Size dependent strengthening mechanisms in carbon nanotube reinforced metal matrix composites

The matrix grain size plays a dual role in metal matrix composites (MMCs). Contrary to enhance the strength of matrix, grain refinement can weaken the thermal expansion mismatch strengthening induced by the reinforcement. In this article, a dislocation density based model is developed to describe the factors affecting the strengthening mechanisms in Carbon nanotube (CNT)-reinforced MMCs with different matrix grain sizes. Two kinds of thermal expansion mismatch strengthening mechanisms are considered, i.e., geometrically necessary dislocations (GNDs) are distributed in entire matrix and GNDs are limited in dislocation punched zones (DPZs). In addition, comparisons between the predictions and some available experimental results are also performed.     

纳米颗粒分布对镁基复合材料强化机制的影响

Orowan强化、 热错配强化和Hall-Petch强化是纳米颗粒增强镁基复合材料的主要强化机制, 纳米颗粒在基体中的分布状态对起主导作用的强化机制具有重要影响。本文中对现有强化机制模型进行了适当修正, 以纳米SiC颗粒增强AZ91D复合材料为例, 通过理论计算分析了纳米颗粒完全分布于晶内、 完全分布于晶界、 在晶内晶界上均有分布的三种状态对镁基复合材料屈服强度的影响, 并与实验结果进行对比。结果表明: 颗粒完全分布于晶内时, 增强效果最好, 主要增强机制为Orowan强化; 颗粒完全分布于晶界上时, 增强效果最差, 主要增强机制为Hall-Petch强化。颗粒在晶内晶界上均有分布时, 多种强化机制共同发挥作用, 增强效果随着晶内与晶界上颗粒比例的减小而逐渐减弱。     

纳米颗粒分布对镁基复合材料强化机制的影响

Orowan强化、热错配强化和Hall-Petch强化是纳米颗粒增强镁基复合材料的主要强化机制,纳米颗粒在基体中的分布状态对起主导作用的强化机制具有重要影响.本文中对现有强化机制模型进行了适当修正,以纳米SiC颗粒增强AZ91D复合材料为例,通过理论计算分析了纳米颗粒完全分布于晶内、完全分布于晶界、在晶内晶界上均有分布的三种状态对镁基复合材料屈服强度的影响,并与实验结果进行对比.结果表明:颗粒完全分布于晶内时,增强效果最好,主要增强机制为Orowan强化;颗粒完全分布于晶界上时,增强效果最差,主要增强机制为Hall-Petch强化.颗粒在晶内晶界上均有分布时,多种强化机制共同发挥作用,增强效果随着晶内与晶界上颗粒比例的减小而逐渐减弱.     

铜基复合材料的构型多功能化

金属基复合材料(MMCs)由于其优异的性能,在空天、交通、电子、能源等领域的应用中具有不可替代的作用。随着科学技术的发展,单一高性能的金属基复合材料常不能满足越来越复杂的使役条件,亟需发展综合性能优异、多功能协同的新型金属基复合材料。然而,不同性能之间由于多种增强机制耦合,其解耦调控困难,使得它们呈现倒置关系。近年来,通过调控增强体在金属基体中的空间分布,即对复合构型开展研究,充分发挥复合材料的可设计性,最终实现金属基复合材料的多性能之间的协同,已成为该领域发展的重要趋势。本文在本研究团队前期关于金属材料构型化复合的研究基础上,以纳米碳/铜基复合材料为典型范例,对我们在该类新型材料研究中通过复合构型设计实现结构-功能一体化的最新进展,即"微纳砖砌"构型对强度-塑韧性-导电性能的协同进行了总结和介绍,并对该领域的发展趋势进行了展望。     

Role of work hardening characteristics of matrix alloys in the strengthening of metal matrix composites

The strengthening of particulate reinforced metal-matrix composites is associated with a high dislocation density in the matrix due to the difference in coefficient of thermal expansion between the reinforcement and the matrix. While this is valid, the role of work hardening characteristics of the matrix alloys in strengthening of these composites is addressed in the present paper. It is found that commercial purity aluminium which has the lowest work hardening rate exhibits the highest strength increment. This effect is due to increased prismatic punching of dislocations. This relationship of decreasing work hardening rate associated with increasing prismatic punching of dislocations in the order 7075, 2014, 7010, 2024, 6061 and commercial purity aluminium leading to increased strength increments is noted.     

