Processing, microstructure and properties of C40 Mo(Si,Al)2/Al2O3 composites

The C40 Mo(Si_0.75Al_0.25)₂/Al₂O₃ composites were prepared by spark plasma sintering (SPS) of mechanically alloyed (MA) powders. The Mo(Si_0.75Al_0.25)₂/0–20 vol.% Al₂O₃ materials, showing micron and submicron composite structure, possess a hardness of 13.9–14.6 GPa but a poor toughness of 1.78–1.80 MPa m^1/2. The addition of 30 vol.% Al₂O₃ leads to the formation of the micron C40 Mo(Si_0.75Al_0.25)₂/Al₂O₃ composite with an intergranular distribution of Al₂O₃, that results in a drop of the hardness to 10.2 GPa and an improvement of the toughness to 3.67 MPa m^1/2. The transition of the cleavage facets to the intergranular fracture with the addition of Al₂O₃ is assumed as the main toughening mechanism.     

In situ synthesis of Mo(Si,Al)2-SiC composites

An in situ reaction was proposed and investigated to produce Mo(Si_{1 – x} Al _x )₂-SiC composites. The starting powders were MoSi₂, Al and C. A direct current hot pressing (DCHP) method was used to prepare these composites. When the mixed powder was hot pressed at temperatures lower than 1500°C, the phase composition was Mo(Si,Al)₂ and -SiC. When the hot pressing temperature was higher than 1600°C, however, Nowotny phase Mo₅Si₃C₁ appeared. The chemical stoichiometry of the proposed in situ reaction becomes difficult because of the formation of solid solution among these phases and the appearance of Mo₅Si₃C phase. The in situ formed SiC phase in the x = 0.3 sample was partly in whisker shape. However, the SiC phase in x = 0.15 sample was in particle shape. These in situ formed SiC particles and whiskers acted as crack deflection and bridging elements and improved the fracture toughness. The Vickers hardness and fracture toughness of the x = 0.3 sample hot pressed at 1700°C for 60 min in vacuum were 15.6 GPa and 5.39 MPa · m^1/2, respectively.     

基于SPS法B4C/6061Al中子吸收复合材料组织及性能

B₄C中B的同位素¹⁰B具有较大的热中子吸收截面,是良好的中子吸收体。采用放电等离子烧结法（SPS）制备了B₄C体积分数为10%~40%的B₄C/6061Al中子吸收复合材料,对B₄C/6061Al中子吸收复合材料的微观组织形貌及物相组成进行了观察分析,并测试了其拉伸性能。结果表明：B₄C颗粒均匀地分布在6061Al基体中,颗粒尖端放电产生的等离子体能够促进B₄C颗粒/6061Al基体界面结合,材料内部的物相主要有Al、B₄C、AlB₂和Al₃BC。随着B₄C体积分数的增加,B₄C/6061Al中子吸收复合材料的致密度降低,抗拉强度先增加后降低,断裂机制主要为6061Al基体及B₄C颗粒/6061Al基体界面的撕裂。     

Microstructure origin of strength and toughness of 2-d C/C composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

液相渗Si工艺制备2D CF/SiC复合材料中SiC晶粒的生长机制

由液相渗硅工艺(LSI)制得了2D CF/SiC复合材料,经过XRD分析得知材料中SiC均为β相。经过同质量比的K_3Fe(CN)_6与KOH混合水溶液对其进行腐蚀,由SEM观察腐蚀后2D CF/SiC复合材料的形貌,发现其中SiC呈现细等轴晶和粗大晶粒两种不同的形貌。分析认为LSI工艺制备2DCF/SiC复合材料中生成的SiC有两种生成机制:Si原子通过空位机制向碳中扩散形成无定型SiC,在保温过程中结晶形成细晶SiC层;C原子扩散进入熔融态硅中形成C—Si基团,由于温度梯度和浓度梯度的存在,在远离C/SiC界面处过饱和析出,通过溶解-沉淀机制形成粗大的SiC晶体,在晶粒的长大过程中伴随着层错的出现。     

Microstructure and thermo-mechanical properties of SiCp/Al composites prepared by pressureless infiltration

In this paper, SiCp/Al composites with high reinforcement content are fabricated by pressureless infiltration with aluminum alloy into porous SiC preforms obtained by cold press forming. Microstructures and particulate distributions are analyzed with scanning electron microscope, X-ray diffraction and energy dispersive spectrometer. The reinforcement volume fraction reaches 65 % by using bimodal particle distributions. The bending strength ranges from 320 to 342 MPa, depending on particle sizes. Due to the intrinsically low thermal conductivity of the matrix, the thermal conductivity of SiCp/Al composites are in the range of 121–143 W m^?1 K^?1.     

