Effect of heat treatment on the mechanical and microstructural characterization of Mg-AZ91E/TiC composites

Mg-AZ91E/TiC_p composite was fabricated using a spontaneous infiltration technique at 950 °C under an argon atmosphere. The composites produced have 37 vol.% of metal matrix and 63 vol.% of TiC-like reinforcement. The obtained composites were subsequently solution heat-treated at 413 °C during 24 h, cold water quenched, and subsequently artificially aged at 168 and 216 °C during 16 h in an argon atmosphere. Effect of heat treatment on the microstructure and mechanical properties was evaluated. Microstructural characterization was analyzed using different techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM). Interface between matrix and reinforcement was examined using transmission electron microscopy (TEM), and mechanical properties were evaluated by measuring the elastic modulus and hardness. Mg, TiC, Al, and Mg₁₇Al₁₂ phases through XRD were detected. Meanwhile, using TEM analysis in heat-treated composites MgAl₂O₄, MgO, and Al₂O₃ were identified. The as-fabricated composite have elastic modulus and hardness of 162 GPa and 316 Hv, respectively. After solution heat treatment and aging at 168 °C during 12 h, the composites reaches values of 178 GPa and 362 Hv for the elastic modulus and hardness, respectively. Time of aging was correlated with measures of elastic modulus and hardness.     

Dry sliding wear characteristics of in situ synthesized Al-TiC composites

Abstract

Aluminum-based composites containing 0.06, 0.09, 0.12 fractions of in situ-synthesized TiC (Titanium carbide) particles have been prepared through in-melt reaction from Ai–SiC–Ti system following a simple and cost-effective stir-casting route. The TiC forms by the reaction of Ti with carbon which is released by SiC at temperatures greater than 1073 K. However, some amount of titanium aluminide (Al₃Ti) is also formed. The formation of TiC has been confirmed through X-ray diffraction studies of the composite. The hardness and tensile strength have been found to increase with increasing amount of TiC. The friction and wear characteristics of the composites have been determined by carrying out dry sliding tests on pin-on-disc machine at different loads of 9.8 N, 19.6 N, 29.4 N, 39.2 N at a constant sliding speed of the 1 m/s speed. The wear rate i.e. volume loss per unit sliding distance has been found to increase linearly with increasing load following Archard’s law. However, both the wear rate and friction coefficient have been observed to decrease with increasing amount of TiC in the composite. This has been attributed to (i) a relatively higher hardness of composites containing relatively higher amount of TiC resulting in a relatively lower real area of contact and (ii) the formation of a well-compacted mechanically mixed layer of compacted wear debris on the worn surface which might have inhibited metal–metal contact and resulted in a lower wear rate as well as friction coefficient.     

Thermoluminescence studies of γ-irradiated nanocrystalline Y3Al5O12

Nanocrystalline yttrium aluminum garnet (Y₃Al₅O₁₂) is synthesized by combustion technique. The X-ray diffraction (XRD) pattern of 900 °C annealed sample revealed a cubic structure. The average crystallite size is found to be 20.5 nm. γ-irradiated Y₃Al₅O₁₂ exhibits two thermoluminescence (TL) glows: a prominent one with a peak at ～410 K and another one with a peak at ～575 K. It is found that the TL glow peak intensity at 410 K increases, while its glow peak temperature is almost steady with an increase in the γ-dose. The effect of the heating rate on the TL glow curve is studied. It is found that T_m1 shifts towards higher temperature region while the I_m1 decreases with an increase in the heating rate. The TL glow curves are analyzed by Chen's peak shape method and the TL parameters are estimated.     

