Phase and microstructural evolution of Ir–Si binary alloys with fcc/silicide structure

To search hardening approach or new probable phases benefiting to high temperature behavior of Ir-based superalloys, Ir alloyed with Si was employed. Investigations on phase and microstructural evolution of a series of Ir–xSi (x=2.5, 5, 15, 20, 30, 36 and 45 mol%) binary alloys were carried out by XRD, EPMA and SEM analysis. A schematic plot of the Ir–Si binary diagram with the nominal Si content ranging from 0 to 50 mol% was primarily drafted. Room temperature mechanical properties, the Vicker hardness and Young's modulus, of bulk material or each kind of phases were also measured. Researches reveal that with Si addition up to 50 mol%, the microstructures are respectively composed of primary Ir solid solution fcc+peritectic Ir₃Si silicide (nominal Si content: 0–25 mol%), primary Ir₃Si+eutectoid silicide (Si: 25–33.3 mol%), Ir₃Si₂+eutectoid silicide (Si: 33.3–40 mol%) and primary IrSi+Ir₃Si₂ silicide (Si: 40–50 mol%). With plastic characteristic, the fcc phase has the low Vickers hardness and Young's modulus, while both of the silicides are high and the silicides behave brittle. For the high temperature applications over 1400 °C, Ir-based alloys with Si dropping must avoid the appearance of any kind of Ir/Si silicides in microstructure because the melting points of silicides (Ir₃Si, Ir₂Si and Ir₃Si₂) are close to 1400 °C; instead, solid solution hardening on Ir by Si is recommended.     

Phase equilibria and mechanical properties of the Ir–W–Al system

Phase equilibria in the Ir–W, Ir–Al and Ir–W–Al systems at temperatures between 1100 °C and 1600 °C were experimentally investigated using diffusion couples and two- or three-phase alloys, and the mechanical properties of γ′ (L1₂) strengthened Ir–W–Al alloys were examined by hardness and compression tests at room and elevated temperatures. The phase boundaries between the γ(A1)/ε′(D0₁₉), ε′/ε(A3) and ε/ε″(B19) in the Ir–W system at 1400 °C–1600 °C and those between the γ/β(B2) and β/Al_2.7Ir in the Ir–Al system at 1100 °C–1400 °C were determined. The phase diagrams in the Ir-rich corner of the Ir–W–Al ternary system at 1300 °C and 1400 °C were also determined. The existence of the γ′ phase of the Ir₃(W,Al) ternary compound was confirmed, and this system was found to consist of the γ, γ′, ε, ε′ and β phases in the Ir-rich portion. It was also found from hardness and compression tests up to 1200 °C that Ir–Al–W alloys having the γ + γ′ structure with a small lattice misfit show high hardness and strength at room and high temperatures.     

Fabrication of Y2Si2O7 coating and its oxidation protection for C/SiC composites

Yttrium silicate (Y₂Si₂O₇) coating was fabricated on C/SiC composites through dip-coating with silicone resin + Y₂O₃ powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y₂Si₂O₇ coating were in–estigated. Y₂Si₂O₇ can be synthesized by the pyrolysis of Y₂O₃ powder filled silicone resin at mass ratio of 54.2:45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and fa–orable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%–140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y–Si–Al–O glassy phase between Y₂Si₂O₇ and Al₂O₃ sample bracket at 1500 °C is disco–ered.     

多壁碳纳米管表面涂层的形成及其抗氧化性

研究以单质硅粉(Si)、金属铝粉和硅粉的混合物(Al+Si)作为2种不同硅源时,在埋碳床中经1000～1500℃处理后多壁碳纳米管(multi-walled carbon nanotubes,MWCNTs)表面涂层的形成及其抗氧化性能的变化。采用X射线衍射仪、高分辨透射电镜及能谱仪和热重-差示扫描量热仪对处理前后的样品进行表征。结果表明:以Si为硅源时,MWCNTs经小于1400℃处理后表面生成了SiC涂层,1500℃处理后大部分MWCNTs蚀变成实心的SiC纳米线;与Si相比,以Al+Si为硅源时相同温度处理后MWCNTs表面涂层的厚度增加,且1500℃处理后MWCNTs并未发生结构蚀变。与原MWCNTs相比,处理后的MWCNTs抗氧化性能提高,且随着涂层厚度的增加而不断提高。非等温动力学进一步表明,原MWCNTs的氧化活化能为157.69kJ/mol,以Si为硅源时1400℃处理后为202.39kJ/mol,而Al+Si为硅源时达到230.70kJ/mol。     

An investigation on the in situ synthesis–sintering and mechanical properties of MoSi2–xSiC composites prepared by spark plasma sintering

