13. Metals and Alloys 1

Question 1

Features of metals (2)

Answer 1

Superior mechanical properties (strength, rigidity, elastic limit, ductility, hardness)
Poor aesthetics

Question 2

Definition of metal

Answer 2

An aggregate of atoms in a crystalline structure

Metals are the building blocks of alloys

Question 3

Definition of alloy

Answer 3

A combination of metal atoms in a crystalline structure

Question 4

Definition of elastic limit

Answer 4

Maximum stress a material can withhold without plastic deformation

Question 5

Definition of ductility

Answer 5

Amount of plastic deformation prior to fracture (measure of the extent a material can be shaped/manipulated)

Question 6

Mechanical properties are based on (2)

Answer 6

Choice of material (each has its own inherent characteristics)
Crystalline structure depends on history (method of production) and shaping (crucial for dental applications - cold working, swaging)

Question 7

Factors affecting mechanical properties (3)

Answer 7

Crystalline structure
Grain size
Grain imperfections

Question 8

Features of crystal growth (3)

Answer 8

Atoms at these sites act as nuclei of crystallisation
Crystals grow to form dendrites (3D branched lattice network(
Crystals (or grains) grown until they impinge on other crystals

Question 9

Definition of grain boundary

Answer 9

Region where grains make contact

Change in orientation of the crystal planes (impurities concentrate here)

Question 10

Types of grain structure (3)

Answer 10

Equi-axed
Radial
Fibrous

Question 11

Definition of equi-axed grains

Answer 11

If crystal growth was of equal dimension in each direction

Question 12

Features of radial grain structure

Answer 12

Molten metal cooled quickly in cylindrical mould

Question 13

Features of fibrous grain structure

Answer 13

Wire pulled through die (cold worked metal/alloy)

Question 14

Features of grains (2)

Answer 14

Varying sizes and shapes

Contain atoms arranged in a lattice-like structure

Question 15

Types of crystal growth (2)

Answer 15

Fast cooling (quenching)
Slow cooling

Question 16

Best type of crystal growth

Answer 16

Fast cooling (quenching)

Question 17

Features of fast cooling, quenching (2)

Answer 17

More nuclei (more grains)
Small fine grains

Question 18

Features of slow cooling (2)

Answer 18

Few nuclei

Large coarse grains

Question 19

How is crystal growth enhanced

Answer 19

Addition of nucleating agents

Question 20

How do nucleating agents enhance crystal growth

Answer 20

Impurities or additives act as foci for enhanced crystal growth

Question 21

Definition of grain

Answer 21

A single crystal (lattice) with atoms orientated in given directions (dendrites)

Question 22

What type of grains are advantageous

Answer 22

Small grains

Question 23

Why are small grains advantageous (3)

Answer 23

High elastic limit
Increased ultimate tensile strength
Increased hardness

Question 24

Why are small grains disadvantageous

Answer 24

Decreased ductility

Question 25

Factors for rapid cooling, quenching (4)

Answer 25

Small bulk Heat metal/alloy just above melting temperature Mould – high thermal conduction Quench

Question 26

Definition of dislocations

Answer 26

Imperfections/defects in the crystal lattice | Dislocations lead to an alteration of lattice structure and shape and are resistible

Question 27

SLIP is due to

Answer 27

Propagation of dislocation | Involves rupture of only a few bonds at a time

Question 28

Features of dislocations

Answer 28

``` Weak points Defect moves along the plane by SLIP Defect will move upon force Leads to a change in material shape Dislocations cannot move from one grain to another and therefore accumulate at grain boundaries ```

Question 29

Impeded movement of dislocation increases (3)

Answer 29

Elastic limit Ultimate tensile strength Hardness

Question 30

Impeded movement of dislocation decreases (2)

Answer 30

Ductility | Impact resistance

Question 31

Factors impeding dislocation movement (3)

Answer 31

``` Grain boundaries (hence fine grains) Alloys (different atom sizes) Cold working (dislocations build-up at grain boundaries) ```

Question 32

Features of cold working (4)

Answer 32

Work hardening/strain hardening Involves work being done on the metal/alloy (bending, rolling, swaging) at low temp. (below recrystallisation temp.) Cold working causes SLIP (dislocations collect at grain boundaries) and a stronger, harder material Cold work results in internal stresses - may lead to appliance distortion over time

Question 33

Cold work modifies grain structure increasing (3)

Answer 33

Elastic limit Ultimate tensile strength Hardness

Question 34

Cold work modifies grain structure decreasing (3)

Answer 34

Ductility Impact strength Corrosion resistance

Question 35

Effects of residual stress (2)

Answer 35

Instability in the lattice and results in distortion over time This is undesirable and is relieved through annealing

Question 36

Features of annealing

Answer 36

Involves heating a metal or alloy so that greater thermal vibrations allow migration of atoms (re-arrangement of atoms)

Question 37

Features of stress relief annealing (2)

Answer 37

Eliminates stresses by allowing atoms to re-arrange within grains Grain structure and mechanical properties are unaltered, and some further cold work is possible (final shaping)

Question 38

Recrystallisation occurs when the metal/alloy is heated causing (3)

Answer 38

New smaller, equi-axed grains Lower elastic limit, ultimate tensile strength and hardness Increased ductility

Question 39

Recrystallisation spoils the benefits of

Answer 39

Cold work and allows further cold work

Question 40

Repeating cold work and recrystallisation

Answer 40

Cold work/recrystallisation can be repeated until the correct shape is obtained

Question 41

Relationship between recrystallisation temperature and cold work (2)

Answer 41

The recrystallisation temperature depends on the amount of cold work The greater the amount of cold work, the lower the recrystallisation temperature

Question 42

Features of excessive temperature rises

Answer 42

Causes large grains to replace smaller coarse grains, yielding poorer mechanical properties

Question 43

Features of grain structure (2)

Answer 43

Influences mechanical properties | Influenced by dislocations (SLIP)

Question 44

Types of dental appliance manipulation (3)

Answer 44

Cold working Stress relief annealing Recrystallisation