Lectures 7-9 Flashcards

(70 cards)

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
What size of atom can the solute in interstitial solid solutions have? Why?
Small solute atoms Large atoms would disrupt the crystal lattice
26
Intermediate compounds are
combination of elements that produce a compound
27
Intermediate compounds when they are all metals are called
intermetallic compounds
28
A stochiometric compound is when
two elements that form a compound have fixed ratios within the composition
29
If you add a single atom of A into a pure substance of B, what happens to the system?
Increases the disorder, the entropy
30
If you keep adding atoms of A into a pure substance of B, what happens to the entropy? Why?
The entropy starts to decrease, as now B is dissolved in A
31
What happens to the B-B bonds when you introduce an atom of of A into the structure?
B-B bonds are broken and replaced by a set of A-B bonds
32
If the A-B bonds are weaker than the original bonds, then change in H will be ----. Why
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
If the A-B bonds are stronger than the original bonds, then change in H will be ----. Why
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
Other than bond strength, what can affect the enthalpy change of mixing?
Number of new bonds created
35
If an atom is added to a lattice and distorts the structure, what happens to the enthalpy?
Increase in energy as some of the bonds have gotten closer to breaking (as they are stretched). Change in enthalpy will be positive
36
Why does increasing temperature lead to a higher solubility? Reference to Free energy equation
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
If two elements are not soluble in each other what happens to them?
They will separate into different phases
38
On a phase diagram, the L represents?
Liquid phase
39
On a phase diagram, the Greek letters of Alpha, Beta represents?
Solid solutions - elements are soluble in eachother
40
On a phase diagram, the symbols of two phases represent?
Where two phases exist together
41
On a phase diagram, intermediate compounds with a fixed composition are represented by?
A vertical line
42
How can we increase diffusion rate?
Increasing temperature as the atoms have more energy required
43
Substitutional diffusion is aka
vacancy diffusion
44
Vacancy diffusion is
when atoms move into neighbouring vacancies
45
Interstitial diffusion is
when small atoms pass through the structure, between the atoms
46
An example of diffusion application:
Carburising (case hardening by diffusion of carbon onto the surface of steels)
47
Solidification is
transforming the arrangement of a material from disordered liquid (near melting point) to a solid (near melting point)
48
What does solidification result in a slight change of?
Volume, slightly decreased, shrinkage
49
Undercooling is
the act of cooling a liquid beneath it's melting temperature (under cooled) for solidification to actually start
50
Solidification commences when several atoms with ---- energy in the liquid meet and are able to form a small ---- region, called an ----.
low, solid, embryo
51
If the nucleus is not broken up, what will happen to it
Surrounding atoms will attach themselves and the nucleus will grow
52
Epitaxial growth is
the additional atoms continuing the crystal structure of the solid
53
Homogenous nucleation takes place in a liquid which
does not have any solid impurities or surfaces
54
Heterogenous nucleation takes place when there are
solids present such as a container wall
55
Why is it easier for materials to nucleate through heterogenous nucleation? Two points
- The container provides a surface upon which a nucleus can more easily form - Container helps to conduct heat away from the liquid
56
Inoculants are
particles added to a liquid to start the nucleation process
57
Liquidus line represents
the temp at which freezing starts / melting is completed
58
Solidus line represents
the temp at which freezing completes / melting starts
59
wt % represents
the percentage of the alloy based on weight
60
at % represents
the percentage of the alloy based on the number of atoms
61
How to find a composition of a two phase region?
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
Lever rule equation to find liquid fraction f L =
(solidus - Composition) / (solidus - liquidus)
63
Lever rule equation to find solid fraction f α =
(Composition - liquidus) / (solidus - liquidus)
64
When using the lever rule in wt% to calculate fractions what do the fractions represent?
Mass of material in each phase
65
Difference between the temperature changes of a pure metal to an alloy when changing phase from liquid to solid?
Pure metal solidifies fully at one temp Alloy solidifies over a temperature change (sloped line "mushy")
66
Cored/segregated alloys are
when the cooling of the alloy does not happen in an equilibrium and so the grains are not homogenous
67
Non-equilibrium solidification process in brief
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
Effects of non-equilibrium cooling
- 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
What does the cooling rate of the alloy determine on the phase diagram?
Difference of composition compared to the original phase diagram
70
How could you even out the composition of an alloy that has solidified in a non-equilibrium?
Heat treatments of the solid