Lecture 2

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Transcript Lecture 2

THE NATURE OF MATERIALS

Atomic Structure and the Elements 2.

Bonding between Atoms and Molecules 3.

Crystalline Structures 4.

Noncrystalline (Amorphous) Structures 5.

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

semi-metals

Periodic Table

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

Bonding between Atoms and Molecules

  Atoms are held together in molecules by various types of bonds

Primary bonds

- generally associated with formation of molecules

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

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

 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

repeated - basic geometric grouping of atoms that is  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).

Line Defects

Connected group of point defects that forms a line in the lattice structure  Most important line defect is a

dislocation

Edge Dislocation

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

 After

Twinning

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

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 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

  Polymers usually carbon plus one or more other elements: H, N, O, and Cl  Many repeating

mers

in molecule held together by covalent bonding Amorphous (glassy) structure or mixture of amorphous and crystalline  Properties: low density, high electrical resistivity, and low thermal conductivity, strength and stiffness vary widely ©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version