Micro-mechanical modeling the densification behavior of titanium metal matrix composites

Vacuum hot pressing has been used for the development of Ti-MMCs using foil–fiber–foil method, and a unified micro-mechanical model has been presented to determine the densification behavior. The effects of processing conditions on the consolidation, together with microstructural evolutions of the materials have been investigated. The explicit representation of fiber array, which is coupled with deformation behavior of matrix materials, is modeled in finite element simulation to determine the effect of geometrical arrangements on densification process. The approach is then used to model the densification behavior of porous plastic materials using the parameters obtained, and comparisons are made with experimental data. As shown by the results, either increasing temperature or pressure leads to increasing densification rate but the conditions should be determined by the precisely controlled geometrical arrangements with processing conditions. Further experimental investigation of the densification behavior of SiC/Ti–6Al–4V composites using thermo-acoustic emission analysis has been performed, and the results obtained are compared with the model predictions. Good comparisons are achieved.     

Sintering of tin oxide using zinc oxide as a densification aid

The physicochemical electronic characteristics of SnO₂ render it useful in many technical applications, including ceramic varistors, stable electrodes used in electric glass-melting furnaces and electrometallurgy of aluminum, transparent windows and chemical sensors. The use of ZnO as a sintering aid was explored in this study to obtain SnO₂ as a dense ceramic. Compacts were obtained by mechanical mixing of oxides, isostatic pressing at 210 MPa and sintering in situ inside a dilatometer at heating rates of 10°C/min. The grain size and microstructure were investigated by scanning and transmission electron microscopy (SEM/TEM). The phases and chemical composition were analyzed by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results indicated that ZnO acts as a densification aid for SnO₂, improving its grain growth with additions of up to 2 mol%. ZnO forms a solid solution with SnO₂ up to 1 mol%, above which SnZnO₃ precipitates in the grain boundary, potentially inhibiting shrinkage and grain growth.     

Effects of deposition temperature on microstructure of laminated (SiC-C) matrix composites

The microstructure of fiber-reinforced laminated SiC-C matrix composites has been studied using scanning and transmission electron microscopy (SEM and TEM) techniques. Different regions of the composites were found to exhibit microstructural differences due to the temperature gradient imposed during composite fabrication by the forced-flow thermal-gradient chemical vapor infiltration process. The matrix layers of alternating Carbon and SiC were found to be thicker but less clearly defined in the higher-temperature regions than in the lower-temperature regions. This feature was found to be more pronounced with increasing distance from the fiber surface. The medium temperature region was found to represent the intermediate point in the microstructural development of this composite. Possible qualitative explanations for the observed microstructural differences are suggested in the context of the temperature gradient and other process parameters employed.     

Ti_2AlC-TiB_2对TiAl基复合材料组织及力学性能的影响

以Ti、Al和B4C为原料,采用真空电弧熔炼的方法制备了含Ti_2AlC-TiB_2增强相的TiAl基复合材料;分析了添加不同含量的Ti_2AlC-TiB_2对复合材料的物相组成、组织结构及力学性能的影响,并探讨了微观组织结构的形成机制。结果表明:Ti_2AlC-TiB_2/TiAl复合材料主要由TiAl、Ti3Al、TiB_2和Ti_2AlC等物相组成,TiB_2和Ti_2AlC分布在层片状的TiAl+Ti3Al基体中;随着原料中B4C含量的增多,复合材料组织中Ti_2AlC-TiB_2含量增多,且TiAl基体的晶粒被明显细化,TiB_2和Ti_2AlC分布于基体晶界或晶内。Ti_2AlC主要为层片状和板条状,尺寸5~15μm,而TiB_2颗粒形态与其含量有关,当Ti_2AlC-TiB_2含量小于20wt%时,TiB_2颗粒呈针棒状,尺寸为0.5~5μm,当Ti_2AlC-TiB_2含量增加到30wt%时,TiB_2颗粒主要呈块状,尺寸为5~20μm。Ti_2AlC由TiC与Ti-Al熔体发生包晶反应生成,Ti_2AlC和TiB_2的形成提高了Ti_2AlC-TiB_2/TiAl复合材料的硬度、塑性和抗压强度。当4Ti+Al+B4C的加入量为10wt%时,复合材料的变形量比纯TiAl提高14%,而抗压强度达到最高值1 591 MPa。Ti_2AlC和TiB_2通过裂纹偏转、颗粒钉扎、拔出等机制对Ti_2AlC-TiB_2/TiAl复合材料起到增强增塑的作用。     

