Metals and composites in context
Creators: name: Chen, Junjie
Publisher: name: Zenodo
Year: 2022
Subjects: Metals; Composites; Alloys; Reinforced concretes; Reinforced masonry; Reinforced plastics; Ceramic matrix composites; Metal matrix composites

description: <strong>Metals and composites in context</strong> Junjie Chen Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China Contributor: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com A metal is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typically ductile (can be drawn into wires) and malleable (they can be hammered into thin sheets). These properties are the result of the metallic bond between the atoms or molecules of the metal. A metal may be a chemical element such as iron; an alloy such as stainless steel; or a molecular compound such as polymeric sulfur nitride. In physics, a metal is generally regarded as any substance capable of conducting electricity at a temperature of absolute zero. Many elements and compounds that are not normally classified as metals become metallic under high pressures. For example, the nonmetal iodine gradually becomes a metal at a pressure of between 40 and 170 thousand times atmospheric pressure. Equally, some materials regarded as metals can become nonmetals. Sodium, for example, becomes a nonmetal at pressure of just under two million times atmospheric pressure. In chemistry, two elements that would otherwise qualify (in physics) as brittle metals, arsenic and antimony, are commonly instead recognized as metalloids due to their chemistry (predominantly non-metallic for arsenic, and balanced between metallicity and non-metallicity for antimony). Around 95 of the 118 elements in the periodic table are metals (or are likely to be such) [1]. The number is inexact as the boundaries between metals, nonmetals, and metalloids fluctuate slightly due to a lack of universally accepted definitions of the categories involved. Metals are shiny and lustrous, at least when freshly prepared, polished, or fractured. Sheets of metal thicker than a few micrometers appear opaque, but gold leaf transmits green light. The solid or liquid state of metals largely originates in the capacity of the metal atoms involved to readily lose their outer shell electrons. Broadly, the forces holding an individual atom's outer shell electrons in place are weaker than the attractive forces on the same electrons arising from interactions between the atoms in the solid or liquid metal. The electrons involved become delocalized and the atomic structure of a metal can effectively be visualized as a collection of atoms embedded in a cloud of relatively mobile electrons. This type of interaction is called a metallic bond. The strength of metallic bonds for different elemental metals reaches a maximum around the center of the transition metal series, as these elements have large numbers of delocalized electrons. Although most elemental metals have higher densities than most nonmetals, there is a wide variation in their densities, lithium being the least dense and osmium the most-dense. Magnesium, aluminum and titanium are light metals of significant commercial importance. Metals are typically malleable and ductile, deforming under stress without cleaving. The nondirectional nature of metallic bonding is thought to contribute significantly to the ductility of most metallic solids. In contrast, in an ionic compound like table salt, when the planes of an ionic bond slide past one another, the resultant change in location shifts ions of the same charge into close proximity, resulting in the cleavage of the crystal. Such a shift is not observed in a covalently bonded crystal, such as a diamond, where fracture and crystal fragmentation occur. Reversible elastic deformation in metals can be described by Hooke's Law for restoring forces, where the stress is linearly proportional to the strain. Heat or forces larger than a metal's elastic limit may cause a permanent (irreversible) deformation, known as plastic deformation or plasticity. An applied force may be a tensile (pulling) force, a compressive (pushing) force, or a shear, bending, or torsion (twisting) force. A temperature change may affect the movement or displacement of structural defects in the metal such as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in both crystalline and non-crystalline metals. Internal slip, creep, and metal fatigue may ensue. The electronic structure of metals means they are relatively good conductors of electricity. Electrons in matter can only have fixed rather than variable energy levels, and in a metal the energy levels of the electrons in its electron cloud, at least to some degree, correspond to the energy levels at which electrical conduction can occur. In a semiconductor like silicon or a nonmetal like sulfur there is an energy gap between the electrons in the substance and the energy level at which electrical conduction can occur. Consequently, semiconductors and nonmetals are relatively poor conductors. Metals are relatively good conductors of heat. The electrons in a metal's electron cloud are highly mobile and easily able to pass on heat-induced vibrational energy. The contribution of a metal's electrons to its heat capacity and thermal conductivity, and the electrical conductivity of the metal itself can be calculated from the free electron model. However, this does not take into account the detailed structure of the metal's ion lattice. Taking into account the positive potential caused by the arrangement of the ion cores enables consideration of the electronic band structure and binding energy of a metal. Various mathematical models are applicable, the simplest being the nearly free electron model. An alloy is a substance having metallic properties and which is composed of two or more elements at least one of which is a metal. An alloy may have a variable or fixed composition. For example, gold and silver form an alloy in which the proportions of gold or silver can be freely adjusted; titanium and silicon form an alloy in which the ratio of the two components is fixed (also known as an intermetallic compound). Most pure metals are either too soft, brittle, or chemically reactive for practical use. Combining different ratios of metals as alloys modifies the properties of pure metals to produce desirable characteristics. The aim of making alloys is generally to make them less brittle, harder, resistant