THE NATURE OF MATERIALS - homes.ieu.edu.trhomes.ieu.edu.tr/oegilmez/oegilmez/Courses_files/CIE 205_Week01d.pdf · THE NATURE OF MATERIALS 1. ... Imperfections (Defects) in ... Difference

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


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



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


Simple Model of Atomic Structure for Several Atoms

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


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


Primary Bonds

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

§  Types: §  Ionic §  Covalent §  Metallic


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


Covalent Bonding

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


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


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


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


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


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


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


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


Three Crystal Structures in Metals

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


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


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


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


Edge Dislocation

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


Screw Dislocation

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


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


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

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“

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


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


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


Twinning

§  After plastic deformation


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


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


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


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


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


Volumetric Effects

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


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


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


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


Documents

THE NATURE OF MATERIALS - homes.ieu.edu.trhomes.ieu.edu.tr/oegilmez/oegilmez/Courses_files/CIE 205_Week01d.pdf · THE NATURE OF MATERIALS 1. ... Imperfections (Defects) in ... Difference