钛铝合金高温氧化机理电子理论研究

为了从电子层面揭示钛铝合金高温氧化的物理本质,采用递归法与Castep相结合的方式, 计算了原子埋置能、亲和能、结合能等电子结构参数,探索合金氧化机理.研究表明: 氧在钛中有较大固溶度,氧原子可以在钛表面的基体内聚集,逐步向深层扩散. 氧与钛具备较强的亲和力,能形成钛的氧化膜.钛基体中铝原子间具有相互吸引力, 能形成铝的原子团簇.铝原子团簇中的钛原子间相互排斥与铝形成化合物. 铝、钛与氧的亲和能相近,不易发生铝的优先氧化,而是同时生成钛的氧化物和铝的氧化物. Al₂O₃比TiO₂的结合能略低,因而更加稳定,铝在TiO₂中有较大的固溶度, 能替换其中的钛形成更稳定的Al₂O₃氧化物.     

Silver macro‐texture substrates fabricated by plasma selective etching for surface‐enhanced Raman scattering

We have demonstrated a novel method to generate the nanostructured substrate that shows a large enhancement with a spatially uniform enhancement factor of approximately 10⁶ in surface enhanced Raman scattering (SERS). The substrates are fabricated using plasma selective etching. First, the Al₂O₃–TiC template which contains mixed Al₂O₃ and TiC grains with the diameters of ~400 nm is selected as a base plate. The Al₂O₃ and TiC grains have different physical properties, such as hardness, which corresponds to different etching rate in a plasma gas. Then, the Al₂O₃–TiC substrate is selectively etched to generate a random macro‐texture (MT) with different depths using the plasma of mixed gas of Ar and C₂H₄. Third, the MT substrate is deposited with a silver film (Ag). We further demonstrate that by varying the thickness of Ag layer, the EF is different which is confirmed by the plasmonic localized electric fields calculations using finite difference time domain. Finally, we combine this novel Ag MT substrate with ultrathin dielectric film, and the prepared substrates are coated with a 10 Å ta‐C film. The 10 Å ta‐C film can protect the oxygen‐free Ag in air and prevent Ag ionizing in aqueous solutions. More importantly, the ultrathin ta‐C can release the strongest plasmonic electric field to the outside of ta‐C layer and get a higher electric field than the uncoated Ag substrate. We expect that this method has more potential applications in analytic assays using SERS technology. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of the KNO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system by differential scanning calorimetry

The structural and the thermodynamic properties of potassium nitrate KNO₃ and its composites with nanosized aluminum oxide Al₂O₃ have been studied by differential scanning calorimetry. It has been found that an amorphous phase forms in composites (1–x)KNO_3–xAl₂O₃. The thermal effect corresponding to this phase has been observed at 316°C. It has been found that the phase transition heats of potassium nitrate decreased as the aluminum oxide fraction increased.     

Simulation of KrF laser ablation of Al2O3-TiC

Previous work by the authors on micromachining of Al₂O₃-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al₂O₃-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al₂O₃, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al₂O₃ rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al₂O₃ surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm.     

Comparative effects of macro-sized aluminum oxide and aluminum oxide nanoparticles on erythrocyte hemolysis: influence of cell source,temperature, and size

Al₂O₃ is the most abundantly produced nanomaterial and has been used in diverse fields, including the medical, military, and industrial sectors. As there are concerns about the health effects of nanoparticles, it is important to understand how they interact with cells, and specifically with red blood cells. The hemolysis induced by three commercial nano-sized aluminum oxide particles (nanopowder 13 nm, nanopowder <50 nm, and nanowire 2–6 × 200–400 nm) was compared to aluminum oxide and has been studied on erythrocytes from humans, rats, and rabbits, in order to elucidate the mechanism of action and the influence of size and shape on hemolytic behavior. The concentrations inducing 50 % hemolysis (HC₅₀) were calculated for each compound studied. The most hemolytic aluminum oxide particles were of nanopowder 13, followed by nanowire and nanopowder 50. The addition of albumin to PBS induced a protective effect on hemolysis in all the nano-forms of Al₂O₃, but not on Al₂O₃. The drop in HC₅₀ correlated to a decrease in nanomaterial size, which was induced by a reduction of aggregation. Aluminum oxide nanoparticles are less hemolytic than other oxide nanoparticles and behave differently depending on the size and shape of the nanoparticles. The hemolytic behavior of aluminum oxide nanoparticles differs from that of aluminum oxide.     

