THE NATURE OF MATERIALS - homes.ieu.edu.trhomes.ieu.edu.tr/oegilmez/oegilmez/Courses_files/CIE...

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Simple Model of Atomic Structure for Several Atoms

§  (a) Hydrogen, (b) helium, (c) fluorine, (d) neon, and (e) sodium

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Primary Bonds

§  Characterized by strong atom‑to‑atom attractions that involve exchange of valence electrons

§  Types: §  Ionic §  Covalent §  Metallic

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Covalent Bonding

§  Electrons are shared (as opposed to transferred) between atoms in their outermost shells to achieve a stable set of eight

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Two Examples of Covalent Bonding

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Metallic Bonding

§  Sharing of outer shell electrons by all atoms to form a general electron cloud that permeates the entire block

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Three Crystal Structures in Metals

§  (a) Body-centered cubic, (b) face-centered cubic, and (c) hexagonal close-packed

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Point Defects

§  Imperfections in crystal structure involving either a single atom or a small number of atoms

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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, which can take two forms: §  Edge dislocation §  Screw dislocation

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Edge Dislocation

§  Edge of an extra plane of atoms that exists in the lattice

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Screw Dislocation

§  Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Elastic Strain

§  When a crystal experiences a gradually increasing stress, it first deforms elastically

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, no permanent change in positions of atoms

Plastic Strain

§  If the stress is higher than forces holding atoms in their lattice positions, then a permanent shape change occurs

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Plastic deformation (slip), in which atoms in the crystal lattice structure are forced to move to new "homes“

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Twinning

§  After plastic deformation

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

Volumetric Effects

§  Characteristic change in volume for a pure metal (a crystalline structure), compared to same volumetric changes in glass (a noncrystalline structure)

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

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

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e