Chapter 9 Flashcards
Why are phase diagrams important?
Phase diagrams have characterize the development of microstructure of alloys which correspond to mechanical properties
What is a component?
Pure metals and/or compounds of which an alloy is composed (like a solute and solvent)
What is a system?
2 meanings
1) A specific body of material under consideration
2) the series of possible alloys consisting of the same components but without regard to alloy composition
What is the solubility limit?
the maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution
What does the addition of solute in excess of the solubility limit result in?
The formation of another solid solution or compound that has a distinctly different composition
What does solubility limit increase with?
Temperature
What is a phase?
A homogenous portion of a system that has uniform physical and chemical characteristics
Is every pure material considered to be a phase?
Yes
Are solid, liquid, and gaseous SOLUTIONS phases?
yes
ex: a sugar-water solution
Are FCC and BCC structures of the same substance in the same phase?
No, they are in separate phases because their respective physical characteristics differ
What is a single-phase system?
Homogenous system
What is a mixture?
Heterogeneous systems
Are most metallic alloys homogenous or heterogenous?
Heterogenous
What does the mechanical behavior of a material depend on?
the microstructure
In metal alloys, what is microstructure characterized by in a phase diagram?
The number of phases present, their proportions, and the manner in which they are distributed or arranged
What does the microstructure of an alloy depend on?
Alloying elements present, their concentrations, and the heat treatment of the alloy
When is a system at equilibrium?
If its free energy is at a minimum under some specified combination of temperature, pressure, and composition
What is free energy?
Is a function of the internal energy of a system and also the randomness or disorder of the atoms and molecules
What is phase equilibrium?
Equilibrium as it applies to systems in which more than one phase may exist. It’s a constancy with time in the phase characteristics of a system.
ex: a solid sugar & sugar-water solution is at a phase equilibrium with a constant (60%- 40%), but then temperature is increased, taking it out of phase equilibrium, and then it settles back into equilibrium with a new ration (50%-50%)
What is a metastable state?
A state of system where equilibrium is never completely achieved because the rate of approach to equilibrium is extremely slow
What is another name of a metastable state?
nonequilibrium
Are metastable structures or equilibrium structures of more practical significance? Why?
metastable structure. some metals’ strength relies on the formation of metastable microstructures
What is another word for one-component phase diagrams?
unary
What’s another name for a phase diagram?
Equilibrium diagram
What are the three externally controllable parameters that affect phase structure?
Temperature, pressure, and composition
What is the simplest phase diagram to understand?
Pressure-temperature phase diagram
one-component system where composition is help constant
temperature (x-axis) versus ln(pressure) (y-axis)
What do the boundaries mean in a Pressure-Temperature phase diagram?
They are called phase boundaries and that means that any point on one of these curves, the two phases on either side of the curve are in equilibrium (or coexist) with one another
What happens when you cross a boundary in a phase diagram?
one phase transforms into another phase
What is the transition from liquid to solid called?
Solidification
What is at the O point of a phase diagram?
Triple point/invariant point
All three phases are simultaneously in equilibrium
– any deviation from this point will cause one of the three phases to disappear
What are on the axis of a one-component (or unary) phase diagram?
pressure versus temperatre
What are on the axis of a binary phase diagram?
temperature versus composition
– pressure is held constant (usually 1atm)
What are binary alloys? What are they present in?
An alloy (material created by mixing two or more elements) composed of two distinct chemical elements.
What is the alpha phase of a binary phase diagram?
a substitutional solid
What is an isomorphous system? Give an example
A system that has complete liquid and solid solubility of the two components
Cu and Ni (since they have the same crystal structure– FCC)
What is the phase line called that separates L from the alpha+ L phase field in a binary phase diagram?
liquidus line
What is the phase line that separates alpha from the alpha+ L line in a binary phase diagram?
solidus line
Where do the solidus and liquidus lines intersect on a binary phase diagram?
At the two composition extremes: corresponding to the melting temperatures of the pure components
How do you find the melting point in a binary phase diagram?
the melting point occurs between the solidus and liquidus lines, so choose a composition (point on x-axis) and then go up until you’re in between the lines and in the L + alpha zone
What are the three kinds of information available in a binary system of known composition and temperature?