The optimized preparation and electrochemical properties of LiMn1.95Co0.05O4 and Al2O3-coated LiMn1.95Co0.05O4

Cathode material LiMn_1.95Co_0.05O₄ for lithium ion battery was synthesized via solid state reaction, and calcination temperature and time were investigated, respectively. Thermogravimetry (TG) and differential thermal analysis (DTA) measurements were utilized to determine the calcination temperature of precursor sample. The optimized calcination temperature and time are 850 °C and 15 h. The surface of LiMn_1.95Co_0.05O₄ cathode is coated using Al₂O₃ coating materials. The phase structures, surface morphologies, and element types of the prepared LiMn_1.95Co_0.05O₄ and Al₂O₃-coated LiMn_1.95 Co_0.05O₄ were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy spectrum analysis (EDS). The 0.5 wt% Al₂O₃-coated compound exhibited better specific capacity and capacity retention than bare sample. The initial discharge capacity was 140.9 mAh/g and capacity retention was 96.7 % after 10 cycles at 0.1 C. Such enhancements are attributed to the presence of a stable Al₂O₃ layer which acts as the interfacial stabilizer on the surface of LiMn_1.95Co_0.05O₄.     

Active metal brazing of Al2O3 to Kovar® (Fe–29Ni–17Co wt.%) using Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%)

The application of an active braze alloy (ABA) known as Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%) to join Al₂O₃ to Kovar® (Fe–29Ni–17Co wt.%) has been investigated. This ABA was selected to increase the operating temperature of the joint beyond the capabilities of typically used ABAs such as Ag–Cu–Ti-based alloys. Silica present as a secondary phase in the Al₂O₃ at a level of ~5 wt.% enabled the ceramic component to bond to the ABA chemically by forming a layer of Si₃Ti₅ at the ABA/Al₂O₃ interface. Appropriate brazing conditions to preserve a near-continuous Si₃Ti₅ layer on the Al₂O₃ and a continuous Fe₃Si layer on the Kovar® were found to be a brazing time of ≤15 min at 1025 °C or ≤2 min at 1050 °C. These conditions produced joints that did not break on handling and could be prepared easily for microscopy. Brazing for longer periods of time, up to 45 min, at these temperatures broke down the Si₃Ti₅ layer on the Al₂O₃, while brazing at ≥1075 °C for 2–45 min broke down the Fe₃Si layer on the Kovar® significantly. Further complications of brazing at ≥1075 °C included leakage of the ABA out of the joint and the formation of a new brittle silicide, Ni₁₆Si₇Ti₆, at the ABA/Al₂O₃ interface. This investigation demonstrates that it is not straightforward to join Al₂O₃ to Kovar® using Copper ABA®, partly because the ranges of suitable values for the brazing temperature and time are quite limited. Other approaches to increase the operating temperature of the joint are discussed.     

Green luminescence and EPR studies on Mn-activated yttrium aluminum garnet phosphor

Yttrium aluminum garnet (Y₃Al₅O₁₂) and Mn activated Y₃Al₅O₁₂ phosphors have been prepared by urea combustion route in less than 5 min. The phosphors are well characterized by powder X-ray diffraction, Scanning electron microscopy and Fourier-transform infrared spectroscopic techniques. Photoluminescence tests on the pure Y₃Al₅O₁₂ showed a strong green emission at 525 nm (2.36 eV) attributed to the strongly allowed transition of F⁺ center whereas in Mn²⁺ activated YAG the green emission at 519 nm is due to the ⁴T₁ (G)→⁶A₁ (S) transition of Mn²⁺ ions. EPR studies have been carried out on Mn²⁺ activated Y₃Al₅O₁₂ phosphor at 300 and 110 K. From EPR spectra the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting (A) indicates that the Mn²⁺ ions are in a moderately ionic environment. The spin concentration (N) and paramagnetic susceptibility (χ) have been evaluated and discussed.     

