the nature of materials manufacturing processes, met 1311 dr simin nasseri southern polytechnic...

THE NATURE OF MATERIALS

Manufacturing Processes, MET 1311Dr Simin Nasseri

Southern Polytechnic State University(© Fundamentals of Modern Manufacturing; Materials, Processes and Systems,

by M. P. Groover)

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Manufacturing Processes, Prof 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

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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

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Atomic Structure and the Elements

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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

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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

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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)

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Bonding between Atoms and Molecules

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Bonding between Atoms and MoleculesAtoms 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

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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

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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.

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Example: Sodium Chloride (NaCl)

Properties:

•Poor Ductility

•Low Electrical Conductivity


Covalent Bonding

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

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

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Examples: Diamond, Graphite

Properties:

•High Hardness

•Low Electrical Conductivity


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.

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Example: Metals Properties:

- Good Conductor (Heat and Electricity)- Good Ductility


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

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Crystalline Structures

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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

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Crystallinity

When 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:

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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!!

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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 (strongest at room temperature)

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Crystal Structures for Common Metals (FYI)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

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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

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Crystalline Structure

Growth of crystals in metals

Grain or phase

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

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Noncrystalline (Amorphous) Structures

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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: Glass =>

Many plastics

Rubber

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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?

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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.

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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

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A pure metal melts at a certain temperature. It absorbs the heat of fusion to change it’s state from solid to liquid.

Glass starts to get soft at “glass temperature”. Temperature keeps changing as glass’ state changes gradually from solid to liquid.


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

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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.

?

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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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the nature of materials manufacturing processes, met 1311 dr simin nasseri southern polytechnic...

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