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BDS2 CDS DMS > 14. Metals and Alloys 2 > Flashcards

14. Metals and Alloys 2 Flashcards

1
Q

Definition of alloy

A

A combination (or mixture) of two or more metals or metal(s) with a metalloid (Fe, C)

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2
Q

Advantages of alloys over metals (2)

A

Improved mechanical properties - elastic limit, ultimate tensile strength, hardness
Improved corrosion resistance
Lower melting point than individual metal

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3
Q

Dental uses of alloys (4)

A

Steel (burs, instruments)
Amalgam (restorative material)
Gold alloy (inlays, crowns, bridges, partial dentures, wires)
Nickel chromium (crowns, bridges, wires)

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4
Q

Definition of phase

A

A physically distinct homogenous structure (can have more than one component)

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5
Q

Definition of one phase (2)

A

Grains composed of one metal only

Two metals in a homogenous mixture (solution) - solid solution (one phase)

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6
Q

Definition of two phases

A

Individual grains of two metals situated in a lattice network

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7
Q

Definition of solution

A

A homogenous mixture at an atomic scale (not a liquid)

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8
Q

Definition of solid solution

A

A common lattice structure containing two metals that are soluble in one another

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9
Q

Metals are normally soluble when

A

Molten

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10
Q

Upon crystallisation (cooling), metals may (3)

A

Be insoluble
Form an intermetallic compound with specific chemical formulation
Be soluble and form a solid solution

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11
Q

Features of insoluble metals upon crystallisation (2)

A

No common lattice

Two phases

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12
Q

Features of soluble metals upon crystallisation (2)

A

Form a common lattice

Two types

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13
Q

Types of solid solution (2)

A

Substitutional

Interstitial

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14
Q

Definition of substitutional solid solution

A

One in which the atoms of one metal replace the other metal in the crystal lattice/gain

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15
Q

Types of substitutional solid solution (2)

A

Random

Ordered

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16
Q

Features of random substitutional solid solution

A

Metal atoms similar in size, valency, crystal structure

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17
Q

Features of ordered substitutional solid solution

A

Metal atoms in regular lattice arrangements, similar in size, valency, crystal structure

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18
Q

Definition of interstitial solid solution

A

One in which atoms are markedly different in size and smaller atoms are located in spaces in the lattice/grain structure of larger atoms

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19
Q

Feature of metal cooling curve

A

Metals crystallise at one temperature

20
Q

Feature of alloy cooling curve

A

Alloys crystallise over a temperature range (TL to TS)

21
Q

Types of solubility of metals (2)

A

Soluble (solid solution formed – homogenous mix of metals in each grain)
Insoluble (grains of individual metals formed)

22
Q

Relationship of ratio of metals in alloys

A

The ratio of metals in alloys can vary – this can change the alloy properties (mechanical and thermal)

23
Q

Definition of liquidus

A

The line representing the temperature at which different alloy compositions begin to crystallise

24
Q

Definition of solidus

A

The line representing the temperature at which different alloy compositions have completely crystallised

25
Q

Features of slow cooling of molten metals (3)

A

Allows metal atoms to diffuse through the lattice
Ensures grain composition is homogenous
Results in large grains (not good)

26
Q

Features of rapid cooling of motel alloys (2)

A

Prevents atoms diffusing through lattice

Causes coring as composition varies throughout the grain

27
Q

Coring conditions (2)

A

Fast cooling of liquid state

Liquidus and solidus must be separated and determines extent of coring

28
Q

Features of coring

A

May reduce the corrosion resistance of the solid form of the alloy

29
Q

Fast cooling generates

A

Small grains

30
Q

Features of small grains produced by fast cooling (2)

A

Impede dislocation movement, improving mechanical properties of alloys
Will cause undesirable coring

31
Q

Definition of homogenising annealing

A

Process which reverses coring

32
Q

Process of homogenising annealing (3)

A

Solid alloy is formed by fast cooling (and coring occurs)
Alloy reheated to allow atoms to diffuse, causing grain composition to become homogenous
Kept below recrystallisation temperature, otherwise grain structures will be altered

33
Q

Alloys forming a solid solution and consisting of metals of different atomic size have a

A

Distorted grain structure

34
Q

Features of distorted grain structure (2)

A

Impedes dislocation movement

Improves mechanical properties (elastic limit, ultimate tensile strength, hardness)

35
Q

Dislocation movement in a metal lattice (2)

A

Defect ‘rolls’ over the atoms in the lattice plane

Little energy/force is required for the defect to move along the slip plane

36
Q

Dislocation movement in a solid solution (3)

A

Defect does not ‘roll’ over lattice plane; instead it falls into the larger space existing between large and small atoms
More energy/force is required for the defect to overcome the different-sized atoms and move along the lattice to the grain boundary
Hence it requires greater stress to move dislocations in a solid solution, making alloys inherently stronger (greater fracture resistance) than metals

37
Q

Features of alloys forming an ordered solid solution - atoms distributed at specific lattice sites (3)

A

Have a distorted grain structure
impedes dislocation movement
Improves mechanical properties (elastic limit, ultimate tensile strength and hardness)

38
Q

Definition of eutectic alloys

A

Alloys containing metals that are soluble in liquid state but insoluble in solid state

39
Q

Features of eutectic alloys (5)

A

Each metal forms physically distinct grains
Eutectic alloys show complete insolubility between the metals composing the alloy
Lowest melting point – at eutectic composition – is used for solder
Hard but brittle
Poor corrosion resistance

40
Q

Composition of eutectic alloys (2)

A

Where liquidus and solidus coincide (where the crystallisation process occurs at a single temperature)
Where grains of individual metals are formed simultaneously

41
Q

Non-eutectic composition of a eutectic alloys (3)

A

Excess metal crystallises first
Liquid reaches eutectic composition
Both metals crystallise (forming separate grains)

42
Q

Features of partially soluble alloy phase diagram (3)

A 
Has two important phases 
a-phase – solid solution
Mostly a (a-rich)
B-phase
Mostly B (B-rich)
43
Q

Features of partially soluble alloys (2)

A

Have a solubility limit line

Upon annealing, a supersaturated alloy will undergo precipitation hardening

44
Q

Definition of partially soluble alloys solubility limit line (2)

A

Indicates that a range of compositions of the metals (corresponding to the horizontal section of the solidus – H1 to H2) are not possible
Hence, the molten alloy composition of Z does not cool rapidly to produce a 50/50 grain compromising of X and Y; instead grains of a and B are formed

45
Q

Why are alloys better than metals

A

Better mechanical properties fracture strength, rigidity, elastic limit, surface hardness) due to solution, order and precipitation hardening

46
Q

Annealing removes

A

A cored structure (from fast/rapid cooling)

BDS2 CDS DMS (15 decks)

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