The interaction mechanism of mechanical activated energetic composites

Herein, in order to study the interaction mechanism of mechanical activated energetic composites, molecular dynamics simulation was carried out to calculate the binding energy and the interaction mode of PTFE with different crystalline surfaces of Al and Al₂O₃. Then the mechanical activated energetic composites were prepared, the microscopic morphology and the surface element content of Al-PTFE composites before and after mechanical activation have been studied. At last, the reactivity was tested. The results show that the adsorption of PTFE on the (0 0 1), (0 1 0) and (1 0 0) surface of Al₂O₃ and Al is stable. PTFE interacts with Al₂O₃ mainly via electrostatic force, and PTFE interacts with Al mainly via Van der Waals’ force. After 40 min ball milling, Al and PTFE melt into a composite aggregate, the adsorption of PTFE on the surface of Al and Al₂O₃ is stable, the experimental results are in good agreement with the simulation results. Mechanical activation can remarkably reduce the diffusion distance, improve the reaction activity and the detonation velocity of Al-PTFE composites.     

The transport and thermal properties of glassy LiPO<Subscript>3</Subscript>/crystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript> (ZrO<Subscript>2</Subscript>) composite electrolytes

Glassy LiPO₃/crystalline Al₂O₃ and glassy LiPO₃/crystalline ZrO₂ (0–12.5 vol.% of oxide fillers) composite solid electrolytes have been prepared by glass matrix softening. Their thermal and transport properties have been investigated by differential scanning calorimetry (DSC) and impedance spectroscopy methods. The addition of ZrO₂ leads to a decrease in the crystallization temperature of LiPO₃ glass. It was found that the conductivity behavior depends on the nature of the dispersed addition. In the case of the Al₂O₃ addition, the increase in the electrical conductivity is observed. The ionic conductivity of the LiPO₃/10% Al₂O₃ composite reaches 5.8 × 10^?8 S/cm at room temperature. In contrast, the conductivity in the LiPO₃/ZrO₂ composite system decreases.     

Structural and Tribological Properties of Polytetrafluoroethylene Composites Filled with Glass Fiber and Al2O3 in Atomic Oxygen Environment

Effects of atomic oxygen (AO) irradiation on the structural and tribological behaviors of polytetrafluoroethylene (PTFE) composites filled with both glass fibers and Al₂O₃ were investigated in a ground-based simulation facility, in which the average energy of AO was about 5 eV and the flux was 5.0 × 10¹⁵/cm² s. It was found that AO irradiation first induced the degradation of PTFE molecular chains on the sample surface, and then resulted in a change of surface morphology. The addition of Al₂O₃ filler significantly increased the AO resistance property of PTFE composites. Friction and wear tests indicated that AO irradiation affected the wear rate and increased the friction coefficient of specimens. The PTFE composite containing 10% Al₂O₃ exhibit the best AO resistance and lower wear rate after long time AO irradiation.     

Swift heavy ion induced photoluminescence studies in Aluminum oxide

Aluminum oxide, a promising material for high temperature applications, is synthesized by combustion route and characterized by X-ray diffraction technique. Photoluminescence of aluminum oxide bombarded with 120 MeV swift Au⁹⁺ ions have been studied at room temperature. The observed PL emission with peak at ～420 nm is attributed to F-center while the two more weak emissions with peaks at ～482 and ～525 nm are attributed to aggregates of F-centers. It is found that PL intensity increases with increase in Au⁹⁺ ion fluence up to ～1×10¹³ ions cm^?2 and thereafter it reaches saturation. The Fourier transform Infrared spectroscopy results show the destruction of Al?O?H bonds whereas the XRD results indicate the surface amorphization of Al₂O₃.     

