©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Periodic Table

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Simple Model of Atomic Structure for Several Atoms  (a) Hydrogen, (b) helium, (c) fluorine, (d) neon, and (e) sodium

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Primary Bonds  Characterized by strong atom ‑ to ‑ atom attractions that involve exchange of valence electrons  Following forms:  Ionic  Covalent  Metallic

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Covalent Bonding  Electrons are shared (as opposed to transferred) between atoms in their outermost shells to achieve a stable set of eight

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Two Examples of Covalent Bonding

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Metallic Bonding  Sharing of outer shell electrons by all atoms to form a general electron cloud that permeates the entire block

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Hydrogen Bonding  Occurs in molecules containing hydrogen atoms covalently bonded to another atom (e.g., H 2 O)  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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Three Crystal Structures in Metals  Three types of crystal structure: (a) body-centered cubic, (b) face-centered cubic, and (c) hexagonal close-packed

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Point Defects Imperfections in crystal structure involving either a single atom or a small number of atoms Point defects: (a) vacancy, (b) ion ‑ pair vacancy, (c) interstitialcy, (d) displaced ion (Frenkel Defect).

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Edge Dislocation Edge of an extra plane of atoms that exists in the lattice

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Screw Dislocation Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Elastic Strain  When a crystal experiences a gradually increasing stress, it first deforms elastically Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, no permanent change in positions of atoms

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Plastic Strain  If the stress is higher than forces holding atoms in their lattice positions, then a permanent shape change occurs Plastic deformation (slip), in which atoms in the crystal lattice structure are forced to move to new "homes“

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Twinning  After

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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 are transition zones, perhaps only a few atoms thick

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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 (a) (b)

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Volumetric Effects  Characteristic change in volume for a pure metal (a crystalline structure), compared to same volumetric changes in glass (a noncrystalline structure)

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version Summary: Characteristics of Ceramics  Most ceramics have crystalline structures, while glass (SiO 2 ) is amorphous  Molecules characterized by ionic or covalent bonding, or both  Properties: high hardness and stiffness, electrically insulating, refractory, and chemically inert

©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version 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, and low thermal conductivity, strength and stiffness vary widely