Thermal properties of Ti-SiC- and Ti-TiB2-reinforced composites

Thermal conductivity data are presented for titanium-based composites reinforced with 10 and 20%, by volume, of SiC and TiB₂ particles and for a composite reinforced by 10%, by volume, of SiC short fibers. In each case the thermal conductivity of titanium would be expected to be enhanced by the presence of the reinforcement. Measurements were made on the composites both as fabricated and after heat treatment. The results clearly show that the thermal conductivity of the Ti-SiC is much lower than predicted and decreases still further after heat treatment. This is attributed to the production of an interfacial contact resistance as a result of crack generation in the reaction products. A smaller effect is noted in the Ti-TiB₂ composites.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

The effect of the interface on the thermal conductivity of titanium-based composites

Thermal conductivity measurements have been made on composites of unalloyed Ti reinforced with short fibre SiC, particulate SiC and particulate TiB₂, and Ti-6Al-4V reinforced with long fibre SiC. Measurements were also made on the unreinforced matrices and on the reinforcements themselves. The results have been compared with predictions from the Hasselman and Johnson model for spherical and long fibre composites and from the Eshelby model for ellipsoids of any aspect ratio. It is shown that, while an enhancement of the thermal conductivity is expected to arise from the presence of both types of particulate, this effect is in fact produced only with the TiB₂ reinforcement. This is explained in terms of the large thermal resistance of the SiC/Ti interface, which is in turn related to the nature of the reaction layer formed there during processing. The transverse conductivity of the Ti-6Al-4V reinforced with long SiC fibres was also impaired by the high thermal resistance of the interface, but in this case the effect was less significant because of the larger diameter of the reinforcement, which reduces the frequency with which transport of heat across an interface must occur. Finally, similar characteristics were exhibited by the short SiC fibre/Ti composite, except that the axial conductivity was rather lower than expected. This effect is attributed to matrix porosity localized at the fibre ends.     

Interface microstructures in Ti-based composites using TiB2/C-coated and uncoated SiCf after short-term thermal exposure

The extent of interfacial reaction after short-term thermal exposure during vacuum plasma spraying (vps) and vacuum hot-pressing (vhp) of Ti-based metal-matrix composites (mmcs) using TiB₂/C-coated and uncoated SiC fibres has been investigated by a combination of scanning and transmission electron microscopies. There is no interfacial reaction after short-term thermal exposure during vps manufacture of SiC_f/Ti mmcs using either TiB₂/C-coated or uncoated SiC_f. There is only limited interfacial reaction after short-term thermal exposure during vhp manufacture of SiC_f/Ti-6Al-4V mmcs using TiB₂/C-coated SiC_f. In the initial stage of the interfacial reaction, TiB needles are formed by preferential nucleation and growth at β particles and grain boundaries in the Ti-6Al-4V matrix.     

Explosive shock-compression processing of titanium aluminide/titanium diboride composites

Explosive shock-compression processing is used to fabricate Ti₃Al and TiAl composites reinforced with TiB₂. The reinforcement ceramic phase is either added as TiB₂ particulates or as an elemental mixture of Ti + B or both TiB₂ + Ti + B. In the case of fine TiB₂ particulates added to TiAl and Ti₃Al powders, the shock energy is localized at the fine particles, which undergo extensive plastic deformation thereby assisting in bonding the coarse aluminide powders. With the addition of elemental titanium and boron powder mixtures, the passage of the shock wave triggers an exothermic combustion reaction between titanium and boron. The resulting ceramic-based reaction product provides a chemically compatible binder phase, and the heat generated assists in the consolidation process. In these composites the reinforcement phase has a microhardness value significantly greater than that of the intermetallic matrix. Furthermore, no obvious interface reaction is observed between the intermetallic matrix and the ceramic reinforcement.     

Titanium Particulate Metal Matrix Composites – Reinforcement,Production Methods,and Mechanical Properties

Titanium metal matrix composites (MMCs) offer potential advantages for structural applications, where they combine the high strength, high temperature capability, and oxidation resistance of titanium with an increase in stiffness provided by the ceramic reinforcement. They have the advantage of being isotropic in behaviour, cheaper to manufacture and more amenable to subsequent processing and component forming operations. Of potential reinforcing phases for titanium, including TiB, TiB₂, SiC, Al₂O₃, and TiC, TiB offers the best balance of stiffness, stability, and similarity of thermal expansion coefficients. The methods used to produce these Ti–TiB MMCs, such as arc melting, gas atomization, rapid solidification, and powder blending have been assessed and the benefits these composites offer over conventional titanium alloys including increased stiffness, good creep performance, fatigue resistance, and wear resistance are highlighted.     

