhibited that the Al₂O₃-TiB₂ nanocomposite was formed after 1.5 h milling time. The results indicate that increasing milling time up to 40 h had no significant effect other than refining the crystallite size.     

The Microstructures and Mechanical Properties of V2O5-Doped Al2O3/TiAl In-Situ Composites by Reactive Hot Pressing Process

Al₂O₃/TiAl composites are successfully fabricated by the in-situ hot pressing method from the elemental powders of Ti, Al, TiO₂, and V₂O₅. The effect of V₂O₅ addition on the microstructure and mechanical properties of the Al₂O₃/TiAl in-situ composites is investigated in detail. It is found that the as-synthesized composites mainly consist of V-dissolved γ-TiAl, α₂-Ti₃Al, and Al₂O₃ particles along with a small amount of V₃Al phase, and the in-situ-formed fine Al₂O₃ particles tend to disperse on the grain boundaries of TiAl matrix. With increasing V₂O₅ content, the density and Vickers hardness of the resulting composites gradually increase, whereas the fracture toughness and flexural strength first increase and then decrease with the increase of V₂O₅ content. The composite with 3.5 wt.% V₂O₅ has the maximum value of 9.35 MPa m^1/2 and 713.36 MPa for the fracture toughness and flexural strength, respectively. The toughening mechanism is also discussed in detail.     

Phase composition,morphology and element contents of micro-arc oxidation ceramic coatings on Ti–6Al–4V alloy under different calcination conditions

Compound ceramic coatings with the main crystalline of Al_2TiO_5(in the as-prepared coating without treatment) were prepared in situ on the surface Ti–6Al–4V alloy by means of pulsed bipolar micro-arc oxidation in Na AlO_2 solution. For the purpose of studying the antioxidation properties of the samples, the coated samples treated in argon and the as-coated samples were calcined in air at 1000 °C. And the related characteristics were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM) and X-ray fluorescence(XRF) spectroscopy, respectively. The results show that, when it was calcined in air for 1 h, Al_2TiO_5in the as-prepared coating decomposed and transformed into α-Al_2O_3 and rutile TiO_2.However, after almost 4 h in argon, Al_2TiO_5in the asprepared coating decomposed and the final coating surface contents are completely α-Al_2O_3, and those of the middle interface are mainly Al_2O_3 and Ti_2O_3. The morphologies of the coatings after calcination in argon and air are different.High-temperature oxidation occurred violently in the alloy substrate without coatings. Furthermore, the weight gain curves of the as-prepared samples and the coated samples treated in argon both show a parabolic shape.