Microstructure and growth mechanism of SiC whiskers on carbon/carbon composites prepared by CVD

The SiC whiskers of good quality are expected to act as the reinforcing element in the ceramic coatings for C/C composites. Using CH₃SiCl₃(MTS) and H₂ as the precursors, SiC whiskers were prepared on the surface of C/C composites by chemical vapor deposition (CVD) at normal atmosphere pressure. XRD, SEM and TEM analyses show that the whiskers are β-SiC structure and their diameters are several-hundred nanometers. With the descending MTS concentration in the depositing room, the purity of the as-prepared whiskers increases and the diameters of the whiskers decrease. The investigation of growth mechanism shows the CVD-SiC whiskers grown up by the vapor–solid (VS) growth process.     

Degradation of Al2O3–SiC–Al composites prepared by the oxidative growth of Al-alloys into SiC particulate

Composites of Al₂O₃–SiC–Al have been prepared by the oxidative infiltration of Si-free aluminium alloys into SiC particulate that were coated with spray-pyrolysed alumina. Moisture degradation studies were carried out to evaluate the extent of formation of aluminium carbide, the formation of which was also inferred by the determination of the residual silicon content in the composite. The efficiency of the coating technique in protecting the SiC from attack by the melt was evaluated at 1000 and 1200 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

SPS制备氮化硅/锌铝基复合材料的组织和性能研究

通过放电等离子烧结工艺制备了氮化硅/锌铝基复合材料,重点探讨了氮化硅添加量对氮化硅/锌铝基复合材料致密度、硬度和摩擦性能的影响.采用扫描电子显微镜(SEM)及电子探针X射线显微分析仪(EPMA)对样品的微观组织进行了分析,并使用显微硬度仪、旋转摩擦试验仪对其性能进行了研究.结果表明:氮化硅在样品中分散均匀,且氮化硅的加...     

Effects of SiC,Al2O3, and ZrO2 particles on the LBMed characteristics of Al/SiC,Al/Al2O3, and Al/ZrO2 MMCs prepared by stir casting process

The effects of SiC, Al₂O₃, and ZrO₂ particles on the characteristics of Al/SiC, Al/Al₂O₃, and Al/ZrO₂ metal matrix composites (MMCs) have been studied in the present research work. The comparison of machining characteristics has been done to analyze the behavior of various reinforced particles with the variation of laser machining variables. The output characteristics such as dross height and kerf deviation have been investigated and compared with each MMCs. SEM and XRD have been used for the investigation of morphological changes in the structure and agglomeration of reinforced particles. The crack and recast layer formation has been examined in the specimens of higher quantity of reinforced particles. It was observed that the MMC material reinforced with SiC particles has shown different behavior as compared to other MMC materials.     

无压烧结Ti3SiC2/铝基复合材料组织与性能

MAX相具有独特的层状晶体结构,不但具备常用铝基复合材料外加陶瓷颗粒的性能特征,同时具有可与石墨媲美的摩擦性能.本文以Al粉、Si粉和典型MAX相Ti_3SiC_2为原料,采用冷压成型-无压烧结方法制备了Ti_3SiC_2/Al-Si复合材料,并通过金相显微镜、X射线衍射仪(XRD)、扫描电镜(SEM)、能谱仪(EDS)等分析手段,研究了烧结温度、Si元素含量对复合材料组织与性能的影响.研究表明:随着烧结温度从500℃提高到700℃,复合材料致密度先上升后下降,摩擦系数先降低后上升,硬度逐渐增大至最大值并基本保持稳定;随着Si质量分数从0增加到20.7%,复合材料的致密度逐渐降低,硬度逐渐增大,摩擦系数先降低后增大,晶粒尺寸随之下降,12.5%Si晶粒最为细小;烧结温度为650℃,Si元素质量分数为12.5%的铝基复合材料具有最低的摩擦系数0.18,相应的硬度为62 HV,致密度为92.12%.XRD物相和扫描电镜组织分析表明,复合材料的主要相组成为Al、Ti_3SiC_2,及由界面反应产生的Al_4C_3和Al的氧化产物Al_2O_3.     

