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The nature of materials

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  1. 1. THE NATURE OF MATERIALS <ul><li>Atomic Structure and the Elements </li></ul><ul><li>Bonding between Atoms and Molecules </li></ul><ul><li>Crystalline Structures </li></ul><ul><li>Noncrystalline (Amorphous) Structures </li></ul><ul><li>Engineering Materials </li></ul>
  2. 2. Why Materials are Important in Manufacturing <ul><li>Manufacturing is a transformation process </li></ul><ul><ul><li>It is the material that is transformed </li></ul></ul><ul><ul><li>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 </li></ul></ul>
  3. 3. Atomic Structure and the Elements <ul><li>The basic structural unit of matter is the atom </li></ul><ul><li>Each atom is composed of a positively charged nucleus, surrounded by a sufficient number of negatively charged electrons so the charges are balanced </li></ul><ul><li>More than 100 elements, and they are the chemical building blocks of all matter </li></ul>
  4. 4. Element Groupings <ul><li>The elements can be grouped into families and relationships established between and within the families by means of the Periodic Table </li></ul><ul><ul><li>Metals occupy the left and center portions of the table </li></ul></ul><ul><ul><li>Nonmetals are on right </li></ul></ul><ul><ul><li>Between them is a transition zone containing metalloids or semi‑metals </li></ul></ul>
  5. 5. <ul><li>Figure 2.1 ‑ Periodic Table of Elements. The atomic number and symbol are listed for the 103 elements </li></ul>
  6. 6. Bonding between Atoms and Molecules <ul><li>Atoms are held together in molecules by various types of bonds </li></ul><ul><ul><li>Primary bonds - generally associated with formation of molecules </li></ul></ul><ul><ul><li>Secondary bonds - generally associated with attraction between molecules </li></ul></ul><ul><li>Primary bonds are much stronger than secondary bonds </li></ul>
  7. 7. Primary Bonds <ul><li>Characterized by strong atom‑to‑atom attractions that involve exchange of valence electrons </li></ul><ul><li>Following forms: </li></ul><ul><ul><li>Ionic </li></ul></ul><ul><ul><li>Covalent </li></ul></ul><ul><ul><li>Metallic </li></ul></ul>
  8. 8. <ul><li>Ionic Bonding </li></ul><ul><li>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 </li></ul>Figure 2.4 ‑ Three forms of primary bonding: (a) ionic
  9. 9. <ul><li>Covalent Bonding </li></ul><ul><li>Electrons are shared (as opposed to transferred) between atoms in their outermost shells to achieve a stable set of eight </li></ul>Figure 2.4 ‑ Primary bonding: (b) covalent
  10. 10. <ul><li>Metallic Bonding </li></ul><ul><li>Sharing of outer shell electrons by all atoms to form a general electron cloud that permeates the entire block </li></ul>Figure 2.4 ‑ Primary bonding: (c) metallic
  11. 11. Secondary Bonds <ul><li>Whereas primary bonds involve atom‑to‑atom attractive forces, secondary bonds involve attraction forces between molecules </li></ul><ul><li>No transfer or sharing of electrons in secondary bonding, and bonds are weaker than primary bonds </li></ul><ul><li>Three forms: </li></ul><ul><ul><li>Dipole forces </li></ul></ul><ul><ul><li>London forces </li></ul></ul><ul><ul><li>Hydrogen bonding </li></ul></ul>
  12. 12. <ul><li>Dipole Forces </li></ul><ul><li>Arise in a molecule comprised of two atoms with equal and opposite electrical charges </li></ul><ul><li>Each molecule therefore forms a dipole that attracts other molecules </li></ul>Figure 2.6 ‑ Types of secondary bonding: (a) dipole forces
  13. 13. <ul><li>London Forces </li></ul><ul><li>Attractive force between nonpolar molecules, i.e., atoms in molecule do not form dipoles </li></ul><ul><li>However, due to rapid motion of electrons in orbit, temporary dipoles form when more electrons are on one side </li></ul>Figure 2.6 ‑ Secondary bonding: (b) London forces
  14. 14. <ul><li>Hydrogen Bonding </li></ul><ul><li>Occurs in molecules containing hydrogen atoms covalently bonded to another atom (e.g., H 2 O) </li></ul><ul><li>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 </li></ul>Figure 2.6 ‑ Secondary bonding: (c) hydrogen bonding