The microstructure and mechanical properties of TiC and TiB2-reinforced cast metal matrix composites

TiC and TiB₂ particles have been spontaneously incorporated into commercial purity aluminum melts through the use of a K-Al-F-based liquid flux that removes the oxide layer from the surface of the melt. The combination of spontaneous particle entry and close crystal structure matching in the Al-TiB₂ and Al-TiC systems, results in low particle-solid interfacial energies and the generation of good spatial distributions of the reinforcing phase in the solidified composite castings. The reinforcement distribution is largely insensitive to the cooling rate of the melt and the majority of the particles are located within the grains. Modulus increases after TiC and TiB₂ particle additions are greater than those for Al₂O₃ and SiC. It is thought that interfacial bonding is enhanced in the TiC and TiB₂ systems due to wetting of the reinforcement by the liquid and particle engulfment into the solid phase. TiC-reinforced composites exhibit higher stiffnesses and ductilities than TiB₂-reinforced composites. This has been attributed to stronger interfacial bonding in the Al-TiC system, due to the increased tendency for nucleation of solid on the particle surfaces.     

高体积分数SiCp/A356复合材料的显微组织和电导率

铝基复合材料在电子封装领域存在着潜在的应用前景。为获得高体积分数的铝基复合材料,利用压力浸渗法制备了高体积分数SiC颗粒增强A356复合材料(SiC_p/A356),通过金相显微镜、XRD、SEM和EDS等分析手段对其物相、显微结构和电导率进行了表征。结果表明:用该方法制备的SiC_p/A356复合材料组织致密,颗粒分布均匀,界面结合性能较好;SiC增强颗粒与A356基体界面反应控制良好,仅有少量Al4C3脆性相生成。SiC粉体经颗粒表面氧化处理在其表面生成一层SiO_2薄膜,虽抑制了界面反应的发生,但也使复合材料的收缩减小,电阻率增大,导电性能变差。     

Effect of boron carbide particle size and volume fraction of TiB–TiC reinforcement on steady state creep behaviour of PM processed titanium matrix composites

Abstract

TiB–TiC reinforced titanium matrix composites (TMCs) were synthesised through pressureless sintering of titanium and boron carbide (B₄C) powder compacts. Effect of boron carbide (B₄C) particle size and volume fraction of TiB–TiC reinforcement on steady state compression creep behaviour of composites was investigated in the temperature range of 773–873 K. The creep rates of composites are found to be about two orders of magnitude lower than those of unreinforced titanium. The creep rates further lowered with decrease in size of B₄C particles (from 16 to 3 μm) used in preparation of composites as well as with increase in volume fraction of the TiB–TiC reinforcement from 10 to 30 vol.%. By using the concept of effective stress as well as incorporation of load transfer and substructural strengthening effect produced by the reinforcement into analysis, the entire creep data of Ti and the composites can be made to merge on to a single line within a scatter band of factor of 2–3 and can be represented by a unified power-law equation.     

Effect of coating on the microstructure and thermal conductivities of diamond-Cu composites prepared by powder metallurgy

Diamond-Cu composites from the direct combination of diamond and Cu show low thermal conductivities due to weak interface and high thermal resistance as a result of chemical incompatibility. In this paper, a new method is proposed to strengthen interfacial binding between diamond and Cu by coating strong carbide-forming elements, e.g., Ti or Cr on the surface of the diamond through vacuum micro-deposition. Interfacial thermal resistance of diamond-Cu composites is greatly decreased when diamond particles are coated by a Cr or Ti layer of a certain thickness before combining with Cu. Thermal conductivity is also increased several times. Cr coating can reduce more effectively interface thermal resistance between diamond and Cu than Ti coating. Moreover, it has a smaller negative impact on the thermal conductivity of the Cu matrix, resulting in higher thermal conductivity of Cr-coated diamond-Cu composites. Through the vacuum micro-deposition technology, Cr on the diamond particle surface is present in the form Cr₇C₃ near diamond and a pure Cr outer layer at 2:1. The optimum thickness is within 0.6-0.9 μm; at this depth, the thermal conductivities of 70 vol% diamond-Cu composites can be increased four times and reach as high as 657 W/m K. In this work, an original theoretical model is proposed to estimate the thermal conductivities of composite materials with an interlayer of a certain thickness. The predicted values from this model are in good agreement with the experimental values.     