Generalised constitutive equations for densification of metal matrix coated fibre composites

Abstract

The present paper completes a study of constitutive equations for the consolidation processing of continuous fibre reinforced metal matrix composite materials. It builds on an earlier paper in which physically based constitutive equations were derived for the case of symmetrical, isostatic loading. In the present paper, constitutive equations are developed for in plane, general stress states. The total deformation of the consolidating composite is expressed as the sum of a conventional deviatoric creep term, together with a dilatational term, which was derived using a variational method previously published. The equations contain only two material parameters, which are the conventional creep coefficient and exponent for the fibre coating material (in this case, Ti-6Al-4V). The resulting equations have been implemented into finite element software enabling the simulation of practical consolidation processes. The model has been verified by comparing predicted results with those obtained from independent micromechanical models. A number of experimental tests have been carried out, and the model is used to predict the rates of densification for a range of experimental pressure and temperature histories. Good comparisons have been achieved.     

Influence of nano-reinforcements on the mechanical properties and microstructure of titanium matrix composites

The goal of this work is the evaluation of nanoscaled reinforcements; in particular nanodiamonds (NDs) and carbon nanotubes (CNTs) on properties of titanium matrix composites (TiMMCs). By using nano sized materials as reinforcement in TiMMCs, superior mechanical and physical properties can be expected. Additionally, titanium powder metallurgy (P/M) offers the possibility of changing the reinforcement content in the matrix within a very wide range. In this work, TiMMCs have been produced from titanium powder (Grade 4). The manufacturing of the composites was done by hot pressing, followed by the characterisation of the TiMMCs. The Archimedes density, hardness and oxygen content of the specimens in addition to the mechanical properties were compared and reported in this work. Moreover, XRD analysis and SEM observations revealed in situ formed titanium carbide (TiC) phase after hot pressing in TiMMCs reinforced with NDs and CNTs, at 900 °C and 1100 °C respectively. The strengthening effect of NDs was more significant since its distribution was more homogeneous in the matrix.     

Temperature dependence of the dielectric properties of metal-polymer composites based on zinc oxide nanoparticles stabilized in low-density polyethylene matrix

We have studied the temperature dependence of the dielectric constant of composites based on low-density polyethylene and zinc oxide nanoparticles. Features related to the appearance of metastable states in the course of sequential heating-cooling cycles are revealed and mechanisms explaining this behavior are proposed.     

碳源组成对化学气相渗透C/C复合材料致密化及热解炭结构的影响

为分析碳源在化学气相渗透过程中的沉积机制,以碳纤维针刺整体毡为预制体,添加丙烯（C₃H₆）的天然气混合气体为碳源,研究了碳源组成对C/C复合材料致密化及热解炭结构的影响。结果表明:相比于以天然气为碳源,以添加了适当比例C₃H₆的天然气为碳源,可有效提高C/C复合材料的致密化速率及密度分布均匀性;同时,有利于生成高织构的热解炭。最优条件（9vol% C₃H₆）下沉积100 h后,C/C复合材料的密度和径向密度偏差分别为1.40 g/cm3和0.04 g/cm3,热解炭为均一的粗糙层结构,石墨化度高;而以天然气作碳源时,密度和径向密度偏差分别为1.17 g/cm3和0.07 g/cm3,热解炭为二元带状结构,石墨化度较低;当C₃H₆比例增加到17vol%时,其密度和径向密度偏差分别为1.28 g/cm3和0.10 g/cm3,密度及密度分布均匀性较最优条件下制备的复合材料明显降低。      

