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‫ 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.