Composites of MoSi₂–x wt.% SiC (x = 5, 10, 15, 20) prepared using spark plasma sintering. The effect of temperature on the in-situ synthesis–sintering was investigated between 1100 °C and 1500 °C. X-ray diffraction patterns showed that at 1100 °C the reactions were incomplete and elementary diffraction peaks of Mo, Si and C still exist. With an increase in temperature from 1100 to 1300 °C the reactions were performed completely. The study showed that the sintering ability at higher temperature at the presence of enough mechanical pressure was better because the heat released from the reaction between Mo, Si and C causes higher temperature than the melting point of Si (1410 °C). Consequently the silicon would melt during the heating process. The molten Si can strengthen the interconnections and it has higher diffusion rate. Therefore, due to the liquid phase sintering and at the presence of mechanical pressure, the sintering ability at higher temperature is better than lower temperature. Scanning electron microscopy showed that with the addition of carbon, there was no silica phase in the microstructure of the synthesized samples, due to the formation of SiC. Therefore, it can be noted that the addition of carbon leads to better mechanical properties due to elimination of silica phase.     

Anti-oxidation mechanism of SiC-B4C-C composites

Mixture of green petroleum coke, B4 C and SiC together with short carbon fiber were employed as starting materials, the mixture was press formed without any binder after grinding, dense and homogeneous binderless SiC-B4 C-C（carbon/ceramic） composites were then obtained after sintering. Composites thus prepared possess excellent anti-oxidation property, that is, mass loss less than IG within the temperature range from 900 to 1 100 ℃ for 10 h. Anti-oxidation mechanism was also discussed from the viewpoint of thermodynamics, excellent anti-oxidation property of materials thus prepared can be discribed to 1） solid SiO2 formed from SiC, which restrains the filtering of oxygen and simultaneously, its volume expansion brought about by the reaction takes roles both walling up the original pores and making the material more compact; 2） liquid B2O3 from the reaction of B4C not only makes the combination with C, B4 C and SiC tighter through forming solid solution, but also effect of reaction SiC（s）＋2CO（g） =SiO2 （s）＋3C（s） is an expansive process, which improves the microstructure of the material.     

机械诱发自蔓延反应合成Ti3SiC2的机理研究

机械合金化3Ti/Si/2C粉体,会诱发自蔓延反应,产生组成相为TiC、Ti3SiC2、TiSi2和Ti5Si3的粉体与块体产物.获得的粉体和块体产物中Ti3SiC2含量分别约为17.6%和58.2%(质量分数,下同).本研究提出了一个机械诱发自蔓延反应合成Ti3SiC2的反应机制,即Ti3SiC2是从固相TiC与Ti-Si液相中形核并长大.最后讨论了机械诱发自蔓延反应与自蔓延高温烧结对合成产物中Ti3SiC2含量及显微形貌的影响.     

氧化钇坩埚对镍三铝基高温合金返回料纯净化熔炼的影响

本文研究了Y2O3坩埚对N3Al基高温合金返回料纯净化熔炼的影响,并对使用Y2O3坩埚和MgO坩埚进行真空感应熔炼的结果进行了对比。结果表明：使用Y2O3坩埚对N3Al基高温合金IC21返回料进行纯净化熔炼有利于降低返回料中的H、N和O含量。和使用MgO坩埚相比分别降低了50%,80%,和80%。最低可以达到0.5 wppm,1 wppm 和2 wppm的数值。当精炼温度高于1550?C或精炼时间超过5分钟时,精炼参数对纯净化熔炼的效果影响很小。     

Properties of iridium-inserted nickel silicides by thermal annealing of the Ni/Ir bilayer on silicon and polysilicon substrates

To improve the thermal stability of the conventional nickel monosilicide, 10 nm-Ni/l nm-Ir/p-Si(100) (or poly-crystalline Si) was thermally annealed using rapid thermal annealing for 40 s at 300–1200°C. The annealed bilayer structure developed into Ni(Ir)Si, and the resulting changes in sheet resistance, microstructure, and composition were investigated using a four-point probe, a scanning electron microscope, a field ion beam, an X-ray diffractometer, and an Auger electron spectroscope. The final thickness of Ni(Ir)Si_x formed on single crystal silicon was approximately 40 nm, and it maintained a sheet resistance of below 20 ω/sq. during the silicidation annealing at 1200°C. The silicide formed on polysilicon had a thickness of 55 nm, and its low resistance was maintained up to 850°C. An additional annealing of silicides at 900°C for 30 min. resulted in a drastic increase in sheet resistance. A possible reason for the improved thermal stability of the silicides formed on single crystal silicion substrate is the role of iridium in preventing NiSi₂ transformation. Iridium also improved the thermal stability of the silicides formed on the polysilicon gate, but this enhancement was lessened due to the formation of NiIrSi_x and also as a result of silicon mixing during high temperature diffusion. In conclusion, the proposed iridium-inserted nickel silicides may be superior to the conventional nickel monosilicides due to improved thermal stability.     

