Microstructure-guided numerical simulations to predict the thermal performance of a hierarchical cement-based composite material

This paper presents a microstructure-guided numerical homogenization technique to predict the effective thermal conductivity of a hierarchical cement-based material containing phase change material (PCM)-impregnated lightweight aggregates (LWA). Porous inclusions such as LWAs embedded in a cementitious matrix are filled with multiple fluid phases including PCM to obtain desirable thermal properties for building and infrastructure applications. Simulations are carried out on realistic three-dimensional microstructures generated using pore structure information. An inverse analysis procedure is used to extract the intrinsic thermal properties of those microstructural components for which data is not available. The homogenized heat flux is predicted for an imposed temperature gradient from which the effective composite thermal conductivity is computed. The simulated effective composite thermal conductivities are found to correlate very well with experimental measurements for a family of LWA-PCM composites considered in the paper. Comparisons with commonly used analytical homogenization models show that the microstructure-guided simulation approach provides superior results for composites exhibiting large property contrast between phases. By linking the microstructure and thermal properties of hierarchical materials, an efficient framework is available for optimizing the material design to improve thermal efficiency of a wide variety of heterogeneous materials.     

Kontaktwerkstoffe auf Silberbasis ‐ Gefüge und mechanische Eigenschaften

Silver‐based contact materials – microstructure and mechanical properties Different silver‐based materials have been used in relays and contactors. Silver‐based composite materials in particular have played an important role. To produce such composite materials on an industrial scale, conventional powder mixing and wet‐chemical methods are used. By means of the powder‐metallurgical route, these materials are processed in a second step into wire material. To produce silver‐based composite materials with a comparable microstructure, the usability of alternative production routes was tested. This article shows the potential of the methods high‐energy ball milling (HEM) and intensive mixing compared to the two above‐named conventional methods. The main focus is on the evaluation of the microstructure of the composite powder and the extrusion wires concerning the dispersion of the reinforcement component and the resulting mechanical properties of the wire material.     

Global optimum of microstructure parameters in the CMWP line-profile-analysis method by combining Marquardt-Levenberg and Monte-Carlo procedures

Line profile analysis of X-ray and neutron diffraction patterns is a powerful tool for determining the microstructure of crystalline materials. The Convolutional-Multiple-Whole-Profile (CMWP) procedure is based on physical profile functions for dislocations, domain size, stacking faults and twin boundaries. Order dependence, strain anisotropy, hkl dependent broadening of planar defects and peak shape are used to separate the effect of different lattice defect types. The Marquardt-Levenberg (ML) numerical optimization procedure has been used successfully to determine crystal defect types and densities. However, in more complex cases like hexagonal materials or multiple phases the ML procedure alone reveals uncertainties. In a new approach the ML and a Monte-Carlo statistical method are combined in an alternative manner. The new CMWP procedure eliminates uncertainties and provides globally optimized parameters of the microstructure.     

晶须及其应用

晶须增强的复合材料是具有高性能的新型复合材料。在这些材料中晶须不仅起着强化作用,而且可以显着地提高材料的韧性,或赋予材料某种特殊的物理性能,成为特殊功能复合材料。本文对晶须的理论和应用进行了全面的讨论,并指出了目前存在的问题和进一步研究方向。      

Microstructural and Nano‐Mechanical Characterisation of Multicomponent Composites Nanocrystalline Matrix

Multicomponent, Ti‐based, in situ formed composites with a nanocrystalline matrix are a promising new type of material for structural applications. The materials exhibit an excellent combination of mechanical properties resulting from the composite microstructure. This paper contains a detailed introduction to such materials and a review of the most recent developments in the specific areas of microstructural and nano‐mechanical characterization.     

Carbon fiber reinforced hafnium carbide composite

Hafnium carbide is proposed as a structural material for aerospace applications at ultra high temperatures. The chemical vapor deposition technique was used as a method to produce monolithic hafnium carbide (HfC) and tantalum carbide (TaC). The microstructure of HfC and TaC were studied using analytical techniques. The addition of tantalum carbide (TaC) in the HfC matrix was studied to improve the microstructure. The microstructure of HfC, TaC and co-deposited hafnium carbide-tantalum carbide (HfC/TaC) were comparable and consisted of large columnar grains. Two major problems associated with HfC, TaC, and HfC/TaC as a monolithic are lack of damage tolerance (toughness) and insufficient strength at very high temperatures. A carbon fiber reinforced HfC matrix composite has been developed to promote graceful failure using a pyrolytic graphite interface between the reinforcement and the matrix. The advantages of using carbon fiber reinforcement with a pyrolytic graphite interface are reflected in superior strain capability reaching up to 2%. The tensile strength of the composite was 26 MPa and needs further improvement. Heat treatment of the composite showed that HfC did not undergo any phase transformations and that the phases comprising composite were are thermochemically compatible.     