烧结制度对莫来石纤维增强多孔玻璃基复合材料的结构及性能的影响

以莫来石纤维为增强体,采用固相烧结法制备了莫来石纤维增强多孔玻璃基复合材料。研究了烧结温度和保温时间对多孔玻璃复合材料的微观结构和性能的影响。采用SEM、XRD等检测手段对复合材料的物相结构及断面的微观形貌进行测试,采用阿基米德法测量其密度,使用万能材料试验机测量其抗折强度。结果表明,随着发泡温度升高、保温时间延长,多孔玻璃的孔径增大,但发泡温度过高或保温时间过长易产生连通孔;抗折强度、密度随着发泡温度的升高或保温时间的延长而降低。当发泡温度为840℃,保温时间为20min时,比强度达到最大值为0.013m2/s2。     

Electrical bistability in zinc oxide nanoparticle-polymer composites

The article reports observed electrical bistability in thin films of ZnO nanoparticles embedded in an insulating polymer matrix. From the current-voltage and impedance characteristics, they studied transport mechanisms involved in the two conducting-states. The electrical bistability in such films has been associated with a memory phenomenon. The bistability, which is reversible in nature, led to read-only and random-access memory applications in the devices based on such nanoparticles.     

Consolidation of polymer-derived SiC matrix composites: processing and microstructure

SiC fiber reinforced SiC matrix composite (SiC/SiC composite) has been developed by polymer impregnation and pyrolysis (PIP) method, which consists of impregnation, curing, consolidation, and re-impregnation and pyrolysis. As a prospective approach to fabricate a high performance composite, consolidation conditions, such as curing temperature to make a green body, pressure and heating rate during consolidation, were systematically controlled for effective consolidation. Because of its advantage in controlling physical characteristic, polyvinylsilane (PVS) that is liquid thermosetting organo-silicic compound was utilized as the matrix precursor. Based on the pyrolytic behavior of PVS, effects of the process conditions on microstructure of the consolidated bodies were accurately characterized. To relate those microstructure with mechanical property, flexural tests were performed for the composites after multiple PIP processing. Consequently, process conditions to make a high performance composite could be appeared. Structural conditions to be optimized for further improvement in mechanical and environmental properties were also discussed.     

Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide

Dense SiC ceramics were obtained by hot pressing of -SiC powders using Al₂O₃-Y₂O₃ and La₂O₃-Y₂O₃ additive systems. The effect of the addition of an amount of ultrafine SiC to commercial silicon carbide powder was evaluated. Sintering behaviour and microstructure depended on type and amount of liquid phase, as densification proceeded via a classical solution-reprecipitation mechanism. A core/rim structure of SiC grains indicated that reprecipitation of a solid solution of SiC containing Al and O occurred on pure SiC nuclei. Grain boundary phase was constituted of crystalline YAG and amorphous silicates. Values of flexural strength up to 750 MPa at RT and up to 550 MPa at 1000 °C were measured. At 1300 °C a strong degradation of strength was attributed to softening of the amorphous portion of grain boundary phase. In highly dense materials toughness ranged from 2.95 to 3.17 MPa·m^1/2 and hardness from 21 to 23 GPa.     

Influence of praseodymium oxide/cobalt oxide ratio on microstructure and electrical properties of zinc oxide varistor ceramics

The microstructure and electrical properties of varistor ceramics, which are composed of (99.5–x–y)ZnO+xPr₆O₁₁+yCoO+0.5Dy₂O₃ system, were investigated with Pr₆O₁₁/CoO mole ratio (x/y=0.5/0.5, 0.5/1.0, 1.0/0.5, 1.0/1.0) and sintering temperature. The density of varistor ceramics with Pr₆O₁₁=1.0 was almost constant with sintering temperature, whereas it was increased noticeably in Pr₆O₁₁=0.5. Increasing Pr₆O₁₁ content enhanced the densification for any CoO content and the density was greatly affected not by CoO content but by Pr₆O₁₁ content. The varistor ceramics with Pr₆O₁₁/CoO=0.5/1.0 exhibited a higher nonlinearity than any other composition ratios. In particular, the varistor ceramics sintered at 1350 °C. exhibited the best electrical properties, with nonlinear exponent of 37.8, leakage current of 7.6 μA, and dissipation factor of 0.059. It was found that Pr₆O₁₁/CoO ratio greatly affects various characteristics of varistor ceramics.     

