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THE NATURE OF MATERIALS
1. Atomic Structure and the Elements
2. Bonding between Atoms and Molecules
3. Crystalline Structures
4. Noncrystalline (Amorphous) Structures
5. Engineering Materials
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Importance of Materials in Manufacturing
§ Manufacturing is a transformation process § It is the material that is transformed § And it is the behavior of the material when
subjected to the forces, temperatures, and other parameters of the process that determines the success of the operation
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Element Groupings
§ The elements can be grouped into families and relationships established between and within the families by means of the Periodic Table § Metals occupy the left and center portions of the
table § Nonmetals are on right § Between them is a transition zone containing
metalloids or semi-metals
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Periodic Table
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Atomic Structure and the Elements
§ The basic structural unit of matter is the atom
§ Each atom is composed of a positively charged nucleus, surrounded by a sufficient number of negatively charged electrons so the charges are balanced
§ More than 100 elements, and they are the chemical building blocks of all matter
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Simple Model of Atomic Structure for Several Atoms
§ (a) Hydrogen, (b) helium, (c) fluorine, (d) neon, and (e) sodium
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Bonding between Atoms and Molecules
§ Atoms are held together in molecules by various types of bonds 1. Primary bonds - generally associated with
formation of molecules 2. Secondary bonds - generally associated with
attraction between molecules § Primary bonds are much stronger than secondary
bonds
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Primary Bonds
§ Characterized by strong atom‑to‑atom attractions that involve exchange of valence electrons
§ Types: § Ionic § Covalent § Metallic
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Ionic Bonding
§ Atoms of one element give up their outer electron(s), which are in turn attracted to atoms of some other element to increase electron count in the outermost shell to eight
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Covalent Bonding
§ Electrons are shared (as opposed to transferred) between atoms in their outermost shells to achieve a stable set of eight
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Two Examples of Covalent Bonding
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Metallic Bonding
§ Sharing of outer shell electrons by all atoms to form a general electron cloud that permeates the entire block
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Secondary Bonds
§ Whereas primary bonds involve atom‑to‑atom attractive forces, secondary bonds involve attraction forces between molecules § No transfer or sharing of electrons § Bonds are weaker than primary bonds § Three forms:
1. Dipole forces 2. London forces 3. Hydrogen bonding
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Dipole Forces
§ Arise in a molecule comprised of two atoms with equal and opposite electrical charges
§ Each molecule therefore forms a dipole that attracts other molecules
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London Forces
§ Attractive force between non-polar molecules, i.e., atoms in molecule do not form dipoles
§ However, due to rapid motion of electrons in orbit, temporary dipoles form when more electrons are on one side
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Hydrogen Bonding
§ Occurs in molecules containing hydrogen atoms covalently bonded to another atom (e.g., H2O)
§ Since electrons to complete shell of hydrogen atom are aligned on one side of nucleus, opposite side has a net positive charge that attracts electrons in other molecules
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Macroscopic Structures of Matter
§ Atoms and molecules are the building blocks of a more macroscopic structure of matter
§ When materials solidify from the molten state, they tend to close ranks and pack tightly, arranging themselves into one of two structures: § Crystalline § Noncrystalline
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Crystalline Structure
§ Structure in which atoms are located at regular and recurring positions in three dimensions § Unit cell - basic geometric grouping of atoms that
is repeated § The pattern may be replicated millions of times
within a given crystal § Characteristic structure of virtually all metals, as
well as many ceramics and some polymers
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Three Crystal Structures in Metals
§ (a) Body-centered cubic, (b) face-centered cubic, and (c) hexagonal close-packed
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Crystal Structures for Common Metals
§ Room temperature crystal structures for some of the common metals: § Body-centered cubic (BCC)
§ Chromium, Iron, Molybdenum, Tungsten § Face-centered cubic (FCC)
§ Aluminum, Copper, Gold, Lead, Silver, Nickel § Hexagonal close-packed (HCP)
§ Magnesium, Titanium, Zinc
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Imperfections (Defects) in Crystals
§ Imperfections often arise due to inability of solidifying material to continue replication of unit cell, e.g., grain boundaries in metals
§ Imperfections can also be introduced purposely; e.g., addition of alloying ingredient in metal
§ Types of defects: (1) point defects, (2) line defects, (3) surface defects
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Point Defects
§ Imperfections in crystal structure involving either a single atom or a small number of atoms
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Point defects: (a) vacancy, (b) ion‑pair vacancy, (c) interstitialcy, (d) displaced ion (Frenkel Defect).