石墨烯负载铜增强铜基块体复合材料的制备及其性能

石墨烯具有独特的二维结构及性能已成为金属基复合材料制备过程中理想的增强相备选材料之一。而铜因具有良好的导热性、导电性和化学稳定性已被广泛应用到电子产品中,但其存在机械强度低、硬度低等缺点成为其应用亟需解决的瓶颈问题。目前,将石墨烯和铜基材料进行结合,虽然在一定程度上可以改善铜基材料的的综合性能。但由于石墨烯易产生团聚,石墨烯与铜之间的润湿性差,使其两者难以形成良好的界面结合,进而导致复合材料的性能变差。因此,为了解决上述问题,本文通过化学还原法在石墨烯上负载铜粒子对石墨烯进行改性处理,成功制备了石墨烯负载铜复合粉体(Cu-rGO),并将其作为增强相,与纳米铜粉混合,运用放电等离子烧结(SPS)工艺制备了石墨烯负载铜增强铜基块体复合材料(Cu-rGO/Cu),研究Cu-rGO复合粉体含量对铜基体组织和性能的影响。研究发现,在50 mg氧化石墨烯(GO)和200 mg硫酸铜(CuSO4·5H2O)时,获得Cu-rGO复合粉体中还原氧化石墨烯较薄且分布均匀。同时结合TEM结构分析发现铜基体与增强相接触界面紧密,且增强相的引入可以有效地细化块体复合材料的晶粒。另外,随着增强相含量的递增,硬度呈...     

石墨烯增强铜基复合材料研究进展

铜(Cu)基复合材料具有优异的力学、热学、电学及耐磨和耐腐蚀等性能,广泛应用于各种工业技术领域。石墨烯(Graphene,Gr)具有二维平面结构和优异的综合性能,是金属基复合材料理想的增强相。石墨烯增强铜基复合材料拓展了铜及其合金的应用范围,适当的制备方法可以使其在保持优异导电导热性能的同时拥有更好的力学性能。石墨烯在铜基体中的存在形式主要以还原氧化石墨烯、石墨烯纳米片或与金属氧化物/碳化物纳米颗粒连接,旨在增强两者之间的界面结合。因此,石墨烯在铜基体中的结构完整性及存在形式直接影响了其性能的优劣。本文综述了Cu/Gr复合材料的制备及模拟方法、复合材料的性能评价及力学性能与功能特性的相互影响规律。指明Cu/Gr复合材料的发展关键在于：(1)分散性与界面结合;(2)三维石墨烯结构的构建;(3)界面结合对力学性能与功能特性的影响及两者间的相互协调。     

石墨烯增强铜基复合材料研究进展

石墨烯是一种新兴的二维碳纳米材料,具有良好的力学、导电以及润滑性能,是铜基复合材料中最具潜力的增强体.本文综述了石墨烯增强铜基复合材料的制备工艺,详细分析并归纳了石墨烯增强铜基复合材料的界面结构对于复合材料力学性能的影响及增强机制,总结了石墨烯增强铜基复合材料摩擦学行为研究的最新进展,并深入阐述了石墨烯增强铜基复合材料...     

Fractographic interpretation of failure mechanisms in titanium matrix composites

Titanium matrix composites (TMC) offer a combination of good mechanical properties and high temperature durability that make them attractive candidate materials for advanced engine components and high temperature structural applications. In such applications the material will be subjected to changing mechanical loads and temperature fluctuations, resulting in complex stress states within the constituents of the composite. This study examines how the various loading conditions on the TMCs are reflected in the fracture behaviour to gain insight into the damage mechanisms active in these materials. A fractographic study was conducted on several different TMC specimens, fabricated from Ti-15-3 and Timetal 21S alloys and SCS-6 fibres, that have been subjected to various thermomechanical loading conditions. The analysis showed that the Ti-15-3 composites were more susceptible to damage during sustained load at elevated temperature than the Timetal 21S composites. For both materials, striations only appear during elevated temperature fatigue when the residual processing stresses are relieved. During the Generic Hypersonic Flight Profile (GHFP) tests, the higher temperatures of the Mission 1 profile induce more damage in the Timetal 21S composites. The striations appear in the composites containing centre holes, unlike the unnotched specimens, indicating that the combined effect of stress concentration due to the hole and an underdeveloped fibre bridging zone may have resulted in crack closure.     