Preparation and characterization of biomorphic SiC hollow fibers from wood by chemical vapor infiltration

Biomorphic SiC hollow fibers were prepared by the reactive infiltration of SiO vapor into basswood-derived charcoal. Gaseous SiO was produced from a SiO₂/Si powder mixture in Ar at elevated temperatures. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform-infrared spectroscopy were employed to characterize the structural morphology and phase compositions of the final products. The results show that the tubular cells in bulk charcoal are converted into lots of SiC hollow fibers with pore diameters of 10–50 μm and lengths ranging from hundreds of μm to several mm. Resulting SiC hollow fibers consist of β-SiC with a minute amount of α-SiC. The formation mechanism of SiC hollow fibers is based on the gas–solid reaction between SiO and carbon.     

Ir–Hf–Zr ternary refractory superalloys for ultra-high temperatures—Phase and microstructural constitution

A phase and microstructural evaluation of Ir–Hf–Zr ternary alloys with a composition below 30 mol% (Hf + Zr) was conducted by microstructural observation using scanning electron microscopy (SEM), composition analysis using electron probe microscopy analysis (EPMA), and phase identification using X-ray diffraction analysis (XRD). Partial isothermal sections of the Ir–Hf–Zr ternary system close to the Ir corner at 1800 °C and 2000 °C were determined. Research revealed that the f.c.c. and L1₂–Ir₃(Hf, Zr) two-phase regions, which are shown in the Ir–Hf and Ir–Zr binary systems, were connected from the Ir–Hf side to Ir–Zr side in the Ir–Hf–Zr ternary system at Hf + Zr contents of less than 25 mol%. The L1₂–Ir₃Hf and L1₂–Ir₃Zr phases were fully soluble with each other. An Ir₃(Hf, Zr)/Ir(Hf, Zr) two-phase structure was found in the Ir–15Hf–15Zr alloy with the Hf + Zr contents of 30 mol%. The potential of the Ir–Hf–Zr ternary alloys as ultra-high-temperature structural materials is discussed from the viewpoints of the microstructure and the lattice misfit between the f.c.c. and the L1₂ phases.     

SiC粉体中Fe元素的赋存形态及其提纯工艺

为了探究SiC块体中Fe元素的赋存形态及其粉体纯化机制,分别以冶炼后SiC块体以及经气流磨破碎后的粉体为主要原料,以HCl、HNO3、H2SO4和HF作为酸洗剂,对SiC粉体进行酸洗。通过XRD、SEM和ICP等检测手段对SiC块体和酸洗前后粉体的物相组成、微观结构及Fe元素含量进行表征,重点研究微观状态下SiC中Fe元素的赋存形态及其酸洗提纯工艺。结果表明:Fe元素以FexSiy固溶体的形式赋存于SiC块体的内部,并且微观状态下呈现出被游离Si所包裹的形态;当控制液/固比为5:1时,最佳酸洗的工艺参数为:HF浓度0.15 mol/L,浸出温度70℃,浸出时间2 h,在此工艺下Fe元素的去除率可达94.1%。     

Synthesis of SiC Powders by Carbothermal Reduction of Enriched Brown Sepiolite with Carbon Black

Ultrafine β-silicon carbide (β-SiC) powders were successfully synthesized by carbothermal-reduction reaction (CRR) of sepiolite. Sepiolite of Turkish deposits as a silica (SiO₂) precursor and carbon black as a reducing agent were mixed with constant C/SiO₂ molar ratio of 4. Mixed powders were subjected to CRR at temperatures of 1450, 1500, and 1550 °C for 1 h in an atmosphere-controlled tube furnace under argon flow of 5 cm³/min. The precursor and resultant powder products were characterized by XRD, SEM, and EDX. Phase transformation was observed in powder products after CRR as a function of the reaction temperature. The results show that the cotton-like nature of sepiolite makes it an effective mineral precursor for synthesis of SiC powders, and that SiC transformation was optimized at 1550 °C with a particle size of approximately 200 nm.     

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.     

Surface morphology evolution and ablation mechanism of SiC–Si multiphase ceramic coating on graphite under oxy-acetylene flame

A dense SiC–Si coating with average thickness of 580 μm was prepared on graphite by pack cementation. The surface morphology evolution and ablation mechanism of the coating at about 1780 °C were investigated by oxy-acetylene flame. After applying SiC–Si coating, the ablation surface temperature dropped about 300 °C. The ablation products SiO₂ presented high viscosity and provided good protection in the early-to-middle stage of the ablation. The mechanical denudation gradually became severe with the ablation time increasing. Finally, the convection between the gases formed in the subsurface and the ablative gases turned out to be the primary ablation barrier.     

