Lectures 7-9 Flashcards
What desire do systems have?
To reach a minimum energy level
What will gases do spontaneously when the boundary separating them is removed?
Mix/combine
Enthalpy is preferred to have —- during a process in a system
decreased
change in S =
change in entropy =
S final - S initial
final entropy - initial entropy
change in G =
change in free energy =
change in H - T X change in S
change in enthalpy - temperature X change in entropy
Free energy is
total energy that has been freed up by the change of a material
Enthalpy change is
total internal energy + pressure X volume of the system
Entropy change is
the amount of disorder that has been created or removed in the system
What type of change in G value will a thermodynamically favoured process have?
a negative change in G
Four situations that can occur when two liquids are mixed together
- Complete solubility
- Partial solubility
- Complete insolubility
- Reaction
Solvent is
the material in greater proportion
Solute is
the material in smaller proportion
Complete solubility is
one liquid mixes completely with the other in all proportions to create a homogenous solution
Partial solubility is
a small amount of liquid A will dissolve into liquid B but an addition of more liquid A causes two layers of mixtures
Complete insolubility is
when two liquids are not soluble at all, eg oil and water
A reaction is
when a new compound is formed
Alloys are
a combination of a metallic element with other metallic or non-metallic elements
Steel is an alloy of what two elements
Fe and C (iron and carbon)
A phase within a solid shows us the
ability of the atoms to mix in different compositions
Substitutional solid solutions are
where the solute atoms replace the solvent atoms in the crystal structure (like substitute)
Two manners that substitutional solid solutions can come in
- Random mixing
- Regular array
Greatest solubility of two elements will occur when 4 points
- Similar in size
- Same crystal structure
- Similar electronegativity
- Same valency
Interstitial solid solution is
where the solute atoms fit into the interstitial sites within the crystal structure
Generally which type of element occupies interstitial spaces?
Non-metals
What size of atom can the solute in interstitial solid solutions have? Why?
Small solute atoms
Large atoms would disrupt the crystal lattice
Intermediate compounds are
combination of elements that produce a compound
Intermediate compounds when they are all metals are called
intermetallic compounds
A stochiometric compound is when
two elements that form a compound have fixed ratios within the composition
If you add a single atom of A into a pure substance of B, what happens to the system?
Increases the disorder, the entropy
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
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
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.
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.
Other than bond strength, what can affect the enthalpy change of mixing?
Number of new bonds created
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
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)
If two elements are not soluble in each other what happens to them?
They will separate into different phases
On a phase diagram, the L represents?
Liquid phase
On a phase diagram, the Greek letters of Alpha, Beta represents?
Solid solutions - elements are soluble in eachother
On a phase diagram, the symbols of two phases represent?
Where two phases exist together
On a phase diagram, intermediate compounds with a fixed composition are represented by?
A vertical line
How can we increase diffusion rate?
Increasing temperature as the atoms have more energy required
Substitutional diffusion is aka
vacancy diffusion
Vacancy diffusion is
when atoms move into neighbouring vacancies
Interstitial diffusion is
when small atoms pass through the structure, between the atoms
An example of diffusion application:
Carburising (case hardening by diffusion of carbon onto the surface of steels)
Solidification is
transforming the arrangement of a material from disordered liquid (near melting point) to a solid (near melting point)
What does solidification result in a slight change of?
Volume, slightly decreased, shrinkage
Undercooling is
the act of cooling a liquid beneath it’s melting temperature (under cooled) for solidification to actually start
Solidification commences when several atoms with —- energy in the liquid meet and are able to form a small —- region, called an —-.
low, solid, embryo
If the nucleus is not broken up, what will happen to it
Surrounding atoms will attach themselves and the nucleus will grow
Epitaxial growth is
the additional atoms continuing the crystal structure of the solid
Homogenous nucleation takes place in a liquid which
does not have any solid impurities or surfaces
Heterogenous nucleation takes place when there are
solids present such as a container wall
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
Inoculants are
particles added to a liquid to start the nucleation process
Liquidus line represents
the temp at which freezing starts / melting is completed
Solidus line represents
the temp at which freezing completes / melting starts
wt % represents
the percentage of the alloy based on weight
at % represents
the percentage of the alloy based on the number of atoms
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
Lever rule equation to find liquid fraction
f L =
(solidus - Composition) / (solidus - liquidus)
Lever rule equation to find solid fraction
f α =
(Composition - liquidus) / (solidus - liquidus)
When using the lever rule in wt% to calculate fractions what do the fractions represent?
Mass of material in each phase
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”)
Cored/segregated alloys are
when the cooling of the alloy does not happen in an equilibrium and so the grains are not homogenous
Non-equilibrium solidification process in brief
- Initial solid forms as alpha 1
- Cooling continues and another layer of solid forms around the alpha 1, but does not allow much diffusion due to it being solid states
- Repeats until there are different compositions of solids, in layer like structures and different grains
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
What does the cooling rate of the alloy determine on the phase diagram?
Difference of composition compared to the original phase diagram
How could you even out the composition of an alloy that has solidified in a non-equilibrium?
Heat treatments of the solid