Effect of different reinforcements on composite-strengthening in aluminium

In the development of metal-matrix composites, reinforcements of aluminium and its alloys with ceramic materials has been pursued with keen interest for quite sometime now. However, a systematic comparison of the effect of different reinforcements in powder-processed aluminium and its alloys is not freely available in the published literature. This study examines the influence of SiC, TiC, TiB₂ and B₄C on the modulus and strength of pure aluminium. B₄C appears slightly superior as a reinforcement when comparing the effect of SiC, TiC, B₄C and TiB₂ on specific modulus and specific strength values of composites. However, TiC appears to be a more effective reinforcement, yielding the best modulus and strength values among those considered in this study. The differences in thermal expansion characteristics between aluminium and the reinforcements do not seem to explain this observation. The other advantage of TiC is that it is economically a more viable candidate as compared to B₄C and TiB₂ for reinforcing aluminium alloys. It is suggested that the superior effect of TiC as a reinforcement is probably related to the high integrity of the bond at the Al-TiC interface.     

Assessment of diffusion barriers for Ti–SiC composites

Abstract

The application of chemical vapour deposition and physical vapour deposition coatings, either singly or in combination, onto SiC fibres is discussed in terms of their ability to enhance the high temperature stability of Ti–SiC composites. The thermal stability and success of potential barrier layers was assessed by studying the fibre-matrix interdiffusion and measurement of the mechanical properties of individual fibres following coating and thermal exposure. Measurements of the level of strength retention have proved to be a reliable method of assessing the effectiveness of potential diffusion barriers. Failures may result from one of three sources. For high strength fibres failures are SiC–core reaction zone initiated, for intermediate strength fibres failures are surface defect (SiC) initiated and for low strength fibres, failures are fibre–matrix reaction zone or coating initiated. To ensure high strength (i.e. core failures) it is essential that a carbon layer is retained at the SiC surface. The most successful barriers have been shown to be TiB₂ and PtAl₂ coatings preventing outward diffusion of carbon and minimising the interaction with the titanium matrix. From these results a life prediction model has been developed based on the fibre–coating interaction, which will predict fibre strength as a function of time at a given temperature.

MST/3001     

Current-activated pressure-assisted sintering (CAPAS) and nanoindentation mapping of dual matrix composites

Titanium boride (TiB_w) whiskers are currently recognized as one of the most compatible reinforcements for titanium (Ti) that have positively affected its wear resistance and stiffness. The fracture toughness and ductility have, however, been reported to deteriorate at increased TiB_w volume fractions, mainly due to the interlocking of these brittle TiB whiskers. This article investigates the processing of dual matrix Ti–TiB_w composites, where microstructures are generated consisting of TiB_w–Ti composite regions separated by a ductile, predominantly Ti, outer matrix. This microstructural design has the potential to prevent the continuous TiB_w interlocking over the scale of the composite (at high TiB_w volume fractions), and promote improved toughness of the material. The processing of these unique composites using current-activated pressure-assisted sintering (CAPAS) is discussed in this article. The effect of processing temperature on the microstructure and hardness of Ti–TiB_w dual matrix composites is also discussed, together with a simultaneous imaging and modulus-mapping nanoindentation technique used to characterize the composites     

Effect of titanium carbide coating on the microstructure and thermal conductivity of short graphite fiber/copper composites

Graphite fiber–Cu composites have drawn much attention in electronic packaging due to its excellent machinability and thermal properties. However, the weak interface bonding between graphite fiber and copper resulted in low thermo-mechanical properties of composites. In this work, a titanium carbide coating with thickness of 0.1 μm or 1 μm was synthesized on the surface of graphite fiber through molten salts method to strengthen interfacial bonding. The enhanced composites present 24–43 % increase in thermal conductivity and achieve the thermal conductivity of 330–365 W m^?1 K^?1 as well as the coefficient of thermal expansion of 6.5 × 10^?6–14 × 10^?6 K^?1. A Maxwell–Garnett effective medium approach on the anisotropic short fiber reinforcement with interfacial thermal resistance was established. The obtained enhancement was in good agreement with the estimates. The results suggest that the major factor that influences the thermal conductivities is not the interfacial thermal resistance but the low thermal conductivity of fiber in transversal direction when a well interfacial bonding is obtained.     