Characterization of the globular oxide inclusion ratings in steel using laser-induced breakdown spectroscopy

Grade assessment of steel is generally performed via the metallographic method, which is timeconsuming and is not able to provide the elemental distribution information. In this paper, we present a method to measure the globular oxide inclusion ratings in steel using laser-induced breakdown spectroscopy (LIBS). The measurement is performed in two basic steps: steel samples are polished using metallographic sand paper and the Al₂O₃ inclusion number and size distribution in a marked area are observed using scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDS) for further LIBS scanning analysis. The threshold intensity that distinguishes soluble aluminum and insoluble aluminum inclusions is determined using LIBS combined with the SEM/EDS statistical data. Carbon steel (the sample number is S9256) and bearing steel (the sample number is GCr15) are analyzed in scanning mode, and the number of Al₂O₃ inclusions in different size ranges is obtained from the statistical information derived from the Al₂O₃ size calibration curve. According to heavy and thin series for globular oxide inclusions grade assessment, the method we propose is comparable to the traditional metallographic method in terms of accuracy; however, the process is simplified and the measurement speed is significantly improved.     

Photoactivity passivation of TiO<Subscript>2</Subscript> nanoparticles using molecular layer deposited (MLD) polymer films

Pigment-grade anatase TiO₂ particles (160 nm) were passivated using ultra-thin insulating films deposited by molecular layer deposition (MLD). Trimethylaluminum (TMA) and ethylene glycol (E.G) were used as aluminum alkoxide (alucone) precursors in the temperature range of 100–160 °C. The growth rate varied from 0.5 nm/cycle at 100 °C to 0.35 nm/cycle at 160 °C. Methylene blue oxidation tests indicated that the photoactivity of pigment-grade TiO₂ particles was quenched after 20 cycles of alucone MLD film, which was comparable to 70 cycles of Al₂O₃ film deposited by atomic layer deposition (ALD). Alucone films would decompose in the presence of water at room temperature and would form a more stable composite containing aluminum, which decreased the passivation effect on the photoactivity of TiO₂ particles.     

Ti和蓝宝石的界面反应

在超高真空中用电子束蒸发在抛光的(1102)取向的蓝宝石(α-Al₂O₃)衬底上蒸镀500nm的Ti膜,在恒温炉中退火,然后用XRD(包括一般的和小角度的X射线衍射),AES(俄歇电子谱,包括深度剖面分布和通过界面的谱形分析)和SIMS(二次离子质谱)等表面分析技术详细研究了从室温至850℃,Ti与α-Al₂O₃的固相界面反应.结果表明室温及300℃,30min退火已有反应,Al₂O_{3< 关键词：}     

Phase composition and the structure of (1–<Emphasis Type="Italic">x</Emphasis>)KNO<Subscript>3</Subscript> + <Emphasis Type="Italic">x</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposites by X-ray diffraction

The phase composition and the structural properties of potassium nitrate KNO₃ and its heterogeneous composites with nanometer-sized powder of aluminum oxide Al₂O₃ have been studied by X-ray diffraction at various concentrations of an Al₂O₃ nanopowder. It is found that, in the (1–x)KNO₃ + xAl₂O₃ nanocomposites, additional high-temperature rhombohedral phase of potassium nitrate (phase III) with lattice parameters a = 5.4644 Å and c = 9.0842 Å. With increasing concentration of Al₂O₃ nanopowder, the content of the main potassium nitrate phase (phase II) is found to significantly decrease, and the relative fraction of the phase III in the total content of the nitrate in the composite composition increases. This phase is assumed to be “frozen” in the nanocomposite at the KNO₃–Al₂O₃ interface. The estimated size of KNO₃ crystallites in the phase III is more than 20 nm.     

Laser-induced etching of polycrystalline Al2O3TiC in KOH aqueous solution

Laser-induced etching of polycrystalline Al₂–O₃TiC material by a tightly-focused cw Ar ion laser has been investigated in a KOH solution with different concentrations. It is found that the KOH concentration can strongly affect the etching quality where low KOH concentration can result in rough and irregular patterns. Laser-induced etching of polycrystalline Al₂O₃TiC in a KOH solution is found to be a photothermal reaction in which a threshold laser power exists. With an appropriate set of etching parameters, well-defined grooves can be obtained with clean side walls and with an etching rate up to several hundred micrometers per second. The etching behavior is also found to depend on laser scanning direction. It is also found that the grains in the polycrystalline Al₂O₃TiC material play an important role in the etching dynamics and etching quality. This etching process is believed to be applicable to the formation of a slider surface of magnetic heads in the future.