metals and alloys 2

Question 1

what is an alloy

Answer 1

A combination (or mixture) of two or more metals
Or
Metal(s) with a metalloid (silicone, carbon)

Question 2

what are the advantages of alloys

Answer 2

• Improved properties
○ Mechanical (EL, UTS, hardness)
○ Corrosion resistance
○ Lower melting point than an individual metal

Question 3

what are the dental uses of alloys

Answer 3

• Steel
○ Burs
○ Instruments

• Amalgam
○ Filling material
○ Special case

• Gold alloy
○ Inlays
○ Crowns
○ Bridges
○ Partial dentures
○ Wires

• Nickel chromium
○ Crowns
○ Bridges
Wires

Question 4

what does phase mean

Answer 4

Phase = physically distinct homogenous structure (can have more than one component

Question 5

what is a solution

Answer 5

homogenous mixture at an atomic scale

2 metals co-exist in a lattice structure

Question 6

if a lattice structure only has one type of metal in it how many phases are there

Answer 6

one phase

Question 7

if a lattice structure has individual grains of metal A and metal B, how many phases are there

Answer 7

2 phases

2 grains, 2 metals but they do not co-exist in the same grain

Question 8

if a lattice structure has 2 metals in a homogenous mixture how many phases are there

Answer 8

one phase

Question 9

when are metals normally soluble

Answer 9

when molten

Question 10

what are the 3 options for metals on crystallisation

Answer 10

On crystallisation, metals may:

A) Be insoluble, no common lattice - 2 phases

B) Form metallic compound with specific chemical formulation (eg Ag3Sn)
- This is the case with amalgam

C) Be soluble and form a solid solution, ie form common lattice

• 3 types of solid solution
• 2 metals are soluble in one another
• 2 types of metals co-exist in the same lattice arrangement

Question 11

what are the types of solid solution

Answer 11

substitutional
> atoms of one metal replace the other metal in the crystal lattice

interstitial
> atoms markedly different in size

Question 12

what are the types of substitutional solid solution

Answer 12

> random

• metal atoms similar in size valency, crystal structure
• can’t tell what the next atom in a row will be

> ordered

• metal atoms in regular lattice arrangement
• can predict what the next atom in a row will be because there is an order to the lattice arrangement

Question 13

what does interstitial solid solution consist of

Answer 13

atoms of different sizes
smaller atoms are located in spaces in lattice / grain structure or larger atoms

cannot predict where the small atom will be
just know it wont be in the lattice structure position

Question 14

how does the cooling curve of an alloy differ from the cooling curve of a metal

Answer 14

metal: crystallises at one temperature
alloy: crystallises over a temperature range

Question 15

what is TL on the alloy cooling curve

Answer 15

the temperature where crystallisation of the alloy begins

Question 16

what is TS on the alloy cooling curve

Answer 16

the temperature when the crystallisation process is complete

Question 17

what happens to soluble metals during the alloy cooling curve

Answer 17

solid solution formed (homogenous mixture of metals in each grain)

Question 18

what happens to insoluble metals during the alloy cooling curve

Answer 18

grains of individual metals formed

Question 19

what is a phase diagram

Answer 19

shows temperature Vs alloy composition

Question 20

what is complete solid solubility

Answer 20

can have any combination of the metals

different compositions of the 2 metals works

Question 21

what is the liquidus

Answer 21

Line representing the temperature which different alloy compositions begin to crystallise

Question 22

what is the solidus

Answer 22

Line representing the temperature which different alloy compositions have completely crystallised

Question 23

what does slow cooling of molten alloy result in

Answer 23

> allows metal atoms to diffuse through lattice
ensure grain composition is homogenous
results in large grains

Question 24

what does rapid cooling result in

Answer 24

> small grains
= good mechanical properties

> concentration gradient
= more likely to corrode

> causes coring - composition varies throughout the grain

> prevents atoms diffusing through lattice

> Generates many small grains which impede dislocation movement, improving its mechanical properties

Question 25

what does coring result in

Answer 25

grains structures of different concentrations

Question 26

what are coring conditions

Answer 26

• Fast cooling of liquid state • Liquidus and solidus must be separated (see phase diagram) and determines extent of coring ○ Eg Au-Pt • May reduce corrosion resistance of the solid form of the alloy

Question 27

how is coring resolved

Answer 27

homogenising anneal

Question 28

what is homogenising anneal

Answer 28

• Once solid cored alloy formed then reheat to allow atoms to diffuse and so cause grain composition to become homogenous • Reheat alloy grains (but not to melting temperature) and this allows the atoms to move around and eliminate the core structure ○ Crucial process • Note: keep below recrystallisation temperature, otherwise grains are altered

Question 29

what is solution hardening

Answer 29

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

Question 30

explain dislocation movement in a metal lattice

Answer 30

defect rolls over the atoms in the lattice plane until it settles at the grain boundary little energy / force is needed for the defect to move along the slip plane

Question 31

explain dislocation movement in a solid solution

Answer 31

defect does not roll over the lattice plane, it has to climb over each atom before it will settle by the grain boundary instead it falls into the larger space existing between large and small atom more energy / force is needed for the defect to overcome the different sized atoms and move along lattice to the grain boundary as greater stress is needed to move dislocations in solid solutions - alloys are more fracture resistant / stronger than metals

Question 32

explain order hardening

Answer 32

Alloys forming an ordered solid solution (atoms distributed at specific lattice sites) have a distorted grain structure (eg Au-Cu) Which impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)

Question 33

what are the properties of eutectic alloys

Answer 33

• Metals are soluble in liquid state • Metals insoluble in solid state (so 2 phases) Ie each metal forms physically distinct grains 2 metals not soluble in one another form physically distinct grains

Question 34

what is eutectic compostion

Answer 34

• Where liquidus and solidus coincide ○ Ie where crystallisation process occurs at a single temperature • Where grains of individual metals formed simultaneously

Question 35

what is binary eutectic

Answer 35

complete solid insolubility becomes melting point of the alloy?

Question 36

what are the advantages and disadvantages of eutectic alloys

Answer 36

Lowest melting point - at eutectic composition: used for solder Hard but brittle Poor corrosion resistance

Question 37

what is non-eutectic composition

Answer 37

* Excess metal crystallises first * Then liquid reaches eutectic composition * And both metals crystallise (forming separate grains)

Question 38

what happens in a partially soluble alloy

Answer 38

has a solubility limit line which indicates the range of compositions of metal a and metal B (corresponding to the horizontal section of the solidus [H1 to H2]) are not possible has alpha and beta solid solutions extremes - the alloy is usually very rich in one metal or the other

Question 39

what is precipitation hardening

Answer 39

heat treatment process that helps make metals stronger. The process does this by producing uniformly dispersed particles within a metal's grain structure that help hinder motion and thereby strengthen it happens in partial soluble alloys on annealing a super saturated alloy will undergo precipitation hardening

Question 40

what has better mechanical properties - alloys or metals?

Answer 40

``` alloys - better fracture strength, ridgidity, elastic limit, surface hardness - due to > solution > order > precipitation hardening ```

Question 41

how should cored structures be dealt with

Answer 41

removed by annealing