Multi-objective Genetic Topological Optimization for Design of Blast Resistant Composites

Composites make it possible to produce materials with properties that are unattainable with single phase materials. This paper examines the use of multi-objective genetic topological optimization to design blast resistant composites. The fundamental problem of the design of a two-layer composite plate that is subjected to blast is considered using the finite element method. Two materials are used to form the microstructure of each layer. The microstructure and thickness of each layer is optimized for the two-layer plate to minimize the weight and stress-to-strength ratio. A set of optimal blast resistant composite microstructures that meet design requirements is demonstrated.     

Novel Ti-base nanostructure-dendrite composite with enhanced plasticity

Single-phase nanocrystalline materials undergo inhomogeneous plastic deformation under loading at room temperature, which results in a very limited plastic strain (smaller than 0-3%). The materials therefore display low ductility, leading to catastrophic failure, which severely restricts their application. Here, we present a new in situ-formed nanostructured matrix/ductile dendritic phase composite microstructure for Ti-base alloys, which exhibits up to 14.5% compressive plastic strain at room temperature. The new composite microstructure was synthesized on the basis of the appropriate choice of composition, and by using well-controlled solidification conditions. Deformation occurs partially through dislocation movement in dendrites, and partially through a shear-banding mechanism in the nanostructured matrix. The dendrites act as obstacles restricting the excessive deformation by isolating the highly localized shear bands in small, discrete interdendritic regions, and contribute to the plasticity. We suggest that microscale ductile crystalline phases might therefore be used to toughen nanostructured materials.     

The characterization of low cost fiber reinforced thermoplastic composites produced by the DRIFT™ process

A new, low cost process for hot-melt impregnation of continuous reinforcing fibers with thermoplastic polymers is described. This technique can be used to fabricate various product forms including discontinuous, long-fiber products for compression molded parts, continuous fiber products for pultrusion, filament winding, and woven fabric applications. Mechanical data are presented for composites with various fiber and polymer combinations. Effects of fiber orientation and length on mechanical properties are discussed, and the effect of fiber–polymer bonding on impact strength and microstructure are shown. It is shown that the low cost and high performance achieved with this approach has the potential to expand applications of thermoplastic composite materials.     

Tailoring nanostructured, graded, and particle-reinforced Al laminates by accumulative roll bonding

Accumulative roll bonding (ARB) is a very attractive process for processing large sheets to achieve ultrafine-grained microstructure and high strength. Commercial purity Al and many Al alloys from the 5xxx and the precipitation strengthened 6xxx alloy series have been successfully processed by the ARB process into an ultrafine-grained state and superior ductility have been achieved for some materials like technical purity Al. It has also been shown that the ARB process can be successfully used to produce multi-component materials with tailored properties by reinforcement or grading, respectively. This allows optimizing the properties based on two or more materials/alloys. For example, to achieve high corrosion resistance and good visual surface properties it is interesting to produce a composite of two different Al alloys, where for example a high strength alloy of the 5xxx series is used as the core material and a 6xxx series alloy as the clad material. It has been shown that such a composite achieves more or less the same strength as the core material although 50% of the composite consists of the significant softer clad alloy. Furthermore, it has been found, that the serrated yielding which typically appears in 5xxx series alloys and limits applications as outer skin materials completely disappears. Moreover, the ARB process allows many other attractive ways to design new composites and graded material structures with unique properties by the introduction of particles, fibres and sheets. Strengthening with nanoparticles for example is a very attractive way to improve the properties and accelerate the grain refining used in the severe plastic deformation process. With an addition of only 0.1 vol.-% Al2O3 nanoparticles a significantly accelerated grain refinement has been found which reduces the number of ARB passes necessary to achieve the maximum in strength. The paper provides a short review on recent developments in the field of ARB processing for producing multicomponent ultrafine-grained sheet materials with tailored properties.     

冷气动力喷涂技术研究进展

作为一种新的涂层方法－冷气动力喷涂因能在较低的温度下喷涂金属、合金以及复合涂层而受到人们的关注。综述了国内外冷气动力喷涂技术领域的最新研究进展，主要包括冷气动力喷涂技术研究、冷喷涂涂层的结构性能和涂层特点及其应用。     

A dual analysis for recycled particulate composites: linking micro- and macro-mechanics

The large amount of disposable bottles produced nowadays makes imperative the search for alternative procedures for recycling them since they are not biodegradable. This paper takes into consideration the thermomechanical recycling of post-consumed plastic bottles, especially the ones made of polyethylene terephthalate (PET) and high-density polyethylene (HDPE), and their use as composite materials for engineering applications. As changes on the composite's microstructure can have an influence on macroscopic behavior, a new type of analysis is needed. To be able to evaluate the composite performance, a dual analysis procedure was developed. It consists of a micro-mechanical analysis where the microstructure is observed by optical microscopy, and variations in morphology are related to composite overall mechanical behavior. The macro-mechanical analysis is performed by ASTM D 3039/3039 M tensile tests. By doing this, the composite effective moduli can be determined. The new composite seems to be encouraging, i.e., an HDPE/PET composite with 40:60 ratio, in weight, experiments a stiffness recovery from the third to the fourth recycle. Moreover, the dual analysis was able to capture this variation.     