to corrosion, or have a more desirable color and luster. Of all the metallic alloys in use today, the alloys of iron (steel, stainless steel, alloy steel) make up the largest proportion both by quantity and commercial value [2]. Iron alloyed with various proportions of carbon gives low-carbon, mid-carbon, and high-carbon steels, with increasing carbon levels reducing ductility and toughness. The addition of silicon will produce cast irons, while the addition of chromium, nickel, and molybdenum to carbon steels results in stainless steels. Other significant metallic alloys are those of aluminum, titanium, copper, and magnesium. Copper alloys have been known since prehistory, bronze gave the Bronze Age its name, and have many applications today, most importantly in electrical wiring. The alloys of the other three metals have been developed relatively recently; due to their chemical reactivity they require electrolytic extraction processes. The alloys of aluminum, titanium, and magnesium are valued for their high strength-to-weight ratios; magnesium can also provide electromagnetic shielding. These materials are ideal for situations where high strength-to-weight ratio is more important than material cost, such as in aerospace and some automotive applications. Alloys specially designed for highly demanding applications, such as jet engines, may contain more than ten elements. The term "ferrous" is derived from the Latin word meaning "containing iron". This can include pure iron, such as wrought iron, or an alloy such as steel. Ferrous metals are often magnetic, but not exclusively. Non-ferrous metals and alloys lack appreciable amounts of iron. While nearly all metals are malleable or ductile, a few, beryllium, chromium, manganese, gallium, and bismuth, are brittle. Arsenic and antimony, if admitted as metals, are brittle. Low values of the ratio of bulk elastic modulus to shear modulus are indicative of intrinsic brittleness. In materials science, metallurgy, and engineering, a refractory metal is a metal that is extraordinarily resistant to heat and wear. Which metals belong to this category varies; the most common definition includes niobium, molybdenum, tantalum, tungsten, and rhenium. A white metal is any of range of white-colored metals (or their alloys) with relatively low melting points. Such metals include zinc, cadmium, tin, antimony (here counted as a metal), lead, and bismuth, some of which are quite toxic. In Britain, the fine art trade uses the term "white metal" in auction catalogues to describe foreign silver items which do not carry British Assay Office marks, but which are nonetheless understood to be silver and are priced accordingly. A heavy metal is any relatively dense metal or metalloid. More specific definitions have been proposed, but none have obtained widespread acceptance. Some heavy metals have niche uses, or are notably toxic; some are essential in trace amounts. All other metals are light metals. A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements [3]. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions. Typical engineered composite materials include: reinforced concrete and masonry, reinforced plastics, such as fiber-reinforced polymer or fiberglass, ceramic matrix composites (composite ceramic and metal matrices), metal matrix composites, and other advanced composite materials. There are various reasons where new material can be favored. Typical examples include materials which are less expensive, lighter, stronger or more durable when compared with common materials. Composite materials are created from individual materials. These individual materials are known as constituent materials, and there are two main categories of it. One is the matrix (binder) and the other reinforcement. A portion of each kind is needed at least. The reinforcement receives support from the matrix as the matrix surrounds the reinforcement and maintains its relative positions. The properties of the matrix are improved as the reinforcements impart their exceptional physical and mechanical properties. The mechanical properties become unavailable from the individual constituent materials by synergism. At the same time, the designer of the product or structure receives options to choose an optimum combination from the variety of matrix and strengthening materials. To shape the engineered composites, it must be formed. The reinforcement is placed onto the mold surface or into the mold cavity. Before or after this, the matrix can be introduced to the reinforcement. The matrix undergoes a melding event which sets the part shape necessarily. This melding event can happen in several ways, depending upon the matrix nature, such as solidification from the melted state for a thermoplastic polymer matrix composite or chemical polymerization for a thermoset polymer matrix. According to the requirements of end-item design, various methods of molding can be used. The natures of the chosen matrix and reinforcement are the key factors influencing the methodology. The gross quantity of material to be made is another main factor. To support high capital investments for rapid and automated manufacturing technology, vast quantities can be used. Cheaper capital investments but higher labor and tooling expenses at a correspondingly slower rate assists the small production quantities. Many commercially produced composites use a polymer matrix material often called a resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common are known as polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, polypropylene, and others. Normally, the fabrication of composite includes wetting, mixing or saturating the reinforcement with the matrix. The matrix is then induced to bind together (with heat or a chemical reaction) into a rigid structure. Usually, the operation is done in an open or closed forming mold. However, the order and ways of introducing the constituents alters considerably. Composites fabrication is achieved by a wide variety of methods. The reinforcing and matrix materials are merged, compacted, and cured (processed) within a mold to undergo a melding event. The part shape is fundamentally set after the melding event. However, under particular process conditions, it can deform. The melding event for a thermoset polymer matrix material is a curing reaction that is caused by the possibility of extra heat or chemical reactivity such as an organic peroxide. The