Dual strengthening mechanisms induced by carbon nanotubes in roll bonded aluminum composites

The dual role of carbon nanotubes (CNTs) in strengthening roll bonded aluminum composites has been elucidated in this study. An increase in the elastic modulus by 59% has been observed at 2 vol.% CNT addition in aluminum, whereas tensile strength increases by 250% with 9.5 vol.% CNT addition. CNTs play a dual role in the strengthening mechanism in Al–CNT composite foil, which can be correlated to the degree of dispersion of CNTs in the matrix. Better CNT dispersion leads to improvement of elastic properties. In contrast, CNT clusters in the aluminum matrix impede dislocation motion, causing strain hardening and thus improvement in the tensile strength. Dislocation density of the composites has been computed as a function of CNT content to show the effect on strain hardening of the metal matrix–CNT composite.     

碳纤维增强铜基复合材料制备方法研究进展

综述了碳纤维增强铜基复合材料的主要制备方法及其发展现状,重点讨论了粉末冶金法、热压扩散烧结法、熔渗法、PVD法、CVD法及电镀法等常用制备工艺的原理及特性,分析了不同制备方法的优缺点及适用领域,提出了现有方法中存在的问题,并展望了碳纤维增强铜基复合材料的发展趋势及在输变电领域的应用前景。     

Thermal conductivity of unidirectional copper matrix carbon fibre composites

The thermal conductivity of the unidirectional copper matrix–carbon fibre composite is characterised and analysed in directions parallel and transverse to the carbon fibre orientation. Unidirectional samples with different fibre content were produced by diffusion bonding of continuous copper-coated carbon fibre tows. The thermal diffusivity was measured in two main orientations to the fibre direction by the laser-flash technique. The longitudinal and transverse thermal conductivity was then calculated and results were compared with simple analytical models. Measurements revealed decreasing thermal conductivity as the fibre volume content in the composite increased and the transverse thermal conductivity of the unidirectional samples presented much lower values in comparison to the longitudinal one.     

A combined microstructure strengthening analysis of SiCp/Al metal matrix composites

A combined microstructure strengthening analysis was made to predict the yield strength of SiC_p/Al metal matrix composites. The modified shear lag theory can be used to predict the yield strength of particlereinforced high strength aluminium composites, but the yield strength is underestimated for composites prepared from pure and cast aluminium matrices because the change of matrix microstructure after particle incorporation is not considered. The increased yield strength due to the change of matrix microstructure represents a large increment of the total increased yield strength of SiC particle-reinforced low strength aluminium composites. Several microstructure strengthening mechanisms are examined to evaluate their effects on the matrix yield strength. The result of the combined microstructure strengthening analysis is consistent with experimental results.     

Mode II delamination failure mechanisms of polymer matrix composites

The failure process of mode II delamination fracture is studied on the basis of the microscopic matrix failure modes (microcracks and hackles) as well as fracture mechanics principles. The crack tip matrix stresses leading to delamination is analysed by examining an adhesive bond with a crack analogous to a delamination crack in the resin layer of a composite. Such crack tip stresses induce matrix microcracks involving two major events: (a) single microcrack initiation and (b) development of multiple microcracks with regular spacing. The microcrack initiation shear stress τ* is found by the use of fracture mechanics to be related to certain resin properties (shear modulus G and mode I fracture toughness GIC) and microcrack length of the order of the resin layer thickness t (related to resin content). The more or less regular microcrack spacing S deduced from shear lag considerations can be related to resin properties GIC, G, τy (resin yield strength) and t. The multiple microcracks reduce the effective resin modulus and strongly affect the subsequent microcrack coalescence process. As a result of the detailed analysis of the failure process, mode II laminate fracture toughness GIIC can be quantitatively expressed as a function of resin GIC and (τ2y/G). The failure process modelled is used to interpret the mode II delamination behaviour of several carbon/epoxy systems studied here and that reported in the literature. This study reveals the critical importance of resin fracture (GIC related) and deformation (yielding) mechanisms in controlling mode II delamination resistance of laminated composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