定向凝固NiAl-Cr(Mo)共晶复合材料的研究进展

综述了定向凝固NiAl-Cr(Mo)共晶自生复合材料的凝固组织特征和力学性能,探讨了添加微量元素如Hf、Ti和Ho等对复合材料组织和力学性能的影响。就目前合金在实际应用中的不足之处提出了改进方法,如提高温度梯度和改变合金成分,试图制备含大体积分数强化相的细密、连续、规则的层片组织以提高其综合力学性能。     

玻璃包覆金属微丝的快速凝固制备及应用

玻璃包覆金属微丝具有耐蚀、耐高温、抗辐照及高绝缘性等特点,作为精密电子元器件用电磁线,具有重要的应用前景.综述了玻璃包覆金属微丝制备技术的发展现状、基本工艺原理、主要工艺参数以及玻璃包覆金属微丝材料的种类、微丝的形状尺寸与组织,介绍了玻璃包覆金属微丝在电气、电子工业以及复合材料制备等领域的用途.     

Preparation and performance of a novel multifunctional plasma electrolytic oxidation composite coating formed on magnesium alloy

Plasma electrolytic oxidation (PEO) in an alkaline phosphate electrolyte was used to produce a novel multifunctional polytetrafluoroethylene (PTFE)-containing oxide composite coatings on AM60B magnesium alloys. The composition and microstructure of the PTFE-containing PEO coatings were analyzed by X-ray photoelectron spectroscope (XPS), X-ray diffraction (XRD), and scanning electron microscope (SEM). The electrochemical corrosion behavior, tribological properties, and wetting properties of the PTFE-containing PEO composite coatings were evaluated using potentiodynamic polarization measurements, a reciprocating ball-on-disk tribometer, and a contact angle meter, respectively. Results show that the PTFE-containing PEO composite coatings exhibited superior corrosion resistance, excellent self-lubricating property, and better hydrophobic property when compared with pure PEO coatings, and will be the attractive advanced materials for a wide range of functional applications.     

In situ synthesis of bone-like apatite/collagen nano-composite at low temperature

A nano-composite of bone-like apatite/collagen was prepared by a new method—low-temperature in situ synthesis using calcium nitrate, diammoniun hydrogen phosphate and cow hide collagen as starting materials. The composite was investigated via X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that bone-like nanoapatite particles were distributed uniformly in collagen fibrils in the composite. The composite with homogeneous microstructure was similar to natural bone in crystallite phase composition and crystal size. The biomimetic composite is expected to exhibit desirable properties in biomedical applications.     

Elaboration of self-coating alumina-based porous ceramics

Materials used for bone substitution (i.e. hydroxyapatite and calcium phosphate) are highly successful, since when implanted they provide an efficient scaffold that can be colonized by the patient’s bone. However, their poor mechanical properties impede their use for load-bearing applications. In contrast, no material with high mechanical properties also presents a high bioactivity. A possible way of finding a material both strong and bioactive is to use a composite. We propose here a composite deriving its strength from its alumina core and its bioactivity from a calcium phosphate surface. Ceramic scaffolds have been produced by infiltration of polymer open-celled foams. Several compositions of the slurries have been tested, leading to the realization of porous pieces with a biocompatibility gradient at a micrometric scale. The mechanical properties of several new materials are presented and correlated to their microstructure.     

功能梯度板壳的力学研究进展

功能梯度材料(FGM)是一种材料组分或微观结构沿一个或几个方向连续变化的新型复合材料。FGM中基本消除了宏观界面,同时具有很好的可设计性,设计人员可以有目的地改变材料组成,以获得所期望的性能。由FGM构成的功能梯度板壳结构在许多工程领域中有着广阔的应用前景,评述了FGM板壳结构的弯曲、屈曲和后屈曲、振动和动力稳定性等力学问题研究的发展现状,阐述了常用方法和理论的优缺点,并提出了未来的发展趋势和需要研究的方向。     

A study on the failure detection of composite materials using an acoustic emission

Fiber reinforced composite materials have been increasingly used as structural material in airplanes, in space applications, and in robot arms because of their high specific stiffness and strength. Structural design and nondestructive test techniques have evolved as increased emphasis has been placed on the durability and damage tolerance of these materials. There are several methods used to detect damaged regions of composite materials. Acoustic emission is one of these. It is a suitable technique for detection of a wide range of micro-structural failures in composite materials.

In this paper, an AE signal analyzer was designed and fabricated with a resonance circuit to extract the specified frequency of an acoustic emission signal. From the tests that were completed, the disturbance noise levels, such as impact or mechanical vibration, of the fabricated AE signal analyzer had a very small value in comparison to those of the conventional AE signal analyzer. Also, the fabricated AE signal analyzer was proven to have generally the same crack detection capabilities as a conventional AE signal analyzer, under static and dynamic tensile tests of the composite materials.