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Y1 Materials and Manufacturing - Sem 1 > Lectures 7-9 > Flashcards

Lectures 7-9 Flashcards

1
Q

What desire do systems have?

A

To reach a minimum energy level

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2
Q

What will gases do spontaneously when the boundary separating them is removed?

A

Mix/combine

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3
Q

Enthalpy is preferred to have —- during a process in a system

A

decreased

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4
Q

change in S =

A

change in entropy =

S final - S initial

final entropy - initial entropy

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5
Q

change in G =

A

change in free energy =

change in H - T X change in S

change in enthalpy - temperature X change in entropy

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6
Q

Free energy is

A

total energy that has been freed up by the change of a material

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7
Q

Enthalpy change is

A

total internal energy + pressure X volume of the system

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8
Q

Entropy change is

A

the amount of disorder that has been created or removed in the system

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9
Q

What type of change in G value will a thermodynamically favoured process have?

A

a negative change in G

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10
Q

Four situations that can occur when two liquids are mixed together

A
  • Complete solubility
  • Partial solubility
  • Complete insolubility
  • Reaction
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11
Q

Solvent is

A

the material in greater proportion

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12
Q

Solute is

A

the material in smaller proportion

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13
Q

Complete solubility is

A

one liquid mixes completely with the other in all proportions to create a homogenous solution

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14
Q

Partial solubility is

A

a small amount of liquid A will dissolve into liquid B but an addition of more liquid A causes two layers of mixtures

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15
Q

Complete insolubility is

A

when two liquids are not soluble at all, eg oil and water

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16
Q

A reaction is

A

when a new compound is formed

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17
Q

Alloys are

A

a combination of a metallic element with other metallic or non-metallic elements

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18
Q

Steel is an alloy of what two elements

A

Fe and C (iron and carbon)

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19
Q

A phase within a solid shows us the

A

ability of the atoms to mix in different compositions

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20
Q

Substitutional solid solutions are

A

where the solute atoms replace the solvent atoms in the crystal structure (like substitute)

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21
Q

Two manners that substitutional solid solutions can come in

A
  • Random mixing
  • Regular array
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22
Q

Greatest solubility of two elements will occur when 4 points

A
  • Similar in size
  • Same crystal structure
  • Similar electronegativity
  • Same valency
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23
Q

Interstitial solid solution is

A

where the solute atoms fit into the interstitial sites within the crystal structure

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24
Q

Generally which type of element occupies interstitial spaces?

A

Non-metals

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25
Q

What size of atom can the solute in interstitial solid solutions have? Why?

A

Small solute atoms

Large atoms would disrupt the crystal lattice

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26
Q

Intermediate compounds are

A

combination of elements that produce a compound

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27
Q

Intermediate compounds when they are all metals are called

A

intermetallic compounds

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28
Q

A stochiometric compound is when

A

two elements that form a compound have fixed ratios within the composition

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29
Q

If you add a single atom of A into a pure substance of B, what happens to the system?

A

Increases the disorder, the entropy

30
Q

If you keep adding atoms of A into a pure substance of B, what happens to the entropy? Why?

A

The entropy starts to decrease, as now B is dissolved in A

31
Q

What happens to the B-B bonds when you introduce an atom of of A into the structure?

A

B-B bonds are broken and replaced by a set of A-B bonds

32
Q

If the A-B bonds are weaker than the original bonds, then change in H will be —-. Why

A

Change in enthalpy will be positive

as more energy has been put in to break the bonds rather than rejected out again when the new weaker bonds are created.

33
Q

If the A-B bonds are stronger than the original bonds, then change in H will be —-. Why

A

Change in enthalpy will be negative

as less energy has been put in to break the bonds rather than absorbed again when the new, stronger bonds are created.

34
Q

Other than bond strength, what can affect the enthalpy change of mixing?

A

Number of new bonds created

35
Q

If an atom is added to a lattice and distorts the structure, what happens to the enthalpy?

A

Increase in energy as some of the bonds have gotten closer to breaking (as they are stretched).

Change in enthalpy will be positive

36
Q

Why does increasing temperature lead to a higher solubility? Reference to Free energy equation

A

Negative change in free energy (energy released out of the solution due to bonds weaker
and broken)

change in G = change in H - T X change in S (- T X change in S will make change in G overall a negative)

37
Q

If two elements are not soluble in each other what happens to them?

