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Glossary of terms specific to Metal Matrix Composites
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Glossary of terms specific to Metal Matrix Composites Composite: A composite (or composite material) is defined as a material that consists of at least two constituents (distinct phases or combinations of phases) which are bonded together along the interface in the composite, each of which originates from a separate ingredient material which pre-exists the composite. The essential elements of this definition are (i) composite refers to a material, as opposed to a structure or a component; as such a composite material is used for the fabrication of components of various shapes or functions, thus it should be distinguished from a wing or other structure made of several components bonded together, and from an electronic device or packaging structure made of layered materials (although one of the materials in the packaging could be considered a composite); (ii) the composite is produced via a physical combination of at least two pre-existing ingredient materials; this distinguishes a composite from other multiphase materials which are produced by bulk processes where one or more phases result from phase transformation: many ”in-situ composites”, such as directionally solidified eutectics or alloys from which a ceramic phase is precipitated, are thus considered to be alloys and not composites (even though their properties can be described using composite theory). Hybrid composite: a composite that consists of at least three (as opposed to two) constituents (distinct phases or combinations of phases) which are bonded together at the atomic level in the composite, each of which originates from a separate ingredient material which pre-exists the composite (i.e., there are, at least three ingredient materials). Metal Matrix Composite (MMC): a composite material in which one constituent is a metal or alloy forming at least one percolating network. The other constituent is embedded in this metal matrix and usually serves as reinforcement. Metal matrix: the percolating metal or alloy forming the constituent of a MMC in which the other constituents are embedded. Ingredient materials: Raw materials put together to produce the composite (e.g., the metal alloy, metal powder, ceramic particles, etc.). During the processing the ingredient materials are transformed into the constituents of the MMC. Reinforcement: a constituent (phase or combination of phases) of the MMC originating from the ingredient material which is combined with a metal or an alloy to produce a metal matrix composite (e.g., alumina fibres, silicon carbide whiskers, steel fibres, or graphite particles, even if, in the case of graphite particles, the reinforcement does not ”reinforce” the matrix). A reinforcement is characterised by its chemical composition, its shape and dimensions, its properties as ingredient material and its volume fraction and spatial distribution in the matrix. Interface: that portion of the composite microstructure that lies between its constituents, i.e. matrix and reinforcement. The interface may be a simple row of atomic bonds (e.g., the interface between alumina and pure aluminium), but may also include matrix/reinforcement reaction products (e.g., aluminium carbide between aluminium and carbon fibres), or reinforcement coatings (e.g., interfacial coatings between SiC monofilaments and titanium matrices). 1) REINFORCEMENT Continuous reinforcement: a constituent of a composite which is continuous in at least one direction through the composite (e.g., continuous fibres or percolating open-celled foam). Continuous fibres (f) : cylindrical ingredient material produced continuously to form an essentially endless reinforcement in the composite; usually delivered on bobbins of multifilament tows, each tow consisting of many individual fibres of diameters typically in the range of 3 to 30 μm. According to the production process such fibres are usually coated by a polymeric sizing and the tows may be slightly twisted. They are typically designated by a brand name, the number of fibres per tow and a symbol of the applied sizing (e.g. T300-6K-E). Monofilaments (m) : essentially endless reinforcement as continuous fibres, except for a larger diameter, typically greater than 100 μm. Monofilaments are generally produced by deposition onto a core fibre (most often of carbon or tungsten), and are delivered as individual fibres instead of tows. Discontinuous reinforcement: a non-percolating constituent of a composite, taking the form of individual elements embedded in the matrix constituent (e.g., particulates, short fibres, whiskers). Preforms produced from discontinuous reinforcements that are mechanically stabilised by a binder or by cold compaction are still considered discontinuous reinforcements. Particulates (p) : roughly equiaxed reinforcement or composite ingredient, usually of aspect ratio (ratio of largest to smallest diameter) less than about 5. Particulates can be both mono- or polycrystalline, can take various shapes (spherical, angular, plate-like) and are typically greater than 1 μm in diameter. Dispersoids (d) : same as particulates, except that the diameter is less than 1μm, hence, being capable of providing Orowan strengthening. Platelets: flat reinforcements of an aspect ratio (diameter to thickness) greater than 2. Platelets of an aspect ratio less than 5 can be considered as a type of particulates. Short fibres (s) : cylindrical reinforcement or composite ingredient with a ratio of length to diameter greater than 5 (but typically greater than 100), and with a diameter typically greater than 1 μm. Whiskers (w) : elongated single crystals, typically produced with a length to diameter ratio greater than 10 and with a diameter typically less than 1micron. 2) CATEGORIES OF MMC Particulate reinforced MMC (PRM): metal matrix composite with a particulate reinforcement occupying a volume fraction greater than 5% in the material (otherwise, the particulates are generally considered to be inclusions). Dispersoid reinforced MMC: metal matrix composite with a dispersoid reinforcement occupying a volume fraction greater than 5% in the material (otherwise, the material is considered to be a dispersion strengthened metal - which incidentally may form the matrix of any type of MMC, i.e., a MMC with dispersion-strengthened matrix). Cermets: a metal matrix with a three-dimensionally percolating ceramic reinforcement, typically with far more ceramic than metal (strictly speaking they contain less than 20% metal per volume and are thus not considered as MMC). According to the percolating structure of both constituents cermets could thus be considered as both a ceramic and a metal matrix composite. Short fibre reinforced MMC (SFRM): metal matrix composite with a short fibre reinforcement. Whisker reinforced MMC (WRM): metal matrix composite with a whisker reinforcement. Continuous fibre reinforced MMC (CFRM): metal matrix composite