Introduction to Phase diagrams

Question 1

What is a phase

Answer 1

A homogeneous structure, a region of a material that has a crystal structure that is the same throughout

Question 2

What do phase diagrams show

Answer 2

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

Question 3

What are equilibrium conditions?

Answer 3

when there is no further driving force or change

Question 4

What are phase diagrams also known as

Answer 4

Equilibrium diagrams, they show the phases present when equilibrium conditions occur e.g. infinitely slow cooling

Question 5

Why is the formation of some phases not represented in an phase diagram

Answer 5

Much of materials processing takes place under non-equilibrium cooling

however useful to explain what phases are likely to form

Question 6

What is the form of phases formed under non-equilibrium conditions determined by

Answer 6

thermodynamics - materials wish to be at their lowest energy states

e.g. reflected in phase transformations

Question 7

Describe the allotropy of pure Fe

Answer 7

• 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

Question 8

What is the simplest form of phase diagram?

Answer 8

Solid solution phase diagram

Question 9

How is a solid solution formed?

Answer 9

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

Question 10

What is an example of a solid solution phase diagram

Answer 10

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

Question 11

Describe the meaning of a Liquid in a SS phase diagram

Answer 11

region in which the 2 atomic species, Cu & Ni, are dissolved in a single liquid solution

Question 12

What is the liquidus?

Answer 12

the line above which everything in the material is liquid

Question 13

What is the alpha + Liquid?

Answer 13

the region where solidification is taken place. Solid is (alpha), is forming within the liquid

Question 14

What is the solidus ?

Answer 14

The line below which everything is solid

Question 15

What is an alpha solid solution

Answer 15

A solid phase in which the atoms of Ni and Cu are intermixed

Question 16

What affects a materials microstructure

Answer 16

The way in which materials change phase affects their microstructure and hence properties

Question 17

What is the line that is drawn across the L+alpha flied, from liquidus to the solidus line

Answer 17

Tie - line

Question 18

What does a tie-line show

Answer 18

the composition of the L & S phases at this temp and composition to be deduced

Question 19

How do you find the Liquid or solid composition in a SS phase diagram?

Answer 19

Given where the tie-line meets the liqiudus

The solid composition is given where the tie-line meets the solidus

Question 20

How does diffusion operate under equilibrium conditions

Answer 20

diffusion operates so that the composition of the solid and the liquid is uniform, and the compositions given by the diagram

Question 21

What dos the collection of data data for fast non-equilibrium conditions result in

Answer 21

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

Question 22

when metal atoms are dissolved in each other it can happen in 2 ways, name and describe the 2 ways

Answer 22

• 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

Question 23

What is the lever rule

Answer 23

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)

Question 24

E.g. for Ni / Cu diagram

how to determine the Ni amounts

Answer 24

• 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

Question 25

How do you find the fraction of liquid

Answer 25

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

Question 26

How do you determine the fraction of solid?

Answer 26

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

Question 27

What is a more complex diagram than the SS phase diagram

Answer 27

Eutectic

Question 28

Give an example of a Eutectic diagram

Answer 28

Cd-Bi alloy, soluble in the liquid state but unlike the Cu-Ni system, completely insoluble in SS

Question 29

What are the phase fields in a Eutectic diagram?

Answer 29

• 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

Question 30

What is the point E

Answer 30

Eutectic point

Question 31

What are hypoeutectic alloys

Answer 31

alloys with compositions too the left of E

Question 32

What are hyoereutectic alloys

Answer 32

Alloys with compositions to the right of E

Question 33

Describe the solidification of a hypo eutectic alloy (20% Cd - 80%Bi)

Answer 33

• 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

Question 34

Describe the solidification of an alloy of eutectic composition (40& Cd - 60% Bi)

Answer 34

• 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

Question 35

What are described as natures composites?

Answer 35

Eutectic structures

Finely divided mixtures 1-10 micron scale, of solid phases with completely mechanical properties

Question 36

What do the properties of the alloys depend on?

Answer 36

size and properties of the primary phases in the microstructures (cuboids and dendrites) , and the amounts

Question 37

Give the equation that describes the formation of a eutectic structure

Answer 37

L S1 +S2

Question 38

Most solids have a degree of solid solubility

Answer 38

The Cd Bi system is simple