Introduction to Phase diagrams Flashcards
What is a phase
A homogeneous structure, a region of a material that has a crystal structure that is the same throughout
What do phase diagrams show
Way of showing what phases are present in materials and phases will change with temp and composition
Describe structure of materials that would occur under equilibrium conditions
What are equilibrium conditions?
when there is no further driving force or change
What are phase diagrams also known as
Equilibrium diagrams, they show the phases present when equilibrium conditions occur e.g. infinitely slow cooling
Why is the formation of some phases not represented in an phase diagram
Much of materials processing takes place under non-equilibrium cooling
however useful to explain what phases are likely to form
What is the form of phases formed under non-equilibrium conditions determined by
thermodynamics - materials wish to be at their lowest energy states
e.g. reflected in phase transformations
Describe the allotropy of pure Fe
- Changes from 1 crystal structure to another on heating BCC–> FCC –> BCC
- changes crystal structure to reduce its free energy
- phase changes can be exploited to manipulate the properties obtainable from Fe, particularly by alloying to form steel
What is the simplest form of phase diagram?
Solid solution phase diagram
How is a solid solution formed?
Materials will dissolve in each other in the liquid state
when they solidify sometimes these materials will remain dissolved in each other in the solid state, framing solid solutions
What is an example of a solid solution phase diagram
Cu-Ni binary system
Shows Cu melts at 1083oC, Ni melts ar 1453oC
There are 3 phases: Liquid, alpha + Liquid, alpha solid solution
lines separating the fields are liquids and solidus lines
Describe the meaning of a Liquid in a SS phase diagram
region in which the 2 atomic species, Cu & Ni, are dissolved in a single liquid solution
What is the liquidus?
the line above which everything in the material is liquid
What is the alpha + Liquid?
the region where solidification is taken place. Solid is (alpha), is forming within the liquid
What is the solidus ?
The line below which everything is solid
What is an alpha solid solution
A solid phase in which the atoms of Ni and Cu are intermixed
What affects a materials microstructure
The way in which materials change phase affects their microstructure and hence properties
What is the line that is drawn across the L+alpha flied, from liquidus to the solidus line
Tie - line
What does a tie-line show
the composition of the L & S phases at this temp and composition to be deduced
How do you find the Liquid or solid composition in a SS phase diagram?
Given where the tie-line meets the liqiudus
The solid composition is given where the tie-line meets the solidus
How does diffusion operate under equilibrium conditions
diffusion operates so that the composition of the solid and the liquid is uniform, and the compositions given by the diagram
What dos the collection of data data for fast non-equilibrium conditions result in
Results in shifting of the temperatures and compositions to the phase fields, to give unpredicted phases, and gradients in compositions within the phases, rather than uniform compositions predicted
when metal atoms are dissolved in each other it can happen in 2 ways, name and describe the 2 ways
- substitutional solid solution (atoms of similar size): atoms of solute metal can be dissolved in the solvent either by substituting for the solvent atoms in the lattice
- interstitial solid solution (atoms of very different sizes): by inserting in the interstices of the lattice
What is the lever rule
the tie line can be used (for known composition) to determine the relative proportions of the solid and liquid in the alpha + liquid region
(the relative amounts of 2 phases in a 2-phase field)
E.g. for Ni / Cu diagram
how to determine the Ni amounts
• Wt.%Ni = (fraction liquid x wt.%Ni in liquid) + (fraction solid x wt.%Ni in solid)
But the fraction solid = 1 – fraction liquid so;
• Wt.%Ni = (fraction liquid x wt.%Ni in liquid) + ((1-fraction liquid) x wt.%Ni in solid)
fl = (Ni a - Ni s)/(Ni l - Ni s)
fl - fraction of liquid
fs = 1 - fl
How do you find the fraction of liquid
Measure the length from the alloy composition to the solidus and divide by the length of the tie-line from the liquidus to the solidus
How do you determine the fraction of solid?
Measure the length from the alloy composition to the liquidus, and divide by the length of the tie-line from the liquidus to the solidus
What is a more complex diagram than the SS phase diagram
Eutectic
Give an example of a Eutectic diagram
Cd-Bi alloy, soluble in the liquid state but unlike the Cu-Ni system, completely insoluble in SS
What are the phase fields in a Eutectic diagram?
- liquid : homogeneous L mixture of Cd and Bi
- Bi + L: some solid Bi has formed in the L
- Cd + L: some solid Cd has formed in the L
- Bi +Cd: mixture of solid pure Cd and solid pure Bi
Only describes composition not shape and sizes
What is the point E
Eutectic point
What are hypoeutectic alloys
alloys with compositions too the left of E
What are hyoereutectic alloys
Alloys with compositions to the right of E
Describe the solidification of a hypo eutectic alloy (20% Cd - 80%Bi)
- first solid forms ~ 260oC, tie line drawn to solidus: pure Cd (forms as dendrites- tree like structures)
- As T decreases, solid fraction increases, existing Cd dendrites grow in size. Simultaneously liquid becomes depleted Cd and enriched in Bi. Composition of L follows ll to left until eutectic temperature (140oC) L has eutectic comp, L supersaturated in Cd and Bi PRECIPITATED simultaneously as a finely divided mixture of the 2 S phases
- Final Microstructure: Large Cd dendrites (formed above 140oC) in a eutectic matrix of finely divided Bi and Cd ‘leaves; = lamellae
Describe the solidification of an alloy of eutectic composition (40& Cd - 60% Bi)
- As temp declines, nothing happens in L until Eutectic T.
@ this point eutectic structure of finely mixed solid pure Cd and solid pure Bi form –> microstructure of entirely eutectic phase
What are described as natures composites?
Eutectic structures
Finely divided mixtures 1-10 micron scale, of solid phases with completely mechanical properties
What do the properties of the alloys depend on?
size and properties of the primary phases in the microstructures (cuboids and dendrites) , and the amounts
Give the equation that describes the formation of a eutectic structure
L S1 +S2
Most solids have a degree of solid solubility
The Cd Bi system is simple