  15. 15. Macroscopic Structures of Matter <ul><li>Atoms and molecules are the building blocks of more macroscopic structure of matter </li></ul><ul><li>When materials solidify from the molten state, they tend to close ranks and pack tightly, arranging themselves into one of two structures: </li></ul><ul><ul><li>Crystalline </li></ul></ul><ul><ul><li>Noncrystalline </li></ul></ul>
  16. 16. Crystalline Structure <ul><li>Structure in which the atoms are located at regular and recurring positions in three dimensions </li></ul><ul><li>Unit cell - basic geometric grouping of atoms that is repeated </li></ul><ul><li>The pattern may be replicated millions of times within a given crystal </li></ul><ul><li>Characteristic structure of virtually all metals, as well as many ceramics and some polymers </li></ul>
  17. 17. <ul><li>Figure 2.7 ‑ Body‑centered cubic (BCC) crystal structure: </li></ul><ul><ul><ul><ul><li>unit cell, with atoms indicated as point locations in a three‑dimensional axis system </li></ul></ul></ul></ul><ul><ul><ul><ul><li>unit cell model showing closely packed atoms (sometimes called the hard‑ball model) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>repeated pattern of the BCC structure </li></ul></ul></ul></ul>
  18. 18. <ul><li>Figure 2.8 ‑ Three types of crystal structures in metals: </li></ul><ul><ul><ul><ul><ul><li>body‑centered cubic </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>face‑centered cubic </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>hexagonal close‑packed </li></ul></ul></ul></ul></ul>
  19. 19. Crystal Structures for Common Metals (at Room Temperature) <ul><li>Body‑centered cubic (BCC) </li></ul><ul><ul><li>Chromium, Iron, Molybdenum, Tungsten </li></ul></ul><ul><li>Face‑centered cubic (FCC) </li></ul><ul><ul><li>Aluminum, Copper, Gold, Lead, Silver, Nickel </li></ul></ul><ul><li>Hexagonal close‑packed (HCP) </li></ul><ul><ul><li>Magnesium, Titanium, Zinc </li></ul></ul>
  20. 20. Imperfections (Defects) in Crystals <ul><li>Imperfections often arise due to inability of solidifying material to continue replication of unit cell, e.g., grain boundaries in metals </li></ul><ul><li>Imperfections can also be introduced purposely; e.g., addition of alloying ingredient in metal </li></ul><ul><li>Types of defects: </li></ul><ul><ul><li>Point defects </li></ul></ul><ul><ul><li>Line defects </li></ul></ul><ul><ul><li>Surface defects </li></ul></ul>
  21. 21. <ul><li>Point Defects </li></ul><ul><li>Imperfections in crystal structure involving either a single atom or a few number of atoms </li></ul>Figure 2.9 ‑ Point defects: (a) vacancy, (b) ion‑pair vacancy, (c) interstitialcy, (d) displaced ion (Frenkel Defect)
  22. 22. Line Defects <ul><li>Connected group of point defects that forms a line in the lattice structure </li></ul><ul><li>Most important line defect is a dislocation , which can take two forms: </li></ul><ul><ul><li>Edge dislocation </li></ul></ul><ul><ul><li>Screw dislocation </li></ul></ul>
  23. 23. <ul><li>Edge Dislocation </li></ul><ul><li>Edge of an extra plane of atoms that exists in the lattice </li></ul>Figure 2.10 ‑ Line defects: (a) edge dislocation
  24. 24. <ul><li>Screw Dislocation </li></ul><ul><li>Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis </li></ul>Figure 2.10 ‑ Line defects: (b) screw dislocation
  25. 25. Surface Defects <ul><li>Imperfections that extend in two directions to form a boundary </li></ul><ul><li>Examples: </li></ul><ul><ul><li>External: the surface of a crystalline object is an interruption in the lattice structure </li></ul></ul><ul><ul><li>Internal: grain boundaries are internal surface interruptions </li></ul></ul>
  26. 26. <ul><li>Elastic Strain </li></ul><ul><li>When a crystal experiences a gradually increasing stress, it first deforms elastically </li></ul><ul><li>If force is removed lattice structure returns to its original shape </li></ul><ul><ul><li>Figure 2.11 ‑ </li></ul></ul><ul><ul><li>Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, with no permanent change in positions of atoms </li></ul></ul>
  27. 27. <ul><li>Plastic Strain </li></ul><ul><li>If stress is higher than forces holding atoms in their lattice positions, a permanent shape change occurs </li></ul><ul><li>Atoms have permanently moved from their previous locations, and a new equilibrium lattice is formed </li></ul><ul><ul><li>Figure 2.11 ‑ </li></ul></ul><ul><ul><li>Deformation of a crystal structure: (c) plastic deformation ( slip ), in which atoms in the lattice are forced to move to new &quot;homes&quot; </li></ul></ul>