纳米SiC颗粒增强反应烧结碳化硼复合材料的研究

采用真空熔渗法,通过对B4C-C素坯于1550 ℃渗Si,得到了较致密的反应结合碳化硼陶瓷复合材料。通过生成SiC纳米颗粒对材料进行强化,并探讨了纳米SiC颗粒对材料组织与性能的影响及其强韧化机理。实验表明,材料包括B4C、Si、SiC和B12(C,Si,B)3四相。结果表明,选取酚醛树脂作为外加碳源,可在材料中成功引入细小的SiC纳米颗粒,使复合材料的抗折强度、断裂韧性和维氏硬度较以炭黑为外加碳源的材料,分别增加了35 %、36 %和15 %,分别高达442 MPa、4.9 MPa?m1/2和23 GPa。     

Microstructure and oxidation behaviour of Ir-rich Ir–Al binary alloys

In the current study, alloys of Ir–11Al, Ir–23Al, Ir–30Al, Ir–41Al and Ir–45Al (at.%) were prepared to investigate the microstructure and oxidation behaviour of Ir-rich Ir–Al alloys. Ir(Al)_ss and/or β-IrAl intermetallic phases were found to exist in the prepared alloys. During isothermal oxidation at 1100 °C, the Ir(Al)_ss and β-IrAl individually changed to porous and dense Al₂O₃. The microstructure of the oxide scale formed on Ir–23Al was similar to that of its former alloy which possessed a dendrite-like configuration. It was found that the mass change of Ir–45Al followed a parabolic law, showing the best oxidation resistance among the Ir–Al alloys.     

Nano silicon top-layer for composite-induced multiphasic enhancement of thermal stability of hardness of diamond-like carbon nanofilm at 900 °C

The effect of 25-nm silicon top-layer on the hardness and thermal stability of 100-nm diamond-like carbon (DLC) film annealed at 750–900 °C has been investigated. The evolution of surface morphology, microstructure and reaction between C and Si was examined by high resolution scanning/transmission electron microscope, Raman and FTIR spectroscopy. The hardness of films was investigated using nano-indentation. After 750–900 °C annealing, the hardness of single carbon layer greatly decreased at 750 °C and then slightly increased at 900 °C due to the formation of SiC at the interface between the single C film and the Si substrate. In contrast, no significant variation occurred on the hardness of two-layer Si/C film under RTA at 750–900 °C. Although the higher annealing temperature resulted in higher sp²/sp³ bonding ratio as well as more sp² bonding formation in the carbon layer to soften the structure, the added Si top-layer can protect DLC from reaction with environmental oxygen and sustain the hardness of the composite film because of the multiphasic formation with extra SiC on the surface and at the interface between the C layer and Si substrate through great interdiffusion between Si and C for extending DLC high-temperature application.     

Influence of submicron SiC particle addition on porosity and flexural strength of porous self-bonded silicon carbide

In the present study, porous self-bonded silicon carbide (SBSC) ceramics were fabricated at temperatures ranging from 1700 °C to 1800 °C using SiC powders, silicon and carbon as starting materials. The amount of (Si + C) powders is fixed and the influence of submicron SiC particle content (0 wt.% to 80 wt.%) on the porosity and strength of the ceramics was studied. The experimental results illustrate that the packing efficiency increased with the increase in submicron particle content to 40 wt.%, and thereafter decreased. The green porosity largely determines the final porosity of the sintered specimens, irrespective of the submicron SiC particle content. The flexural strength increased with the addition of submicron particle content and sintering temperature. Typically, SBSC ceramics prepared using 80 wt.% submicron SiC particle content possess 46% porosity and 42 MPa flexural strength when sintered at 1800 °C.     

Y2O3对反应烧结Si3N4/SiC复相陶瓷组织和性能的影响

以低压铸造用升液管为研究目的,以Y2O3-Al2O3-Fe2O3为复合烧结助剂,磨切单晶硅废料Si粉和SiC为主料,反应烧结法制备Si3N4/SiC复相陶瓷。研究了Y2O3含量对复合材料结构和力学性能的影响,采用XRD、SEM对复合材料的相组成、微观形貌进行分析。结果表明,反应烧结后试样生成Si3N4结合SiC晶粒为主相的烧结体,并含有少量Sialon晶须及未反应的Si。Y2O3含量对复相陶瓷力学性能影响很大,在分析稀土Y2O3作用机理的基础上,得到2.5%Y2O3优化试样的力学性能优良,相对密度达到88%,维氏硬度达到1.1 GPa,常温抗弯强度50 MPa。