THE NATURE OF MATERIALS Manufacturing Processes, 1311 Dr Simin Nasseri Southern Polytechnic State...

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THE NATURE OF MATERIALS

Manufacturing Processes, 1311

Dr Simin Nasseri

Southern Polytechnic State University

Manufacturing ProcessesProf Simin Nasseri

THE NATURE OF MATERIALS

1. Atomic Structure and the Elements

2. Bonding between Atoms and Molecules

3. Crystalline Structures

4. Noncrystalline (Amorphous) Structures

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

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

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 tableNonmetals are on rightBetween them is a transition zone containing metalloids or semi‑metals

Metals Metalloids or Semimetals

NonMetals

Beryllium – Be Boron – B Helium – He

Lithium – Li Silicon – Si Neon – Ne

Magnesium – Mg Arsenic – As Argon – Ar

Cadmium – Cd Antimony – Sb Krypton – Kr

Copper- Cu Polonium - Po Xenon – Xe

Iron – Fe Tellurium - Te Radon – Rn

Zinc – Zn Germanium - Ge Fluorine – F

Titanium – Ti Chlorine – Cl

Gold – Au Oxygen – O

Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.

Periodic Table

Question?What are the noble metals?

CopperSilverGold

Noble metals (precious metals) are metals that are resistant to corrosion or oxidation, unlike most base metals.

Platinum (Pt), Palladium (Pd)

Bonding between Atoms and Molecules

Atoms are held together in molecules by various types of bonds1. 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 Bonding

SecondaryBonding

IonicCovalentMetallic

Dipole forcesLondon forcesHydrogen bonding

Primary Bonds

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

Following forms: Ionic Covalent Metallic

The ones on the outer shell

Ionic Bonding

Figure 2.4 First form of primary bonding: (a) Ionic

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.

Properties:

Poor Ductility

Low Electrical Conductivity

Example: Sodium Chloride (NaCl)

Covalent Bonding

Figure 2.4 Second form of primary bonding: (b) covalent

Outer electrons are shared between two local atoms of different elements.

Properties:

High Hardness

Low Electrical Conductivity

Examples: Diamond, Graphite

Metallic Bonding

Figure 2.4 Third form of primary bonding: (c) metallic

Outer shell electrons are shared by all atoms to form an electron cloud.

Properties:- Good Conductor (Heat and Electricity)- Good Ductility

Secondary Bonds

Secondary bonds involve attraction forces between molecules (whereas primary bonds involve

atom‑to‑atom attractive forces), No transfer or sharing of electrons in

secondary bonding Bonds are weaker than primary bonds Three forms:

1. Dipole forces

2. London forces

3. Hydrogen bonding

Macroscopic Structures of Matter

Atoms and molecules are the building blocks of 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 Structures

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

CrystallinityWhen the monomers are arranged in a neat orderly manner, the

polymer is crystalline. Polymers are just like socks. Sometimes they are arranged in a neat orderly manner.

An amorphous solid is a solid in which the molecules have no order or arrangement. Some people will just throw their socks in the drawer in one

big tangled mess. Their sock drawers look like this:

Question?

"What is glass... is it a liquid or a solid?"

What about glass?! Does glass

have a crystalline structure?!

• Antique windowpanes are thicker at the bottom, because glass has flowed to the bottom over time!

• Glass has no crystalline structure, hence it is NOT a solid. • Glass is a supercooled liquid. • Glass is a liquid that flows very slowly. • Glass is a highly viscous liquid!!

Three Crystal Structures in Metals

1. Body-centered cubic (BCC)

2. Face centered cubic (FCC)

3. Hexagonal close-packed (HCP)

Figure 2.8 Three types of crystal structure in metals.

# of atoms in unit cell: 9 # of atoms: 14 # of atoms: 17

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, (Iron at 1670oF) 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

It is in fact: Deviation in the regular pattern of the crystalline lattice structure.

Studying about imperfections is important:Imperfection is bad: a perfect diamond (with no flaws) is

more valuable than one containing imperfections.Imperfection is good: the addition of an alloying

ingredient in a metal to increase its strength (this is

an imperfection which is introduced purposely).

Types of defects or imperfections

Point defects, Line defects, Surface defects.

Point Defects

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

Figure 2.9 Point defects: (a) vacancy, (b) ion‑pair vacancy (Schottky), (c) interstitialcy, (d) displaced ion (Frenkel Defect).

Extra atom present

Dislocation of an atom

Line Defects

Defect happens along a line ( 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

Figure 2.10 Line defects: (a) edge dislocation

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

Screw Dislocation

Figure 2.10 Line defects: (b) 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

If force is removed lattice structure returns to its original shape

Figure 2.11 Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, with no permanent change in positions of atoms.

Plastic Strain

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

Figure 2.11 Deformation of a crystal structure: (c) plastic deformation (slip), in which atoms in the lattice 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 involves the relative movement of atoms on the opposite sides of a plane in the lattice, called slip plane.

Figure 2.12 Effect of dislocations in the lattice structure under stress.

Slip on a Macroscopic Scale

When a lattice structure with an edge dislocation is subjected to a shear stress, the material deforms much more readily than in a perfect structure.

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

Figure 2.13 Twinning, involving the formation of an atomic mirror image on the opposite side of the twinning plane: (a) before, and (b) 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

Crystalline Structure

Growth of crystals in metals

How do polycrystalline structures form? As a block 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

Grain boundaries are transition zones, perhaps only a few atoms thick

Grainboundary

Noncrystalline (Amorphous) Structures

Many materials are noncrystalline

Water and air have noncrystalline structures

A metal loses its crystalline structure when melted

Some important engineering materials have noncrystalline forms in their solid state:

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

What are the differences

between them?

Crystalline versus Noncrystalline

Figure 2.14 Difference in structure between: (a) crystalline and (b) noncrystalline materials.

The crystal structure is regular, repeating, and denser

The noncrystalline structure is random and less tightly packed.

Solidification

Alloy Metal Pure Metal

Volumetric Effects

Figure 2.15 Characteristic change in volume for a pure metal (a crystalline structure), compared to the same volumetric changes in glass (a noncrystalline structure).

Tg=glass temperatureTm=melting temperature

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

Refractory materials retain their strength at high temperatures. They are used to make crucibles and linings for furnaces, kilns and incinerators.

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

THE NATURE OF MATERIALS Manufacturing Processes, 1311 Dr Simin Nasseri Southern Polytechnic State...

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