专业英语作业.docx

Metal-Matrix Composites: Challenges and OpportunitiesScope and DefinitionsA metal-matrix composite (MMC) combines into a single material a metallic base with a reinforcing constituent, which is usually non-metallic and is commonly a ceramic. By definition, MMCs are produced by means of processes other than conventional metal alloying. Like their polymer-matrix constituents (e.g. a metal and a ceramic fibre). Processes commonly used include powder metallurgy, diffusion bonding, liquid phase sintering, squeeze infiltration and stir-casting. Alternatively, the typically high reactivity of metals at processing temperatures can be exploited to form the reinforcement and/or the matrix in situ, i.e by chemical reaction within a precursor of the composite. 字典窗体底端将文件或链接拖放到此处以翻译文档或网页。将链接拖放到此处以翻译网页。我们不支持您拖放的文件类型，请尝试其他文件类型。我们不支持您拖放的链接类型，请尝试其他类型的链接。 There are several reasons why MMCs have generated considerable interest within the materials community for nearly 30 years.(1) The “composite” approach to metallurgical processing is the only pathway for the production of entire classes of metallic materials. Only in this way can aluminium, copper, or magnesium be combined with significant volume fractions of carbide, oxide or nitride phases because, unlike iron, the solubility of carbon, nitrogen in the molten metal is (with the exception of O in Cu) far too low.(2) The approach facilitates significant alterations in the physical properties of metallic materials. Composites offer scope for exceeding the specific elastic modulus value of about 26 J/kg, which is exhibited by all the main engineering metals. Composites also offer the only path-way for producing materials with tailored physical combinations:an example is that of low thermal expansivity combined with high thermal conductivity, a combination of importance for electronic packaging.(3) MMCs offer significant improvements over their polymer matrix counterparts with regard to several properties, including tolerance of high temperature, transverse strength, chemical inertness hardness and wear resistance, while significantly outperforming ceramic matrix composites in terms of toughness and ductility.(4) Exceptional properties can be obtained in some cases. An example is that of 3Ms Nextel-reinforced aluminium composites, which exhibit along the fibre direction a tensile strength of 1.5 Gpa, a compressive strenght of 3 Gpa and a transverse strength above 200 Mpa, in a material of density only slightly above 3 g/cm-3.MMCs come in several distinct classes, generally defined with reference to the shape of their reinforcement.Particle-reinforced metals (PRMs) contain approximately equiaxed reinforcements, with an aspect less than about 5. There are generally ceramic (SiC, Al2O3, etc.). PRMs commonly contain below 25 vol.% ceramic reinforcement when used for structural applications, but can have as much as 80 vol.% ceramic when used for electronic packaging. In general, PRMs are at least approximately isotropic. They are produced using both solid state (powder metallurgy) and liquid metal techniques (stir casting, infiltration). Their mechanical properties, while often inferior to those of fibre-reinforced metals, are more or less isotropic and often represent, at moderate cost, significant improvements over those of corresponding unreinforced metais.Short fibre and whisker-reinforced metals. These contain reinforcements with an aspect ratio of greater than 5, but are not continuous. These composites are commonly produced by squeeze infiltration. They often form part of a locally reinforced component, generally produced to net or near-net shape. Their use in automotive engines is now well established.Continuous fibre-reinforced metals contain continuous fibres (of alumina, SiC, carbon, etc.) with a diameter below about 20um. The fibres can either be parallel, or prewoven before production of the composite; this is generally achieved by squeeze infiltration. Monofilament-reinforced metals contain jibres that are relatively large in diameter (typically around 100um), available as individual elements. Due to their thickness, the bending flexibility of monofilaments is low , which limits the range of shapes that can be produced . Monofilament-reinforced metals can be produced by solid state processes requiring diffusion bonding: they are commonly based on titanium alloy matrices, which are well-suited to such techniques. Interpenetrating phase composites are ones in which the metal is reinforced with a three-dimensionally percolating phase, for example ceramic foam.Liquid phase sintered metallic materials, include the cemented carbides, in which carbide particles are bonded together by a metal such as cobalt, and the tungsten heavy alloys.Challenges and opportunites (1)There is a need to advance our understanding of processing fundamentals, particularly concerning established processes such as squeeze infiltration , liquid phase sintering, and powder metallurgy .progress in this area is required ,both to drive innovation and to enable quantitative process simulation , optimization , and control. In particular, progress in this area is critical for controlling internal defects-an important goal with these materials, given that they are more brittle than unreinforced metals. (2) Property improvements must be sought, particularly in ductility and toughness. Systematic investigations are required of the fundamentals links between microstructure and properties. Much work to date has focused on only a few commercial or near- commercial materials, which have been characterized in detail, but do not provide full insight into basic microstructure-property relations, such as the link between particle size or spatial distribution and mechanical properties. (3) There is clearly scope for improvements in the properties of reinforcements. Substantial advances in fibres for MMCS have been achieved at the 3M company: in terms of strength, for example, the performance of alumina fibre-reinforced aluminium has doubled over the past decade. Recent work has also shown that significant differences exist between ceramic particles that can be used as reinforcements for aluminium. Research on the economical production of high strength, low-cost, ceramics for the reinforcement of metals would be very timely. (4) An important issue concerns secondary processing. Operations such as welding and machining, and also the definition of recycling strategies, are challenging when applied to MMCs. Research in this area is critical for certain applications and for the life-cycle engineering of these materials. (5) Much work to date has focused on aluminium matrix composites, but copper, magnesium, and iron-based matrix composites do offer promise in specific applications. These include electronic applications for copper-matrix composites, and chemical processing environments for steel matrix composites. These systems deserve exploration, again with emphasis on fundamentals, rather than the development of this or that spe