with reinforcement of continuous fibres. Monofilament reinforced MMC (MFRM): metal matrix composite with reinforcement of monofilaments. MMC designation: method of indicating the composition and at times also the processing of the MMC. As proposed by the American Aluminum Association for the designation of aluminium composites, MMC should be designated as follows by their constituents: accepted designation of the matrix / abbreviation of the reinforcement’s designation / arrangement and volume fraction in % with symbol of type (shape) of reinforcement, such as “p” for particle, “s” for short fibres, “w” for whiskers, “f” for continuous fibres or “m” for monofilaments (The “f” or “m” is assumed to describe unidirectional reinforcement unless otherwise specified). Some examples are: matrix of a magnesium alloy AM10 reinforced by continuous carbon fibres of type T300, perpendicular oriented, amounting to a volume fraction of 65% is designated as: ”AM10/C-T300/65 f-0/90° ”; or matrix of the aluminium alloy AA6061 reinforced by particulates of alumina of 22 % by volume is designated as: ”AA6061/Al2O3/22 p”. Additional information is supplied as needed for more specific reference to properties with following basic form: … / form / heat treatment Form is simply an indicator of final forming step as “extruded” or “pmc” for permanent mold casting, while the heat treatment is specified as its standard for the matrix alloy such as “t6” or “O”. Thus a more complete designation of the second example above is: ”AA6061/Al2O3/22 p / extruded / T6 ”. MMC product names used by companies vary, hence their use indicates the processing route employed by implicit reference to the producer. Cellular metals (metal foams): similar to MMC as they are often formed by combining two separate ingredients yielding a metal matrix and the second constituent being the gas in closed or open pores: however, there is no clear bond between the gas phase and the matrix. 4) PROCESSING STEPS Pre-processing: all steps which precede primary processing (e.g., surface treatment of ingredient materials, or preform fabrication for infiltration processing). Preform: a shaped porous assembly of ingredient material elements (such as fibres, whiskers or particles). Typically, preforms are produced for subsequent infiltration with liquid metal, or foil-fibre-foil stacks prior to diffusion bonding. The mechanical stability of the shape may be provided by the adjunction of a binder, potentially but not always fugitive, and amounting to only a few mass % of the preform. Hybrid preform: a preform containing a t least two types of ingredient material elements which form at least two distinct ”reinforcing” constituents in the composite (e.g. particulate-fibre mixtures, hollow micro-spheres). Primary processing of composites: production of composite material by combining ingredient materials (e.g., powdered metal and loose ceramic particles, or molten metal and fibre preforms), but not necessarily to final shape or final microstructure. Continuous MMC primary process: a process for the production of MMC material that is essentially continuous (e.g., foil-fibre-foil diffusion bonding using a rolling mill, or continuous infiltration). Secondary processing: processing steps which follow primary processing, and aim to alter the shape or microstructure of the material ( e.g., shape casting, forging, extrusion, heat-treatment, machining). Secondary processing may change the constituents (phases, shape) of the composite. Consolidation of MMC: a secondary composite manufacturing process during which composite elements are consolidated (e.g., the consolidation of composite elements consisting of the reinforcement coated with the matrix metal). MMC insertion casting: a secondary process for production of partially reinforced components, in which a pre-fabricated MMC is placed within a mould, and incorporated into a metal or alloy by a casting process. Partially reinforced metal component: semi-products or components which contain MMC material metallurgically bonded to a conventional metal or alloy (e.g. by diffusion bonding, insertion casting, co-extrusion). In-situ MMC: a MMC in which the ingredient reinforcement material changes its phase or shape during primary composite processing (e.g. TiB2 reacts to form TiC in some metals by adding carbon). To be an MMC, the constituents must, however, retain its local identity, although they change in shape and/or composition (otherwise, it is an alloy). 5) LIQUID METAL PROCESSING Stir casting: a primary process of composite production whereby the reinforcement ingredient material is incorporated into the molten metal by stirring. Compocasting: a variant of stir-casting in which the metal is semi-solid, i.e. a rheocasting process incorporating the reinforcing ingredient material to form the composite. (The semisolid state aids incorporation of the particles, in particular because the metal is more viscous.) Semi-solid forming: a family of secondary forming processes which use semi-solid alloy or metal matrix in the case of a MMC. Thixocasting: a family of secondary casting or forming processes, which shape partially remelted thixotropic (semi-solid) metallic materials by deformation. Infiltration: a primary process of composite production whereby the molten metal is made to fill, spontaneously or using external mechanical work, pores within a preform of the reinforcement ingredient. Spontaneous infiltration: an infiltration process in which the matrix penetrates spontaneously the preform without added force (e.g., copper into tungsten because of direct wetting, or by coating nickel onto the reinforcement for infiltration with aluminium, or by control of the atmosphere and the alloy composition as in the Lanxide- Primexprocess). Forced infiltration: an infiltration process which is not spontaneous, i.e., which requires application of mechanical force onto matrix or reinforcement. Pressure infiltration: an infiltration process in which hydrostatic pressure is applied onto the molten matrix surface to drive the liquid into the preform. Vacuum infiltration: an infiltration process in which a hydrostatic pressure of one bar applied by the atmosphere onto the molten matrix drives it into an evacuated preform. In some processes the vacuum can be self-generated, for example by reaction of magnesium with air or oxygen in the preform. Gas pressure infiltration: a pressure infiltration process which uses pressurized gas to apply pressure on the metal surface driving the liquid into a preform. Pressure assisted investment casting: an investment casting process where a preform is integrated into the pattern and the assembly is cast while the mould and the melt are under hydrostatic gas pressure. Mechanical pressure infiltration: a pressure infiltration process which uses a moving solid part to apply pressure on the melt surface driving the liquid into a preform. [search]Glossary of terms specific to Metal Matrix Comp ...[/search] |
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