14. Metals and Alloys 2

Question 1

Definition of alloy

Answer 1

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

Question 2

Advantages of alloys over metals (2)

Answer 2

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

Question 3

Dental uses of alloys (4)

Answer 3

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

Question 4

Definition of phase

Answer 4

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

Question 5

Definition of one phase (2)

Answer 5

Grains composed of one metal only

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

Question 6

Definition of two phases

Answer 6

Individual grains of two metals situated in a lattice network

Question 7

Definition of solution

Answer 7

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

Question 8

Definition of solid solution

Answer 8

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

Question 9

Metals are normally soluble when

Answer 9

Molten

Question 10

Upon crystallisation (cooling), metals may (3)

Answer 10

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

Question 11

Features of insoluble metals upon crystallisation (2)

Answer 11

No common lattice

Two phases

Question 12

Features of soluble metals upon crystallisation (2)

Answer 12

Form a common lattice

Two types

Question 13

Types of solid solution (2)

Answer 13

Substitutional

Interstitial

Question 14

Definition of substitutional solid solution

Answer 14

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

Question 15

Types of substitutional solid solution (2)

Answer 15

Random

Ordered

Question 16

Features of random substitutional solid solution

Answer 16

Metal atoms similar in size, valency, crystal structure

Question 17

Features of ordered substitutional solid solution

Answer 17

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

Question 18

Definition of interstitial solid solution

Answer 18

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

Question 19

Feature of metal cooling curve

Answer 19

Metals crystallise at one temperature

Question 20

Feature of alloy cooling curve

Answer 20

Alloys crystallise over a temperature range (TL to TS)

Question 21

Types of solubility of metals (2)

Answer 21

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

Question 22

Relationship of ratio of metals in alloys

Answer 22

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

Question 23

Definition of liquidus

Answer 23

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

Question 24

Definition of solidus

Answer 24

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

Question 25

Features of slow cooling of molten metals (3)

Answer 25

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

Question 26

Features of rapid cooling of motel alloys (2)

Answer 26

Prevents atoms diffusing through lattice | Causes coring as composition varies throughout the grain

Question 27

Coring conditions (2)

Answer 27

Fast cooling of liquid state | Liquidus and solidus must be separated and determines extent of coring

Question 28

Features of coring

Answer 28

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

Question 29

Fast cooling generates

Answer 29

Small grains

Question 30

Features of small grains produced by fast cooling (2)

Answer 30

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

Question 31

Definition of homogenising annealing

Answer 31

Process which reverses coring

Question 32

Process of homogenising annealing (3)

Answer 32

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

Question 33

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

Answer 33

Distorted grain structure

Question 34

Features of distorted grain structure (2)

Answer 34

Impedes dislocation movement | Improves mechanical properties (elastic limit, ultimate tensile strength, hardness)

Question 35

Dislocation movement in a metal lattice (2)

Answer 35

Defect ‘rolls’ over the atoms in the lattice plane | Little energy/force is required for the defect to move along the slip plane

Question 36

Dislocation movement in a solid solution (3)

Answer 36

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

Question 37

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

Answer 37

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

Question 38

Definition of eutectic alloys

Answer 38

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

Question 39

Features of eutectic alloys (5)

Answer 39

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

Question 40

Composition of eutectic alloys (2)

Answer 40

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

Question 41

Non-eutectic composition of a eutectic alloys (3)

Answer 41

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

Question 42

Features of partially soluble alloy phase diagram (3)

Answer 42

``` Has two important phases a-phase – solid solution Mostly a (a-rich) B-phase Mostly B (B-rich) ```

Question 43

Features of partially soluble alloys (2)

Answer 43

Have a solubility limit line | Upon annealing, a supersaturated alloy will undergo precipitation hardening

Question 44

Definition of partially soluble alloys solubility limit line (2)

Answer 44

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

Question 45

Why are alloys better than metals

Answer 45

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

Question 46

Annealing removes

Answer 46

A cored structure (from fast/rapid cooling)