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SCE3103 EXPLORING MATERIALS

TOPIC 4: MANUFACTURED MATERIAL ALLOYS

Alloys

1. Pure metals are malleable and ductile.

2. The high malleability and ductility of metals can be explained by thearrangement of their atoms.

3. Metals are composed of identical atoms. The atoms in a metal are packedtogether as close as possible and arranged in an orderly manner.

4. Metal atoms are arranged orderly from one layer to another.

5. When a force is applied, the layers of atoms start to slide or slip over each other.

6. The ability of the layers of atoms slip easily over each other makes metalsmalleable and ductile.

7. Thus, a metal can be hammered into different shapes or pulled into a wire.

8. Pure metals are weak and soft due to their ductility and malleability.

9. To improve the properties of a pure metal, it is made into an alloy.

10. An alloy is a material that contains a mixture of elements and has metallic

properties.

11. Most alloys are mixtures of metals. Some alloys may contain mixtures of metal

Learning Outcomes: State the composition of metals in alloys State the properties of alloys

State the use of alloys in our daily life Carry out PCK activities to teach relevant curriculum area on alloys

Investigate and compare the rate of rusting of iron, steel and stainless steel

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and non-metal such as carbon.

12. When metals are converted to alloys, they become harder, stronger, moreresistant to corrosion and have better appearance.

Composition, properties and uses of alloys

Alloy Composition Properties Uses

Steel Iron 99%Carbon 1% High strength Car bodies, bridges,ships

Stainless

steel

Iron 80.6%, carbon 0.4%,

chromium 18%, Nickel 1%

Great resistance to corrosion Knives, sinks, cutlery

Pewter Tin 91%, antimony 7%,

copper 2%

High strength

Resistance to corrosion

Bright shiny surface

Ornaments, souvenirs

Bronze Copper 90%, tin 10% High strength

Resistance to corrosion

Medals, art objects

Duralumin Aluminium 95%Copper 5%

High strengthResistance to corrosion

Low density

Aircraft, racingmountain bicycle

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Why make alloys?

1. To improve the appearance of the pure metal

Metals have lustrous surface.

However, the exposed metal surface quickly loses its shine due to theformation of an oxide layer.

Alloying helps to prevent formation of the oxide layer and enables the

metal to keep its surface shiny.

Example: Pewter

2. To increase the strength and hardness of the pure metal

For example, carbon atoms which are smaller than iron atoms are added

into iron atoms during the making of steel.

As a result, the uniformity of the arrangement of iron atoms is disrupted

and it is more difficult for the layers of the iron atoms to slide over one

another.

This makes steel harder and stronger than pure iron.

The presence of the extra bonds provided by the carbon atoms also causes

the metal lattice to become harder, stronger, and less ductile.

3. To increase the resistance to corrosion

Most metals such as iron and copper corrode readily in the air.

Alloying can prevent metals from corrosion.

Example: Carbon, chromium and nickel are added to iron to make stainlesssteel.

Why doesn't stainless steel rust?

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Alloys

1. Alloys can be classified as(a) solution alloys(b) heterohebeous alloys(c) intermetallic compounds

2. Solution alloys are homogeneous mixtures in which the components aredispersed uniformly.

(a) There are two types of solution alloy: substitutional alloys

interstitial alloys

Substitutional alloys

atoms must have similar atomicradii,

elements must have similar

bonding characteristics.

Interstitial alloys

Solute atoms smaller than solventatoms (in order to fit into the

interstitial site), e.g. a nonmetal.

Chemistry in Our Life

1. Pure gold is termed as 24 carats and it is too soft to be used in making jewelry.

2. Alloys of gold, with copper or silver, have suitable hardness to be used in

jewelry.

3. 18-carat gold contains 75 percent (18

24x 100) of gold.

24-carats gold 99.9%

18-carats gold 75%

22-carats gold 91.6%

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(b) Interstitial alloys: The presence of the extra bonds provided by the interstitial component

causes the metal lattice to become harder, stronger, and less ductile.

Example: Steel is much harder and stronger than pure iron.

Mild steel < 0.2% carbon Cables, nails, chains

Medium steel 0.2 0.6% C Girders, rails

High-carbon steel 0.6 1.5% C Cutlery, tools, springs

Stainless steel (Alloy steel) 0.4% C, 18% Cr, 1% Ni Knives, sinks, cutlery

3. In a heterogeneous alloy the components are not dispersed uniformly.

For example: In the form of steel known as pearlite, two distinct phasesalmost pure Fe and the compound Fe3C (cementite) are present in

alternating layers.

4. Intermetallic compounds are homogeneous alloys that have definite propertiesand compositions.

For example, copper and aluminium form a compound, CuAl2, known asduraluminum.

The intermetallic compound Ni3Al is a major component of jet aircraftengines because of its strength and low density.

Razor blades are often coated with Cr3Pt, which adds hardness, allowingthe blade to stay sharp longer.

The compound Co5Sm is used in the permanent magnets in lightweightheadsets and high-fidelity speakers because of its high magnetic strength

per unit weight.