Phase Diagrams Flashcards
What is meant by degrees of freedom?
How much can change and the phases remains constant
Ie 2 degrees of freedom means that T and Pressure can independently change and phase constant, 1 degree = T and P change together, 0 degree = nothing can change or phases will change
What is the Gibbs phase rule?
P + F = C + 2
Phases + freedom = components + 2
( Police force = cops +2)
What is the difference in alloys and pure elements phase diagrams?
Pure element = clear phase transition temps = liquidus and solidus regions
Alloy = phase transition temps range = L, S and L+S region are present
Draw the idealised cure for an alloyed element (which makes up phase diagram)
Pure element A transition (|_ ), then sloping - lower
|
down, until reach pure B = same as A but - much Lower down
This is true when A Tm > B Tm
Draw the idealise phase diagram for alloy AB and describe common mistakes to avoid
Binary phase diagram
- Solidus line must always be below liquidus line
- both line highest point on left, lowest point on right
- joins at pure A and pure B
What are tie lines?
Join areas with the same temp (always straight) that are on equilibrium (In same phase), never cross, used to work out the composition of the phase - by reading off comp where tie line hits solidus and liquidus lines - Cs = comp where solidus and tie line meet = solid phase of L+S region has X amount of B
What is the lever rule?
Calculates phase fractions of a material
Fs = Cl-Co/Cl-Cs
Co = Fl.Cl + Fs.Cs
What is the linear rule of mixtures?
Co = FlCl + FsCs
Define the eutectic point
Where freedom = 0 so 3 phases exist (L, α,β)
Draw a eutectic phase diagram and label the points
Trapezium with bird wings on top
Top of trapez = eutectic line
Define what is meant by:
Eutectic alloy
Hypo-eutectic alloy
Non - eutectic alloy
Eutectic alloy - alloy which goes through eutectic point
Hypo-eutectic alloy - alloy below eutectic point but still goes through eutectic line (hyper = above and goes through line)
Non - eutectic alloy - alloy that doesn’t go through eutectic line
Define the eutectic reaction, monotectic reaction and the peritectic reaction
L -> α +β
L -> L +α
L + α -> β
With no intermittent phases
Define the eutectoid reaction, monotectoid reaction and the peritectoid reaction
S -> α + β
S -> S + α
S + α -> β
What’s the difference between solubility and miscibility ?
Solubility - when one component dissolves within another
Miscibility - when two components combine to form a homogeneous mixture
What are the rules for solubility? And what happens when solubility limit is reached?
Same crystal structure, similar size, similar valence e-, similar electronegativity
When solubility level is reached = solute of one of the atoms is formed so that element B forms a phase (Si has low solubility in most things)