连续碳化硅纤维增强钛基(SiCf/Ti)复合材料的制备技术及界面特性研究综述

连续碳化硅纤维(SiCf)由于具有比强度、比模量高,耐磨性、热稳定性好等性能优点,常作为增强体制备SiC纤维增强钛基复合材料。与钛合金基体相比,其具有密度更低、强度更高、疲劳蠕变性能大幅提升等优点,但横向性能却明显下降。因此,该类材料常被设计制作成单向增强性部件,广泛应用在航空航天等领域,如发动机的传动轴、整体叶环、盘类及风扇叶片等多种复合材料的结构件。碳化硅纤维增强钛基复合材料的性能主要由碳化硅纤维的性能、基体性能及纤维与基体之间的结合界面性能决定。目前批量生产的SiC纤维性能较差,界面结合状态与复合材料性能之间关系的研究开展较少,还不能为钛基复合材料构件设计提供足够的数据支持。因此,近年来研究者们主要从SiCf/Ti基复合材料力学行为的研究角度出发,探究不同基体及纤维类型、复合材料制备工艺方法、界面特性及产物对SiCf/Ti基复合材料界面结合力及破坏机制的影响,获得了大量有价值的数据,以期开发出成本低、产物稳定性好、可批量生产SiCf/Ti基复合材料的制造工艺方法。目前较为成熟的碳化硅纤维有英国DERA-Sigma公司提供的Sigma系列SiCf及美国Textron公司提供的SCS系列SiCf,后者强度最高达到6 200 MPa。SiCf/Ti基复合材料的制备工艺包括金属箔-纤维-金属箔工艺(FFF)、单层带工艺(MT)、基体-涂层纤维工艺(MCT)等,制备复合材料的工艺根据零部件的用途来定,FFF适用于制备板材等大尺寸构件,MCT适用于制备叶环、轴、管、叶片等复杂结构件。界面是增强体与基体之间的纽带和桥梁,界面结构设计、界面反应控制及反应产物均影响着界面的力学特性。在SiCf/Ti基复合材料的纤维和基体之间添加过渡层能够减缓它们之间的相互扩散及化学反应,过渡层选用反应层和惰性涂层组成的双层涂层较好。界面反应产物受涂层成分、基体组织、复合和热处理工艺、环境因素等的影响,增强纤维及基体性能、优选制备工艺、控制界面反应及产物有利于提高复合材料的力学性能。本文总结了连续SiC纤维(SiCf)增强钛基复合材料的应用研究现状,详述了SiCf/Ti基复合材料的钛合金基体材料、SiCf的种类及性能,SiCf与SiCf/Ti基复合材料的制备方法,分析了SiCf/Ti基复合材料界面结构设计及反应产物,阐明了界面力学特性与复合材料性能的关系,指出国内SiCf/Ti基复合材料发展的重点应放在高性能SiC纤维的研究与开发、界面层设计及界面与性能的关系以及复合材料分析检测手段三个方面,为SiCf/Ti基复合材料的制备及其今后的实际应用提供了参考。     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

Evaluation of the efficiency of TiB2 and TiC as protective coatings for SiC monofilament in titanium-based composites

The vigorous interfacial reactions in SiC/Ti-6Al-4V composites at elevated temperatures lead to the deterioration of the mechanical properties of the composites. TiB₂ and TiC were selected as potential protective coatings for SiC fibres in titanium-based composites. These coatings were deposited on to fibres by the chemical vapour deposition technique. Comparisons and evaluations have been made of the effectiveness of these ceramics as protective coatings for SiC fibres by incorporating the coated fibres into a Ti-6Al-4V matrix using the diffusion bonding method. Emphasis has been placed on the chemical compatibility of the candidate coating with SiC and Ti-6Al-4V by examining the interfaces of the fibre/coating/matrix using microscopic methods and chemical analysis. A stoichiometric TiB₂ coating was found to be stable with SiC and has proved an effective barrier to prevent the SiC fibre from reacting with the Ti-6Al-4V. The TiC coating showed no apparent reaction with a titanium-alloy matrix under the conditions studied, but was found to react with the SiC fibre substrate.     

Fracture toughness assessment of silicon carbide-based ceramics and particulate-reinforced composites

The fracture toughness of sintered silicon carbide (α-SiC) and silicon carbide reinforced with particulate titanium diboride (TiB₂/SiC) has been evaluated using specimens in bending containing chevron notches and through-thickness precracks at ambient and elevated temperatures in air and in vacuum. Fracture toughness values measured from through-thickness precracked test-pieces are lower at all test temperatures. The particulate reinforcement has been shown to toughen the matrix significantly at room temperature only. At the test temperature of 1200°C the difference in toughness between the two materials is reduced and increasing the temperature to 1600°C further reduces this difference, to the extent that the two materials have values of fracture toughness which are indistinguishable. This provides strong evidence that the dominant toughening mechanism in the composite is the effect of thermal residual stresses which are relieved as the temperature is increased. Fractographic observations suggest that the bonding between the SiC and TiB₂ particulate is relatively weak because interfacial decohesion of particles is observed at all test temperatures. Nevertheless, surface roughness measurements indicate that there may also be a contribution to the toughness from increased crack deflection in the composite material at room temperature only.     

