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Transcript
السنة االولى/ قسم الهندسة الميكانيكية المحاضرة االولى/ مادة هندسة المعادن م اسراء فيصل غازي. م Classification of Materials Solid materials have been conveniently grouped into basic classifications: Metals Materials in this group are composed of one or more metallic elements (such as iron, aluminum, copper, titanium, gold, and nickel), and often also nonmetallic elements (for example, carbon, nitrogen, and oxygen) in relatively small amounts. Atoms in metals and their alloys are arranged in a very orderly manner, in comparison to the ceramics and polymers, are relatively dense, have good electrical and thermal conductivities, regard to mechanical characteristics, these materials are relatively stiff and strong yet are ductile. Ceramics Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. For example, some of the common ceramic materials include aluminum oxide (or alumina, Al2O3), silicon dioxide (or silica, SiO2), silicon carbide (SiC) and, in addition, what some refer to as the traditional ceramics—those composed of clay minerals (i.e., porcelain), as well as cement, and glass. With regard to mechanical behavior, ceramic materials are relatively stiff and strong also, ceramics are typically very hard. On the other hand, they are extremely brittle (lack ductility). These materials have low electrical conductivities, and are more resistant to high temperatures and harsh environments than metals and polymers. With regard to optical characteristics, ceramics may be transparent, translucent, or opaque Polymers Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (O, N, and Si). Some of the common and familiar polymers are polyethylene (PE), nylon, and silicone rubber. These materials typically have low densities. Whereas their mechanical characteristics are generally dissimilar to the metallic and ceramic materials—they are not as stiff nor as strong as these other material types. In addition, many of the polymers are extremely ductile (i.e., plastic. In general, they are relatively inert chemically and unreactive in a large number of environments. Furthermore, they have low electrical conductivities and are nonmagnetic. Composites A composite is composed of two (or more) individual materials, which come from the categories discussed above—metals, ceramics, and polymers. The design goal of a composite is to achieve a combination of properties that is not displayed by any single material, and also to incorporate the best characteristics of each of the component materials. One of the most common and familiar composites is fiberglass, in which small glass fibers are embedded within a polymeric material (normally an epoxy or polyester). The glass fibers are relatively strong and stiff (but also brittle), whereas the polymer is ductile (but also weak and flexible).The resulting fiberglass is relatively stiff, strong, flexible, and ductile. Also, it has a low density. Advanced materials Materials that are utilized in high-technology (or high-tech) applications are sometimes termed advanced materials. These advanced materials are typically traditional materials whose properties have been enhanced, and, also newly developed, high-performance materials. Furthermore, they may be of all material types (e.g., metals, ceramics, polymers), and are normally expensive. Advanced materials include semiconductors, biomaterials, and what we may term “materials of the future” (that is, smart materials and nanoengineered materials). for example, materials that are used for lasers, integrated circuits, magnetic information storage, liquid crystal displays (LCDs), and fiber optics. The structure of atoms It would be difficult to study metallurgy meaningfully without relating mechanical properties to the elementary forces acting between the atoms of which a metal is composed. Most of the elements are chemically reactive, so that we find very few of them in their elemental state in the Earth's crust—oxygen and nitrogen mixed together in the atmosphere are the most common, whilst a few metals such as copper, gold and silver, also occur uncombined. Most of the substances we encounter are either chemical compounds or mixtures. The difference between the two is that a compound is formed when there is a chemical join at the surfaces of two or more different atoms, whilst in a mixture only mechanical 'entangling' occurs between discrete particles of the two substances. For example, the powdered element sulphur can be mixed with iron filings and easily separated again by means of a magnet, but if the mixture is gently heated a vigorous chemical reaction proceeds and a compound called iron sulphide is formed. Each atom consists of a very small nucleus composed of protons and neutrons, which is encircled by moving electrons. Both electrons and protons are electrically charged, the charge magnitude being which is negative in sign for electrons and positive for protons; neutrons are electrically neutral. Each chemical element is characterized by the number of protons in the nucleus, or the atomic number this atomic number ranges in integral units from 1 for hydrogen to 92 for uranium .The atomic mass (A) of a specific atom may be expressed as the sum of the masses of protons and neutrons within the nucleus. Although the number of protons is the same for all atoms of a given element, the number of neutrons (N) may be variable. Thus atoms of some elements have two or more different atomic masses, which are called isotopes.