A comparison of strengthening mechanisms in rolled and axisymmetrically deformed Ti-20Y composites

Two Ti-20%Y metal-metal composites were deformation processed: one axisymmetrically and the other by rolling. The microstructures, preferred crystallographic orientations, and tensile strengths of each were measured periodically as the deformation progressed. The axisymmetrically deformed Ti matrix developed a [10 0] fiber texture, and the rolled composite acquired a texture with the <0001> tilted 31° from the sheet normal toward the transverse direction with [10 0] parallel to the rolling direction. The orientations of the {10 0}<11 0> and (0002)<11 0> slip systems in Ti with these textures were used in conjunction with the maximum possible slip distances for dislocation travel in each specimen to demonstrate that the axisymmetrically deformed material should be stronger than the rolled material for composites of equal phase thickness and spacing. The strengths of the two composites measured in this study were compared at similar microstructural phase sizes and spacings, and the axisymmetrically deformed composite was indeed found to be somewhat stronger, although the difference in strengths was not large.     

基于扩展有限元的陶瓷复合材料多重增韧机制

为了提高陶瓷材料的断裂韧性和可靠度,改善材料抵御破坏的能力,将优化的多重增韧机制应用到氧化铝基陶瓷材料的开发中。相变增韧机制可以耗散部分能量,降低裂纹尖端处的应力集中程度,阻止或延缓裂纹扩展速率。当增强相分布较为合理、材料的致密度较高时,裂纹偏转与桥接增韧机制可以有效地削弱裂纹扩展动力,提高材料的断裂韧性。利用扩展有限元（X-FEM）手段讨论了裂纹扩展问题,为分析陶瓷复合材料的多重增韧机制提供了新思路。     

Nanoclay assisted strengthening of the fiber/matrix interface in functionally filled polyamide 6 composites

This study describes the contribution of a nano-filler, i.e. an organically modified layered silicate, in respect of the fiber/matrix interfacial shear strength (IFSS) of a short glass fiber reinforced and functionally filled polyamide 6, given an exfoliated nanocomposite morphology. Apparent IFSS values are determined using a continuum micromechanical method. Polymer chains in the nanoconfined environment of an exfoliated clay nanocomposite are found to crystallize preferentially in the form of a specific crystalline phase with increased degree of crystallinity. Strong dependence of IFSS on polymer crystallinity is found which is related to enhanced shrinkage stresses associated with increased degree of crystallinity.     

颗粒增强铜基复合材料的选区激光烧结制备

利用选区激光烧结制备了亚微米WC-10%Co颗粒增强Cu基复合材料。利用X射线衍射仪、 扫描电镜及原子力显微镜表征了激光烧结试样的显微组织。结果显示: WC增强颗粒或部分熔化且圆滑化, 或完全熔化且原位析出; 与基体具有连续相容的冶金结合界面。 研究了工艺参数(激光功率、 扫描速率、 铺粉厚度)对烧结试样组织及性能的影响。结果表明, 增加激光功率能改善增强颗粒与基体的界面结合性能。激光扫描速率大于0.05m/s 时, 能提高增强颗粒分散均匀性。铺粉厚度降至0.30mm以下, 有利于提高烧结成形致密度。      

Fabrication and properties of short carbon fibers reinforced copper matrix composites

Short carbon fibers (SCFs) reinforced copper matrix composites have been produced by a new electrodeposition plus cold press and sintering technique. SCFs were copperized directly by the new method, and the electrodeposit had a loose porous structure. The coating thickness is uniform, and can be controlled by appropriate parameters. A model representing the growth process of these electrodeposits was presented. SCFs were distributed homogeneously, and no defects were found in the Cu/SCFs composites. The effects of SCFs volume fraction on mechanical, physical, thermal, and tribological properties of the composites were discussed.