A

They will separate into different phases

38
Q

On a phase diagram, the L represents?

A

Liquid phase

39
Q

On a phase diagram, the Greek letters of Alpha, Beta represents?

A

Solid solutions - elements are soluble in eachother

40
Q

On a phase diagram, the symbols of two phases represent?

A

Where two phases exist together

41
Q

On a phase diagram, intermediate compounds with a fixed composition are represented by?

A

A vertical line

42
Q

How can we increase diffusion rate?

A

Increasing temperature as the atoms have more energy required

43
Q

Substitutional diffusion is aka

A

vacancy diffusion

44
Q

Vacancy diffusion is

A

when atoms move into neighbouring vacancies

45
Q

Interstitial diffusion is

A

when small atoms pass through the structure, between the atoms

46
Q

An example of diffusion application:

A

Carburising (case hardening by diffusion of carbon onto the surface of steels)

47
Q

Solidification is

A

transforming the arrangement of a material from disordered liquid (near melting point) to a solid (near melting point)

48
Q

What does solidification result in a slight change of?

A

Volume, slightly decreased, shrinkage

49
Q

Undercooling is

A

the act of cooling a liquid beneath it’s melting temperature (under cooled) for solidification to actually start

50
Q

Solidification commences when several atoms with —- energy in the liquid meet and are able to form a small —- region, called an —-.

A

low, solid, embryo

51
Q

If the nucleus is not broken up, what will happen to it

A

Surrounding atoms will attach themselves and the nucleus will grow

52
Q

Epitaxial growth is

A

the additional atoms continuing the crystal structure of the solid

53
Q

Homogenous nucleation takes place in a liquid which

A

does not have any solid impurities or surfaces

54
Q

Heterogenous nucleation takes place when there are

A

solids present such as a container wall

55
Q

Why is it easier for materials to nucleate through heterogenous nucleation? Two points

A
  • The container provides a surface upon which a nucleus can more easily form
  • Container helps to conduct heat away from the liquid
56
Q

Inoculants are

A

particles added to a liquid to start the nucleation process

57
Q

Liquidus line represents

A

the temp at which freezing starts / melting is completed

58
Q

Solidus line represents

A

the temp at which freezing completes / melting starts

59
Q

wt % represents

A

the percentage of the alloy based on weight

60
Q

at % represents

A

the percentage of the alloy based on the number of atoms

61
Q

How to find a composition of a two phase region?

A

Draw a tie line crossing both the liquidus and solidus. Where the line crosses each provides us with the composition of each solid and liquid

62
Q

Lever rule equation to find liquid fraction

f L =

A

(solidus - Composition) / (solidus - liquidus)

63
Q

Lever rule equation to find solid fraction

f α =

A

(Composition - liquidus) / (solidus - liquidus)

64
Q

When using the lever rule in wt% to calculate fractions what do the fractions represent?

A

Mass of material in each phase

65
Q

Difference between the temperature changes of a pure metal to an alloy when changing phase from liquid to solid?

A

Pure metal solidifies fully at one temp

Alloy solidifies over a temperature change (sloped line “mushy”)

66
Q

Cored/segregated alloys are

A

when the cooling of the alloy does not happen in an equilibrium and so the grains are not homogenous

67
Q

Non-equilibrium solidification process in brief

A
  1. Initial solid forms as alpha 1
  2. Cooling continues and another layer of solid forms around the alpha 1, but does not allow much diffusion due to it being solid states
  3. Repeats until there are different compositions of solids, in layer like structures and different grains
68
Q

Effects of non-equilibrium cooling

A
  • Solidus line to decrease on the phase diagram leading to a lower melting point
  • Reduction of tensile strength due to different properties from the inside grain compared to the outside of grain
69
Q

What does the cooling rate of the alloy determine on the phase diagram?

A

Difference of composition compared to the original phase diagram

70
Q

How could you even out the composition of an alloy that has solidified in a non-equilibrium?

A

Heat treatments of the solid

Y1 Materials and Manufacturing - Sem 1 (8 decks)

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