A comparative study of the coated filler method and the admixture method of powder metallurgy for making metal–matrix composites

Copper–matrix composites were made by powder metallurgy (PM). The reinforcements were molybdenum particles, silicon carbide whiskers and titanium diboride platelets. The coated filler method, which involves a reinforcement coated with the matrix metal, was used. In contrast, conventional PM uses the admixture method, which involves a mixture of matrix powder and reinforcement. For all the composite systems, the coated filler method was found to be superior to the admixture method in providing composites with lower porosity, greater hardness, higher compressive yield strength, lower coefficient of thermal expansion (CTE), higher thermal conductivity and lower electrical resistivity, though the degree of superiority was greater for high than low reinforcement contents. In the coated filler method, the coating on the reinforcement separated reinforcement units from one another and provided a cleaner interface and stronger bond between reinforcement and matrix than the admixture method could provide. The highest reinforcement content attained in dense composites (<5% porosity) made by the coated filler method was 70 vol% Mo, 60 vol% TiB2 and 54 vol% SiC. The critical reinforcement volume fraction above which the porosity of composites made by the admixture method increases abruptly is 60% Mo, 42% TiB2 and 33% SiC. This fraction increases with decreasing aspect ratio of the reinforcement. Among Cu/Mo, Cu/TiB2 and Cu/SiC at the same reinforcement volume fraction (50%), Cu/Mo gave the lowest CTE, highest thermal conductivity and lowest electrical resistivity, while Cu/SiC gave the greatest hardness and Cu/TiB2 and Cu/SiC gave the highest compressive yield strength. Compared to Cu/SiC, Cu/TiB2 exhibited much higher thermal conductivity and much lower electrical resistivity. This revised version was published online in November 2006 with corrections to the Cover Date.     

A comprehensive study on thermal conductivities of wavy carbon nanotube-reinforced cementitious nanocomposites

The thermal conductivities of cementitious nanocomposites reinforced by wavy carbon nanotubes (CNTs) are determined by the effective medium (EM) micromechanics-based method. The nanocomposite is composed of sinusoidally wavy CNTs as reinforcement and cement paste as matrix. The interfacial region between the CNTs and cementitious material is considered in the analysis. The effects of volume fraction and waviness parameters of CNTs, interfacial thermal resistance, type of CNTs placement within the matrix including aligned or randomly oriented CNTs, cement paste properties on the thermal conductivity coefficients of the nanocomposite are studied. The estimated values of the model are in very good agreement with available experimental data. Two parameters of CNT waviness and interfacial region contributions should be included in the modeling to predict realistic results for both aligned and randomly oriented CNT-reinforced nanocomposites. The results reveal that thermal conductivities K₂₂ (transverse in-plane thermal conductivity) and K₃₃ (longitudinal in-plane thermal conductivity) of the nanocomposites are remarkably dependent on the CNT waviness. Also, it is found that the CNT waviness moderately affects the thermal conductivity of a cementitious nanocomposite containing randomly oriented CNTs. However, the non-straight shape of CNTs does not influence the value of thermal conductivity K₁₁ (transverse out of plane thermal conductivity). The achieved results can be useful to guide the design of cementitious nanocomposites with optimal thermal conductivity properties.     

TEM investigations of interfacial processes in SCS-6 SiC/TiB2/Super α2 composites

Interfacial processes in SCS-6 SiC/TiB₂/Super α₂ composites were investigated by means of analytical transmission electron microscopy (TEM). No reaction between the TiB₂ coating and the carbon coating of the fibres can be found in the as-processed specimen. At the TiB₂/matrix interface the TiB₂ transforms into both a homogeneous layer and needles of TiB which are distributed in the matrix. This process takes place according to the reaction TiB₂+Ti=2TiB. The TiB₂ coating is consumed completely in the specimen heat treated at 900°C for 64 h. At this time TiC and Ti₃AlC form at the TiB/C-coating and the TiB/matrix interface, respectively. Prolonging the holding time at 900°C, reaction layers thicken continuously following parabolic reaction kinetics. The paper additionally discusses the formation mechanisms of the reaction products.