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

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


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

Following forms: 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

Three types of crystal structure: (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


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 are transition zones, 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

(a) (b)


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, and low thermal conductivity, strength and stiffness vary widely

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Technology

molecules atoms

electrons bonds

hydrogen atoms

molecules primary bonds

ionic bonding atoms

lattice structure

line defects

types of crystal structure