Effects of matrix characteristics on diamond composites

A series of cobalt-matrix diamond composites was fabricated by hot pressing, and their microstructure, physical properties, transverse rupture strength and resulting fracture surface were studied in detail. Segments of the diamond composites were manufactured, and a one-segment circular sawblade was used for the evaluation of the sawing performance. Results show that the fracture surface of composites containing a cobalt matrix exhibits an excellent ductile appearance, while the fracture surface of composites containing an additive of tin powder in the cobalt matrix displays a less ductile behaviour due to the existence of a tin-rich brittle phase. It is also found that a diamond composite having low porosity, high hardness, and less surface attack of diamond particles will result in a low value of radial sawblade wear.     

氧化石墨烯增强水泥基复合材料的力学性能及微观结构

本文采用改进的Hummers法制备了氧化石墨烯（Graphene oxide,GO）悬浮液,通过FTIR、XRD和AFM等测试技术对GO晶体结构和尺寸形态进行了表征,考察了GO掺量和水灰比的变化对GO增强水泥基复合材料力学性能和微观结构的影响。结果表明：GO增强水泥基复合材料抗折抗压强度随GO掺量增加而先提高后降低,且对于抗折强度增强效果远超过抗压强度,当GO掺量为0.03%时,抗折强度达到最大值13.72 MPa;高水灰比条件下掺入GO对水泥胶砂强度的提高更显著;通过SEM对GO增强水泥基复合材料微观结构进行表征,发现GO能够优化水泥水化产物的微观结构形态,细化晶体尺寸,形成更加致密均匀的网络结构,从而改善水泥基复合材料的宏观性能。     

Thermal expansion of composites with shape memory alloy inclusions and elastic matrix

Incorporation of shape memory alloy (SMA) inclusions into a continue matrix can make composites with various thermal dilatation behavior, and this depends sensitively on the microstructural parameters of the composite. A micro-mechanical method is proposed to relate quantitatively the overall thermal dilatation with the microstructures and the transformation characteristics of SMA materials. Composites with aligned and with two populations of perpendicularly oriented SMA inclusions are analyzed in detail in this paper. It is found that the composite with SMA fibers can have a large transformation temperature range during the heating process, and a linear shrinkage may take place during the cooling because of the large difference between the austenite finish and martensite start temperatures of the composite. Design aspects to minimize the overall thermal dilatation during a full thermal cycle are also discussed.     

Influence of the microstructure on the deformation behaviour of metal–matrix composites

In this contribution, investigations of the influence of the microstructure on the macro-, meso- and micro-effects in metal–matrix composites are presented by direct combination of experiment and simulation. The aim of the presented work consists in an improved understanding of the mechanical behaviour of heterogeneous materials by combining information of different length scales.     

Influence of microstructure and strain rate on the compressive strength of whisker-reinforced ceramic matrix composites

The compressive strength of pyroceramic reinforced with a wide variety of SiC whiskers was characterized at loading rates which range from quasi-static to dynamic. It was found that strength is inversely related to whisker size, and essentially strain rate insensitive. The same strain rate independence was obtained for unreinforced matrix, but the strength of the latter lies below that for small ( 1 m diameter) whisker-reinforced composites, and above that for large ( > 3 m diameter) whisker material. Whisker/crack interaction and (to a lesser extent) whisker pull-out seem to be responsible for the beneficial influence of small whiskers, while the apparently detrimental large whiskers serve as microcrack-nucleating inclusions.