  28. 28. <ul><li>Figure 2.12 ‑ Effect of dislocations in the lattice structure under stress </li></ul><ul><li>In the series of diagrams, the movement of the dislocation allows deformation to occur under a lower stress than in a perfect lattice </li></ul>
  29. 29. Slip on a Macroscopic Scale <ul><li>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 </li></ul><ul><li>Dislocations are a good‑news‑bad‑news situation </li></ul><ul><ul><li>Good news in manufacturing – the metal is easier to form </li></ul></ul><ul><ul><li>Bad news in design – the metal is not as strong as the designer would like </li></ul></ul>
  30. 30. <ul><li>Twinning </li></ul><ul><li>A second mechanism of plastic deformation in which atoms on one side of a plane (called the twinning plane ) are shifted to form a mirror image of the other side </li></ul>Figure 2.13 ‑ Twinning, involving the formation of an atomic mirror image (i.e., a &quot;twin&quot;) on the opposite side of the twinning plane: (a) before, and (b) after twinning
  31. 31. The Polycrystalline Nature of Metals <ul><li>A block of metal may contain millions of individual crystals, called grains </li></ul><ul><li>Such a structure is called polycrystalline </li></ul><ul><li>Each grain has its own unique lattice orientation; but collectively, the grains are randomly oriented in the block </li></ul>
  32. 32. Grains and Grain Boundaries in Metals <ul><li>How do polycrystalline structures form? </li></ul><ul><ul><li>As a block (of metal) cools from the molten state and begins to solidify, individual crystals nucleate at random positions and orientations throughout the liquid </li></ul></ul><ul><ul><li>These crystals grow and finally interfere with each other, forming at their interface a surface defect ‑ a grain boundary </li></ul></ul><ul><ul><li>Grain boundaries are transition zones, perhaps only a few atoms thick </li></ul></ul>
  33. 33. Noncrystalline (Amorphous) Structures <ul><li>Many materials are noncrystalline </li></ul><ul><ul><li>Water and air have noncrystalline structures </li></ul></ul><ul><ul><li>A metal loses its crystalline structure when melted </li></ul></ul><ul><li>Important engineering materials have noncrystalline forms in their solid state </li></ul><ul><ul><li>Glass </li></ul></ul><ul><ul><li>Many plastics </li></ul></ul><ul><ul><li>Rubber </li></ul></ul>
  34. 34. Features of Noncrystalline (Amorphous) Structures <ul><li>Two features differentiate noncrystalline from crystalline materials: </li></ul><ul><ul><li>Absence of long‑range order in molecular structure </li></ul></ul><ul><ul><li>Differences in melting and thermal expansion characteristics </li></ul></ul>
  35. 35. <ul><li>Figure 2.14 ‑ Illustration of difference in structure between: (a) crystalline and (b) noncrystalline materials. The crystal structure is regular, repeating, and denser; while the noncrystalline structure is more loosely packed and random </li></ul>
  36. 36. <ul><li>Figure 2.15 ‑ Characteristic change in volume for a pure metal (a crystalline structure), compared to the same volumetric changes in glass (a noncrystalline structure) </li></ul>
  37. 37. Characteristics of Metals <ul><li>Crystalline structures in the solid state, almost without exception </li></ul><ul><li>BCC, FCC, or HCP unit cells </li></ul><ul><li>Atoms held together by metallic bonding </li></ul><ul><li>Properties: high strength and hardness, high electrical and thermal conductivity </li></ul><ul><li>FCC metals are generally ductile </li></ul>
  38. 38. Characteristics of Ceramics <ul><li>Most ceramics have crystal structure, while glass (SiO 2 ) is amorphous </li></ul><ul><li>Molecules characterized by ionic or covalent bonding, or both </li></ul><ul><li>Properties: high hardness and stiffness, electrically insulating, refractory, and chemically inert </li></ul>
  39. 39. Characteristics of Polymers <ul><li>Many repeating mers in molecule held together by covalent bonding </li></ul><ul><li>Polymers usually carbon plus one or more other elements: H, N, O, and Cl </li></ul><ul><li>Amorphous (glassy) structure or mixture of amorphous and crystalline </li></ul><ul><li>Properties: low density, high electrical resistivity, and low thermal conductivity, strength and stiffness vary widely </li></ul>