Chapter 9 Part 2 Flashcards
What are lamellae?
Alternating layers of alpha and beta phases that form simultaneously during transformation at the eutectic composition
What forms when a material transitions from the liquid phase to the alpha+ beta phase at the eutectic temperature?
The microstructure result consists of alternating layers of the alpha and beta phases (called lamellae)
What is the eutectic structure?
Microstructure of the solid alpha+ beta phase after transformation from liquid at the eutectic composition. It’s alternating alpha and Beta phases and looks like black and white zig zags
What motivates one component of the alloy to redistribute towards a certain phase in the eutectic structure?
They will move towards the phases that they have higher compositions in
ex: if Sn is on the x-axis, it’ll move towards the beta phase on the right
Why does eutectic structure form for Sn and Pb?
Atomic diffusion of lead and tin only has to occur over relatively short distances
- think about it as if alpha and beta particles are all mixed around in the liquid. Instead of the alpha particles having to all go to one side, and the beta the other side, the atoms can just move a little to the left or right to form rows.
In the transition of a material from the alpha+L phase to the alpha+beta phase, how do you differentiate the alpha phases?
Before crossing the eutectic isotherm (Te) it’s called primary alpha, after it’s called eutectic alpha
What does Te stand for
eutectic isotherm
What is a microconstitutent?
An element of the microstructure having an identifiable and characteristic structure
Is the eutectic structure considered a microconstituent?
yes, even though it’s a mixture of two phases, it has a distinct lamellar structure with a fixed ratio
After transitioning from a solid+L phase to a solid+solid phase, what formed the eutectic microconstituent?
What can you assume the eutectic composition of the microconstituent to be?
The liquid phase formed the eutectic microconstituent
Assume it’s the same composition at the eutectic point
Where do you take the lever rule to calculate the eutectic microconstituent and liquid-phase mass fractions?
First off, those are the same thing. The eutectic microconstituent composition is the same as the one from the liquid it transforms from
Take it from the alpha phase to the eutectic point
How do you determine the fractions of total alpha, Walpha (both eutectic and primary) and total beta?
Use the lever rule entirely across the alpha + beta phase field
What happens if (in the fourth case: L+solid to solid+solid) conditions of equilibrium are not maintained while passing through the alpha(or beta) + L phase region?
1) Coring– nonuniform distribution of solute across grains
2) The fraction of the eutectic microconstituent formed will be greater than for the equilibrium situation
Why are the alpha and beta phases sometimes termed terminal solid solutions?
What is the other type of solution termed?
They exist over composition ranges near the concentration extremes of the phase diagram
Intermediate solid solutions/intermediate phases
What are intermetallic compounds?
Intermediate compounds that have distinct chemical formulas
They are represented with a vertical line on the diagram, rather than a phase of area, meaning they can only exist at the precise composition
What’s one way to think about dealing with complex phase diagrams?
Separating them into smaller-component units that are back-to-back
What is a eutectoid reaction?
one solid phase transforms into two other solid phases
γ→α+β or other ones
– must be directly touching at a point below
What is the eutectoid isotherm?
the point where the single phase solid area touches the two-phase solid area
What is a peritectic reaction?
one solid phase transforms into a liquid phase and another solid phase
What is a congruent transformation? What are some examples?
a phase transformation for which there are no compositional for the phases involved
allotropic transformations and melting or pure materials
What are examples of incongruent transformations?
At least one phase experiences a change in composition
eutectic (L- S+S) and eutectoid (S- S+S) reactions are incongruent
What is Gibbs phase rule?
P + F = C + N
It represents the criterion for the number of phases that coexist within a system at equilibrium and is expressed by that equation
What are both steel and cast iron’s primary structural materials?
iron-carbon alloys
What is the stable form (at room temperature) of pure iron called? What structure does it have?
Ferrite/ α-iron
BCC crystal struture
What is the FCC form of ferrite called?
austenite/ γ-iron
After heating ferrite to austenite, then heating autunite, what phase does Iron form?
BCC phase known as δ-ferrite
What is Fe3C?
Cementite, an intermediate compound iron carbide
Is carbon an interstitial or substitutional impurity in iron?
Interstitial in α- and δ-ferrites and also with austenite– indicated by the α, δ, and γ single-phase fields
What is pearlite?
Microstructure for the eutectoid steel that has alternating layers (lamellae) or the alpha-ferrite and Fe3C (cementite) phases (of Iron) that form
What are the mechanical properties of pearlite?
They are intermediate between soft, ductile ferrite and the hard, brittle cementite
Why do they alternating layers in pearlite form?
In the eutectoid transformation, the composition of austenite (γ) is different than the product phases (α ferrite and cementite (Fe3C). The transformation requires a redistribution of the carbon by diffusion and the layered pearlite forms because carbon atoms only have to diffuse minimal distances
What is a hypoeutectoid alloy?
An alloy with a composition to the left of the eutectoid
Which phases are which in an iron-carbon alloy phase diagram?
Ferrite (α): solid solution of carbon in iron/ low-temp/ BCC
Austenite (γ-gamma): solid solution of carbon in iron/ stable at higher temperatures/ FCC
Cementite (Fe3C): iron and carbon with a fixed composition of 6.7% carbon. Often in pearlite form- eutectoid mixture of ferrite and cementite.
When the transformation from austenite(γ)+ferrite(α) to ferrite(α)+cemenite(Fe3C), what transforms to what?
The ferrite remains the same. The austenite phase transforms into pearlite (containing eutectoid ferrite and Fe3C)
What is the ferrite called in pearlite? Before Te?
eutectoid ferrite
proeutectoid ferrite (still present past Te– right underneath Te)
What is proeutectoid cementite?
cementite (Fe3C) that forms before the eutectoid reaction
What two microconstituents are present in hypoeutectoid iron-carbon alloys that are cooled to a temperature right below the eutectoid?
proeutectoid ferrite and pearlite
What is a hypereutectoid alloy?
alloy after the eutectic point
How do you determine the relative amounts of the proeutectoid ferrite and pearlite?
Same as the primary and eutectic microconstituents lever rule
What happen forms as you travel from a cementite+austenite phase to an alpha+cemenite phase
The cementite remains the same, and the remaining austenite is converted into pearlite
pearlite and proeutectoid cementite are the remaining microconstituents
What is the difference between traveling downwards through the eutectic point versus from a single phase versus single, through (+) phase region?
through the eutectic point only forms pearlite
passing through a two phase region forms pearlite (formed from Fe3C & eutectoid alpha or gamma) AND a proeutectoid alpha or gamma (right below Te)
If asked to calculate the fraction of eutectoid ferrite, what should you think to do?
Know all ferrite is either proeutectoid or eutectoid (in pearlite), therefore the sum of the two ferrite fractions equals the total ferrite
total- found below Te
proeutectoid- above Te
eutectoid- calculate with rest
For what two reasons are steels usually alloyed?
To improve their corrosion resistance or to render them amenable to heat treatment
What effects can alloy additions have to iron-iron carbide?
1) shift in the position of the eutectoid with respect to temperature and carbon concentration
2) relative fractions of pearlite and the proeutectoid phase that form
