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BDS2 CDS DMS > 13. Metals and Alloys 1 > Flashcards

13. Metals and Alloys 1 Flashcards

1
Q

Features of metals (2)

A

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

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2
Q

Definition of metal

A

An aggregate of atoms in a crystalline structure

Metals are the building blocks of alloys

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3
Q

Definition of alloy

A

A combination of metal atoms in a crystalline structure

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4
Q

Definition of elastic limit

A

Maximum stress a material can withhold without plastic deformation

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5
Q

Definition of ductility

A

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

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6
Q

Mechanical properties are based on (2)

A

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)

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7
Q

Factors affecting mechanical properties (3)

A

Crystalline structure
Grain size
Grain imperfections

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8
Q

Features of crystal growth (3)

A

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

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9
Q

Definition of grain boundary

A

Region where grains make contact

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

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10
Q

Types of grain structure (3)

A

Equi-axed
Radial
Fibrous

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11
Q

Definition of equi-axed grains

A

If crystal growth was of equal dimension in each direction

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12
Q

Features of radial grain structure

A

Molten metal cooled quickly in cylindrical mould

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13
Q

Features of fibrous grain structure

A

Wire pulled through die (cold worked metal/alloy)

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14
Q

Features of grains (2)

A

Varying sizes and shapes

Contain atoms arranged in a lattice-like structure

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15
Q

Types of crystal growth (2)

A 
Fast cooling (quenching)
Slow cooling
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16
Q

Best type of crystal growth

A

Fast cooling (quenching)

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17
Q

Features of fast cooling, quenching (2)

A 
More nuclei (more grains)
Small fine grains
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18
Q

Features of slow cooling (2)

A

Few nuclei

Large coarse grains

19
Q

How is crystal growth enhanced

A

Addition of nucleating agents

20
Q

How do nucleating agents enhance crystal growth

A

Impurities or additives act as foci for enhanced crystal growth

21
Q

Definition of grain

A

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

22
Q

What type of grains are advantageous

A

Small grains

23
Q

Why are small grains advantageous (3)

A

High elastic limit
Increased ultimate tensile strength
Increased hardness

24
Q

Why are small grains disadvantageous

A

Decreased ductility

25
Q

Factors for rapid cooling, quenching (4)

A

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

26
Q

Definition of dislocations

A

Imperfections/defects in the crystal lattice

Dislocations lead to an alteration of lattice structure and shape and are resistible

27
Q

SLIP is due to

A

Propagation of dislocation

Involves rupture of only a few bonds at a time

28
Q

Features of dislocations

A 
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
29
Q

Impeded movement of dislocation increases (3)

A

Elastic limit
Ultimate tensile strength
Hardness

30
Q

Impeded movement of dislocation decreases (2)

A

Ductility

Impact resistance

31
Q

Factors impeding dislocation movement (3)

A 
Grain boundaries (hence fine grains)
Alloys (different atom sizes)
Cold working (dislocations build-up at grain boundaries)
32
Q

Features of cold working (4)

A

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

33
Q

Cold work modifies grain structure increasing (3)

A

Elastic limit
Ultimate tensile strength
Hardness

34
Q

Cold work modifies grain structure decreasing (3)

A

Ductility
Impact strength
Corrosion resistance

35
Q

Effects of residual stress (2)

A

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

36
Q

Features of annealing

A

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

37
Q

Features of stress relief annealing (2)

A

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)

38
Q

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

A

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

39
Q

Recrystallisation spoils the benefits of

A

Cold work and allows further cold work

40
Q

Repeating cold work and recrystallisation

A

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

41
Q

Relationship between recrystallisation temperature and cold work (2)

A

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

42
Q

Features of excessive temperature rises

A

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

43
Q

Features of grain structure (2)

A

Influences mechanical properties

Influenced by dislocations (SLIP)

44
Q

Types of dental appliance manipulation (3)

A

Cold working
Stress relief annealing
Recrystallisation

BDS2 CDS DMS (15 decks)

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