melding event for a thermoplastic polymeric matrix material is a solidification from the melted state. The melding event for a metal matrix material such as titanium foil is a fusing at high pressure and a temperature near the melting point. Usually, the composite's physical properties are not isotropic in nature. But they are typically anisotropic [4]. For instance, the composite panel's stiffness will usually depend upon the orientation of the applied forces and moments. The composite's strength is bounded by two loading conditions. If both the fibers and matrix are aligned parallel to the loading direction, the deformation of both phases will be the same. This iso-strain condition provides the upper bound for composite strength, and is determined by the rule of mixtures. The lower bound is dictated by the iso-stress condition, in which the fibers and matrix are oriented perpendicularly to the loading direction. The iso-strain condition implies that under an applied load, both phases experience the same strain but will feel different stress. Comparatively, under iso-stress conditions both phases will feel the same stress but the strains will differ between each phase. Though composite stiffness is maximized when fibers are aligned with the loading direction, so is the possibility of fiber tensile fracture, assuming the tensile strength exceeds that of the matrix. The majority of commercial composites are formed with random dispersion and orientation of the strengthening fibers, in which case the composite Young's modulus will fall between the iso-strain and iso-stress bounds. However, in applications where the strength-to-weight ratio is engineered to be as high as possible, fiber alignment may be tightly controlled. Panel stiffness is also dependent on the design of the panel. For instance, the fiber reinforcement and matrix used, the method of panel build, thermoset versus thermoplastic, and type of weave. In contrast to composites, isotropic materials, in standard wrought forms, possess the same stiffness typically despite the directional orientation of the applied forces and moments. The relationship between forces-moments and strains-curvatures for an isotropic material can be described with the following material properties: Young's Modulus, the Shear Modulus and the Poisson's ratio, in relatively simple mathematical relationships. For the anisotropic material, it needs the mathematics of a second-order tensor and up to 21 material property constants. For the special case of orthogonal isotropy, there are three distinct material property constants for each of Young's Modulus, Shear Modulus and Poisson's ratio, a total of 9 constants to express the relationship between forces-moments and strains-curvatures. In general, particle reinforcement is strengthening the composites less than fiber reinforcement. It is used to enhance the stiffness of the composites while increasing the strength and the toughness. Because of their mechanical properties, they are used in applications in which wear resistance is required. For example, hardness of cement can be increased by reinforcing gravel particles, drastically. Particle reinforcement a highly advantageous method of tuning mechanical properties of materials since it is very easy implement while being low cost. In general, continuous fiber reinforcement is implemented by incorporating a fiber as the strong phase into a weak phase, matrix. The reason for the popularity of fiber usage is materials with extraordinary strength can be obtained in their fiber form. Non-metallic fibers are usually showing a very high strength to density ratio compared to metal fibers because of the covalent nature of their bonds. The most famous example of this is carbon fibers that have many applications extending from sports gear to protective equipment to space industries. The stress on the composite can be expressed in terms of the volume fraction of the fiber and the matrix. Although the stress-strain behavior of fiber composites can only be determined by testing, there is an expected trend, three stages of the stress-strain curve. The first stage is the region of the stress-strain curve where both fiber and the matrix are elastically deformed. In reality, the derivative of stress with respect to strain is not always returning the modulus because of the binding interaction between the fiber and matrix. The strength of the interaction between these two phases can result in changes in the mechanical properties of the composite. The compatibility of the fiber and matrix is a measure of internal stress. The covalently bonded high strength fibers experience mostly elastic deformation before the fracture since the plastic deformation can happen due to dislocation motion. Whereas, metallic fibers have more space to plastically deform, so their composites exhibit a third stage where both fiber and the matrix are plastically deforming. Metallic fibers have many applications to work at cryogenic temperatures that is one of the advantages of composites with metal fibers over nonmetallic. A change in the angle between the applied stress and fiber orientation will affect the mechanical properties of fiber-reinforced composites, especially the tensile strength. This angle can be used predict the dominant tensile fracture mechanism. Anisotropy in the tensile strength of fiber reinforced composites can be removed by randomly orienting the fiber directions within the material. It sacrifices the ultimate strength in the aligned direction for an overall, isotopically strengthened material. Shock, impact, or repeated cyclic stresses can provoke the laminate to separate at the interface between two layers, a condition known as delamination. Individual fibers can separate from the matrix, for example, fiber pull-out. Composites can fail on the macroscopic or microscopic scale [5]. Compression failures can happen at both the macro scale or at each individual reinforcing fiber in compression buckling. Tension failures can be net section failures of the part or degradation of the composite at a microscopic scale where one or more of the layers in the composite failure in tension of the matrix or failure of the bond between the matrix and fibers. Some composites are b
description: Contributor: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com, Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China
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