1) the phases that are present
2) the compositions of these phases
3) the percentages or fractions of the phases
How do you determine the composition of a single phase region?
The composition of the region is just the overall composition of the alloy
How do you determine the composition of a two-phase region?
1) draw a tie line at the given temperature
2) point at where it intersects each boundary line
3) draw two lines down from those points to the x-axis of composition
4) create two compositions, of the liquid phase and the alpha phase
- the liquidous boundary intersection creates the composition of the liquid phase (C sub L)
- the solidus boundary composition intersection creates the composition of the solid-solution phase (C sub alpha)
What is a tie line? What’s another name for it?
A horizontal line across the alpha + L region that intersects the two phase boundary lines
Isotherm
What do you call the rule that you have to use if the composition and temperature position is located within the two-phase region?
lever rule
How do you calculate the volume fractions of alpha and beta phases?
V(alpha)=Va/(Va+Vb)
and inverse…
When will there be a difference between mass and volume fractions?
when there is a large difference in density
What is equilibrium cooling?
1) occurs very slowly to maintain phase equilibrium
2) slow alloy cooling gives sufficient time for compositional adjustments
3) microstructure and compositional changes occur gradually along the liquidous and solidus lines
4) final product is uniform with an alloy composition that matches the overall composition
What is nonequilibrium cooling?
When a material isn’t given sufficient time to cool and therefore the readjustments in composition’s can’t be reached in sufficient time
What happens to the solidus line on a phase diagram of Ni-Cu in nonequilibrium cooling?
The solidus line is shifted to higher Ni contents
The liquidus line remains the same because the liquid phase allows for more rapid diffusion
In nonequilibrium cooling, what is present at the point on the solidus line?
There is still liquid present, as well as the other relevant phases
In equilibrium cooling there would be no liquid phase
What does the degree of displacement of the nonequilibrium solidus curve from the equilibrium one depend on?
The rate of cooling:
slower cooling rate- smaller the displacement
Also, the higher the diffusion rate in the solid phase, the closer they are
What is segregation?
An uneven distribution of the two elements within the grains due to nonequilibrium solidification
What does it mean for a material to have a cored structure?
High-melting element (first to freeze) is in the center of each grain
The low-melting element is closer to the grain boundary
What consequences are caused by a cored structure? How can you fix this?
The grain boundary melts first, which produces a loss in mechanical integrity
Coring is fixed by a heat treatment below the equilibrium solidus temperature of the alloy– atomic diffusion occurs
What is an isomorphous alloy?
A solid solution of two or more metals that have a similar crystal structure, allowing them to mix in any proportion without forming separate phases
For all isomorphous alloys, what exists below the melting temperature of the lowest-melting component?
Only a single solid phase exists
What is solid-solution strengthening?
An increase in strength and hardness by additions of the other component (increases tensile strength in a curve)
What is a binary eutectic phase diagram?
3 single-phase regions are found on the diagram: alpha, beta, and liquid
In a binary eutectic phase diagram, what is the solute in the alpha and beta phases? So, what are the alpha and beta phases?
1st component: x-axis
2nd: other
alpha phase - pure 2nd component
- the x-axis component is the solute
beta phase- pure 1st component
- the 2nd component is the solute
What does eutectic mean?
“easily melted”
What is the eutectic reaction?
L→α+β
L(Ce) = a(CaE) + B(CBE)
*** They are all in terms of the component on the x-axis
a= alpha
b= beta
What is CαE and CβE?
The composition of the respective phases
What is the eutectic isotherm?
The horizonal line across at Te
How many phases may be in equilibrium within a phase field?
One and two at most
What are single-phase regions always separated from each other by?
Two-phase regions
What does it mean for a system to be at equilibrium?
Its phase characteristics do not change over time
What are the types of phases found on a one-component (unary) phase diagram?
solid, liquid, and vapor regions
What composition is held constant for binary phase diagrams?
external pressure
How can you identify a binary isomorphous system?
There is only one solid phase present (ex: only alpha exists)