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Line Defects
§ Connected group of point defects that forms a line in the lattice structure
§ Most important line defect is a dislocation, which can take two forms: § Edge dislocation § Screw dislocation
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Edge Dislocation
§ Edge of an extra plane of atoms that exists in the lattice
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Screw Dislocation
§ Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis
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Surface Defects
§ Imperfections that extend in two directions to form a boundary
§ Examples: § External: the surface of a crystalline object is
an interruption in the lattice structure § Internal: grain boundaries are internal surface
interruptions
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Elastic Strain
§ When a crystal experiences a gradually increasing stress, it first deforms elastically
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Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, no permanent change in positions of atoms
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Plastic Strain
§ If the stress is higher than forces holding atoms in their lattice positions, then a permanent shape change occurs
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Plastic deformation (slip), in which atoms in the crystal lattice structure are forced to move to new "homes“
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Effect of Dislocations on Strain
§ In the series of diagrams, the movement of the dislocation allows deformation to occur under a lower stress than in a perfect lattice
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Slip on a Macroscopic Scale
§ Slip occurs many times over throughout the metal when subjected to a deforming load, thus causing it to exhibit its macroscopic behavior in the stress-strain relationship
§ Dislocations are a good‑news‑bad‑news situation § Good news in manufacturing – the metal is easier to
form § Bad news in design – the metal is not as strong as
the designer would like
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Twinning
§ A second mechanism of plastic deformation in which atoms on one side of a plane (the twinning plane) are shifted to form a mirror image of the other side
§ Before
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Twinning
§ After plastic deformation
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Polycrystalline Nature of Metals
§ A block of metal may contain millions of individual crystals, called grains
§ Such a structure is called polycrystalline
§ Each grain has its own unique lattice orientation
§ But collectively, the grains are randomly oriented in the block
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Grains and Grain Boundaries in Metals
§ How do polycrystalline structures form? § As a volume of metal cools from the molten state and
begins to solidify, individual crystals nucleate at random positions and orientations throughout the liquid
§ These crystals grow and finally interfere with each other, forming at their interface a surface defect - a grain boundary, which is a transition zone, perhaps only a few atoms thick
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Noncrystalline (Amorphous) Structures
§ Water and air have noncrystalline structures
§ A metal loses its crystalline structure when melted
§ Some engineering materials have noncrystalline forms in their solid state
§ Glass
§ Many plastics
§ Rubber
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Features of Noncrystalline Structures
§ Two features differentiate noncrystalline (amorphous) from crystalline materials: 1. Absence of long-range order in molecular
structure 2. Differences in melting and thermal expansion
characteristics
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Crystalline versus Noncrystalline Structures of Materials
§ Difference in structure between: (a) crystalline and (b) noncrystalline materials
§ Crystal structure is regular, repeating; noncrystalline structure is less tightly packed and random
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Volumetric Effects
§ Characteristic change in volume for a pure metal (a crystalline structure), compared to same volumetric changes in glass (a noncrystalline structure)
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Summary: Characteristics of Metals
§ Crystalline structures in the solid state, almost without exception
§ BCC, FCC, or HCP unit cells
§ Atoms held together by metallic bonding
§ Properties: high strength and hardness, high electrical and thermal conductivity
§ FCC metals are generally ductile
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Summary: Characteristics of Ceramics
§ Most ceramics have crystalline structures, while glass (SiO2) is amorphous
§ Molecules characterized by ionic or covalent bonding, or both
§ Properties: high hardness and stiffness, electrically insulating, refractory, and chemically inert
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Summary: Characteristics of Polymers
§ Many repeating mers in molecule held together by covalent bonding
§ Polymers usually carbon plus one or more other elements: H, N, O, and Cl
§ Amorphous (glassy) structure or mixture of amorphous and crystalline
§ Properties: low density, high electrical resistivity, low thermal conductivity, strength and stiffness vary widely
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