Microstructure and mechanical properties of short carbon fibre/SiC multilayer composites prepared by tape casting

Silicon carbide multilayer composites containing short carbon fibres (C_sf/SiC) were prepared by tape casting and pressureless sintering. The C fibres were dispersed in solvents with dispersant (Triton X-100) firstly and then mixed with the SiC slurry to make green C_sf/SiC tapes. Fibres were homogeneously distributed in the tape and tended to align fairly well along the tape casting direction. The addition of short C fibre hindered the shrinkage in the plane containing the fibres as well as the grain growth of SiC during sintering. The weight loss occurring during oxidation tests of C_sf/SiC multilayer composites increased with fibre amount and material porosity. Elastic modulus of C_sf/SiC multilayer composites decreased linearly with fibre amount. Bending strength presented clear relationship with the relative density, that is with the total porosity.     

Fabrication and characterization of Ti3SiC2-SiC nanocomposite by in situ reaction synthesis of TiC/Si/Al powders

The microstructure and mechanical properties of Ti₃SiC₂-SiC nanocomposite fabricated by in situ hot pressing (HP) synthesis process were studied. The results show that dense Ti₃SiC₂-SiC composite contained minor TiSi₂ obtained by hot sintering at 1350°C for 1 h. The average grain size of Ti₃SiC₂ was 4 μm in length, and the size of SiC grains is about 100 nm. With its fine microstructure, the Ti₃SiC₂-SiC nanocomposite shows good mechanical properties.     

Creep properties of (Si–Al–O–N)SiC whisker composites

(Si–Al–O–N) (sialon)–SiC whisker (SiC_w) composites containing up to 10 mass% SiC_w were prepared by hot isostatic pressing. The strengths and the fracture toughness of composites remained relatively unchanged with the amount of SiC_w. The addition of SiC_w enabled us to improve the creep properties of sialon ceramics. The total creep strain and steady-state creep rate at 1473 K under a stress of 400–500 MPa decreased with increasing the amount of SiC_w. The experimental creep exponent values of monolithic sialon and sialon–SiC_w composites were nearly 1. It is supposed that the creep of both monolithic sialon and sialon–SiC_w composites are dominated by the viscous flow of the interglanular glassy phase.     

Microstructure and mechanical behaviour of (Fe88Si12)95Al5 alloy prepared by aluminothermics

Abstract

The (Fe₈₈Si₁₂)₉₅Al₅ alloy was prepared by an aluminothermics. The (Fe₈₈Si₁₂)₉₅Al₅ alloy is composed of spheroidic α-Fe(Si,Al) precipitate with size of 20–50 nm and γ-Fe(Si,Al) matrix. The yield strength and fracture strain in compression of the (Fe₈₈Si₁₂)₉₅Al₅ alloy are 1500 MPa and 23% respectively. The shear bands propagating in the compressive deformation are arrested by the precipitation particles that resulted in large ductility and high strength simultaneously.     

具有木材结构Al/C、Al/(SiC+C)复合材料的制备及热学性能

该研究以柳桉木材结构为模板,通过对木材的碳化、硅树脂的浸渍与反应、铝合金的浸渍处理,制备了具有木材结构的Al/C、Al/(SiC C)两种复合材料,并对复合材料的微观结构、热膨胀性能及导热性能进行了研究.结果表明:该复合材料保持了所选木材的天然结构特征,并且其热膨胀系数明显低于铝合金,导热系数远高于由木材转化的多孔碳.     

Enchanced high-temperature performances of SiC/SiC composites by high densification and crystalline structure

We report the enchanced in situ performances of tensile strength and thermal conductivity at elevated temperatures of the PCS-free SiC/SiC composite with a high fiber volume fraction above 50% fabricated by NITE process for nuclear applications. The composite was fabricated by the optimized combination of the fiber coating, the matrix slurry and the pressure-sintering conditions, based on our previous composites’ study history. The composite showed the excellent tensile strength up to 1500 °C, that it retained approximately 88% of the room-temperature strength. Also, the thermal conductivity of the composites represented over 20 W/m K up to 1500 °C, which was enough high to take the advantage of the assumed design value for nuclear applications. Microstructural observation indicated that the excellent high-temperature performances regarding tensile strength and thermal conductivity up to 1500 °C were the contribution to the high densification and crystalline structure in matrix.