Metals and Alloys 2

Question 1

What advantages does an alloy have over a metal

Answer 1

Superior mechanical properties - EL stress, FS, rigidity and hardness
Higher corrosion resistance
Lower melting point

Question 2

What are some common alloy uses in dentistry

Answer 2

Steel - burs and instruments
Gold alloy - inlays, crowns, bridges, partial dentures
CoCr - partial dentures
Amalgam - restorative material

Question 3

What is a phase

Answer 3

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

Question 4

What is a solution

Answer 4

A homogenous mixture at an atomic scale

Question 5

What is a single phase

Answer 5

A phase with grains consisting of atoms of one metal

Question 6

Describe 2 phases with 2 metals

Answer 6

2 metals in a lattice - each grain is a separate entity so there are 2 phases

Question 7

What is a solid solution

Answer 7

2 different metals co-existing in the same grain

Question 8

What are the different possible outcomes of crystallisation of an alloy

Answer 8

Be insoluble - will exist as 2 phases
Form an intermetallic compound with a specific chemical formula
Be soluble and form a solid solution

Question 9

How may metal atoms be substituted in a solid solution and what conditions are necessary for this to take place

Answer 9

A random way
An ordered way
Atoms musty be similar in size, valency and crystal structure

Question 10

What are the different types of solid solution

Answer 10

Substitutional

Interstitial

Question 11

Describe an interstitial solid solution

Answer 11

Atoms are markedly different in size so smaller atoms will be located in spaced in the grain structure of larger atoms

Question 12

Describe their cooling curve of a pure metal

Answer 12

Plateau when the metal changes state
Crystallisation of the metal happens without a change in temperature
Only when all the metal has crystallised will its temperature gradually decrease

Question 13

Describe the cooling curve of an alloy

Answer 13

Crystallisation begins when temp reaches TL
First few metal atoms are cooled sufficiently to form individual nuclei of crystallisation
On further cooling, other metal atoms will have different mp and crystallise when these are reached
As crystallisation takes more time, there is a slight drop in temperature from TL to Ts

Question 14

What does a phase diagram show

Answer 14

If you change the ratio of each metal in an alloy, it will produce different cooling curves with different TL and Ts values
This gathers data for the cooling combinations of the different metals

Question 15

Describe the liquidus and solidus in a phase diagram

Answer 15

Above the liquidus, the alloy is in a liquid state
Below the solidus, the alloy is in a solid state
Between the two curves, the allow is partly liquid and partly solid
From this you can determine the TL and Ts values for any alloy ratio

Question 16

What is the liquidus

Answer 16

The line representing the temperatures which different alloy compositions begin to crystallise

Question 17

What is the solidus

Answer 17

The line representing the temperatures which different alloy compositions have completely crystallised

Question 18

What happens if alloys are cooled slowly

Answer 18

Allows metal atoms to diffuse through the lattice and ensures grain composition is homogenous
BUT - this results in large grains which is undesirable

Question 19

What happens if alloys are cooled rapidly

Answer 19

Prevents atoms diffusing through the lattice causing coring so the composition of each metal will vary throughout the grain creating a concentration gradient

Question 20

What are the necessary conditions for coring and how does this affect it

Answer 20

Phase diagram for the alloy must have the liquidus and solidus separated
The greater the separation, the more coring and the larger the concentration gradient

Question 21

What is the disadvantage of coring

Answer 21

May reduce corrosion resistance of the alloy

Question 22

How is coring of an alloy resolved

Answer 22

Homogenising annealing

Question 23

What is homogenising annealing

Answer 23

Once a solid cored alloy is formed, it is reheated to allow atoms to diffuse and cause the grain composition to become homogenous
Must keep temperature below recrystallisation temperature

Question 24

What is solution hardening

Answer 24

Alloys in a solid solution consisting of metals of different atomic size have a distorted grain structure
This impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)

Question 25

Describe defect movement in a metal lattice

Answer 25

Defect rolls over the atoms in the lattice plane | Little energy is needed for the defect to move along the slip plane

Question 26

Describe defect movement in a solid solution and how this enhances the alloy

Answer 26

Defect falls into the larger space existing between the large and small atom More energy is needed for the defect to overcome the different sized atoms and move along the lattice to the grain boundary So it requires more stress to move dislocations - making alloys more fracture resistant than metals

Question 27

What is order hardening

Answer 27

Alloys forming an ordered solid solution with atoms distributed at specific lattice sites have a distorted grain structure which impedes dislocation movement and so improves mechanical properties

Question 28

What is a eutectic alloy

Answer 28

An alloy in which the metals are soluble in a liquid state but insoluble in a solid state so 2 phases are formed

Question 29

What is the eutectic composition

Answer 29

The point at which the liquidus and solidus collide and the crystallisation process occurs at a single temperature

Question 30

What are the disadvantages of eutectic alloys

Answer 30

They are brittle | Poor corrosion resistance

Question 31

What is the dental application for eutectic alloys

Answer 31

Soldering alloy parts together

Question 32

Describe the phase diagram for a partially soluble alloy

Answer 32

The liquidus and solidus are separate There are dashed lines emanating from points H1 and H2 - solubility limit lines The alloy cannot form grains with a composition between H1 and H2 Instead the alloy forms grains either less than H1% - alpha grains or grains more than H2% - beta grains

Question 33

Give an example of a partially soluble alloy

Answer 33

Ag-Cu

Question 34

Describe alpha and beta grains of AgCu

Answer 34

Alpha grains contain both Ag and Cu but are Ag rich | Beta grains contain both Ag and Cu but are Cu rich

Question 35

What are solubility limit lines

Answer 35

Found in partially soluble alloy phase diagrams | Indicates that a range of compositions of the metals are not possible and instead, alpha and beta grains are formed

Question 36

What is precipitation hardening

Answer 36

When annealing a partially soluble alloy, its temperature is raised causing one of the metals to be pushed to the grain boundary This makes the alloy stronger and increases surface hardness