THE NATURE AND PROPERTIES OF

MATERIALS

www.carsparefinder.co.uk Contd… from lecture 2

Edge DislocationEdge of an extra plane of atoms that

exists in the lattice

Symbol

Screw DislocationSpiral within the lattice structure wrapped

around an imperfection line, like a screw is wrapped around its axis

Screw Dislocation

Planar Defects

Imperfections that extend in two directions to form a boundary are called Planar Defects

• Examples:– Twining (Ti, Zn, Mg)– grain boundaries are internal surface

interruptions

Twinning

Twinning can be defined as a mechanism of deformation in which atoms in one side of a plane are shifted to form amirror image of the other side of the plane.

Twinning: microstructure

Non-crystalline Materials

Absence of long-range order in the molecular structure in a non-crystalline materials

Crystalline Non-Crystalline (Amorphous)

Non-crystalline Materials: Examples

Metallic glass

Crystalliene vs. Non-crystalline Materials: difference in behaviour

Supercooledliquid

m

Tg : Glass transitiontemperature

Tm : Melting temperature

ENGINEERING MATERIALS

Materials

Metal Non-metal

Ferrous

Non-Ferrous

CeramicsComposites

Plastics

ENGINEERING MATERIALS

Ferrous Materials

Steels

Fe-C alloy with C content varying from 0.02 – 2 wt%

Classification• Plain C steels• Alloy steels

Fe-C phase diagram: basis for steels and cast irons

Wt % C

•

•

20 40 60 80 10001000

1100

1200

1300

1400

1500

1600T(°C)

L (liquid)

α (FCC solid solution)

L + αliquidus

solidus

Wt% Ni

Phase Diagram

Solid Solution

Tetrahedral Void

Octahedral Void

Plain C Steels

Classification• Low C steel: < 0.3% C• Medium C Steel: 0.3 – 0.8% C• High C Steel: > 0.8% C

Small amounts of other elements (0.4% Mn, 0.05% S, 0.04% P)

↑ C content ↑ hardness and ↓ ductility

Alloy Steels

Definition• Alloy of iron and carbon containing high

amount of alloying element other than elements in carbon steels.

Properties• Gives better strength , ductility and

toughness compared to plain carbon steels.

Types of alloy steels• Stainless steel

– Large family of steel containing at least 11.5% Cr . Corrosion resistance due to formationof Cr2O3

• Tool steels– Contains elements such as C (0.8-1.3 %), Mn

(0.2-1.6 %) , Cr , W , Va , Mo , Co .– A class of high C alloy steels that is both

shock and wear resistant. – Withstand high temperatures.

Cast IronManufacture• Product that comes out of the BF is Pig

Iron– Composition: 3-4 % C, 1.5-2.5 % Mn , 1-3 %

Si + S and P• When it is cast directly it is called Cast Iron

• Good corrosion resistance– Corrodes readily under atmospheric conditions, then

protective film will form– Used in water pipes.

• Good vibration damping capacity– More than 100 times better than steel and 2000 times

better than Al– Fatigue resistant

• Machinability: Good to bad

UsageEngine blocksMachine frames