WK16 - Kinetics & Phase Diagrams

Question 1

What is diffusion? What’s the difference between inter-diffusion and self-diffusion?

Answer 1

Diffusion is the transport of material via atomic motion (energy is required to overcome barriers for atomic diffusion)

Inter - diffusion of impurities between two regions in response to a concentration gradient

Self - diffusion of atoms in a one-element material, where atoms of the same type exchange

Question 2

What is vacancy and interstitial diffusion?

Answer 2

Vacancy - atomic bonds around the vacancy break allowing a new atom to fill the vacancy (atom migration moves in the opposite direction to the vacancy)

Interstitial - atoms can diffuse fairly quickly as their bonding is weaker and there are more interstitial sites to jump to (atoms need to be small)
• tends to be quicker than vacancy (smaller atoms, less distortion to lattice)

Question 3

What is steady state diffusion? What is the concentration gradient also known as?

Answer 3

Flux does not change with time

Concentration profile is a function of position

Concentration gradient also known as the driving force of diffusion (minus sign indicates diffusion occurs down the concentration gradient)

Question 4

What is Non-steady state diffusion? (Flick’s Second Law)

Answer 4

Both concentration profile and concentration gradient change with time

Most of the cases in reality

Question 5

What is the activation energy? How is it described?

Answer 5

For atoms to jump to a vacancy or interstitial to jump to another site, it needs enough thermal energy to break its bond and move

Probability of a jump is described by the Arrhenius equation*
* Arrhenius dependence is just stating a parameter has an EXP(x) dependence on temperature

Question 6

What are the factors that make diffusion faster? (6 total)

Answer 6

1. Open crystal structures/smaller diffusing atoms
2. Lower density materials
3. Materials with low melting temperatures
4. Materials with secondary bonding
5. Cations (in ionic materials)
6. Polycrystalline materials

Question 7

What is creep? What are the three main phases of creep? What is it dependent on?

Answer 7

Tendency of a solid material to slowly deform under long-term mechanical stresses (slow deformation)

1. Primary (creep rate decreases with time), 2. Secondary creep (steady state creep with a constant slope), 3. Tertiary Creep (creep increases with time, accelerating to rupture)

Time | temperature | load

Question 8

Diffusional Creep and Dislocstion Creep are the two main mechanisms of Creep, what are they?

Answer 8

Diffusional Creep - Occurs when material is transported across the specimen by diffusion through several possible pathways
• Nabarro-Herring Creep (through bulk crystal lattice
• Coble Creep (Diffusion along grain boundaries)

Dislocation (Power-Law) Creep - due to dislocations
• dislocations can climb from one slip plane to another making it easier to move past obstacles (at elevated temps).

Question 9

What are the main material factors that can affect creep?

Answer 9

1. Stress-assisted vacancy diffusion
2. Grain boundary sliding
3. Grain boundary diffusion
4. Dislocation motion

Increasing grain size can help remove some of these mechanisms

Question 10

How can creep be minimised? Main temp at which creep will occur for a material?

Answer 10

1. High melting temps
2. High elastic modules
3. Large grain sizes (inhibits grain boundary sliding)

Creep occurs at > 0.4Tm (where Tm is melting temp)

Question 11

What does it mean if a microstructure has texture?

Answer 11

Texture - distribution of crystallographic orientations of a polycrystalline sample in a specific direction

Question 12

What is annealing?

Answer 12

Hearing a material to certain temperatures allows the atoms within to move around through diffusion

Diffusions processes can remove the effect of cold-working by annihilating dislocations and vacancies, then new grains forms

Cold worked grains - many small grains

Annealed grins - less larger grains
• ductility increases, tensile strength decreased

Question 13

Describe the process that drives recrystallisation

Answer 13

Cold worked material have many small grains resulting in lots of internal strain energy

Strain energy gives a driving force to recrystallise new grains with less strain

Driving force = difference between the internal energy in the strained and unstrained grains

Question 14

Define recrystallisation temp. How is affected by cold working?

Answer 14

“The temperature required to completely recrystallise the material in an hour”

0.3 to 0.7 of Tm (typically)

Temperature decreases as %CW increases

Question 15

What is Quenching?

Answer 15

Rapid cooling, by immersion in water or oil, of a metal from the high temperature at which it has been shaped

• cooling the metal slowly will cause the microstructure to change as it cools

Question 16

What is the solubility limit? What happens above this level?

Answer 16

“The maximum amount of a component that can be dissolved into a phase”

• Above this level the component will precipitate
C has limit of 6.67wt.% in Fe

Question 17

What affects the formation of another phase (in reference to the atoms involved)

Answer 17

1. Crystal structure of the main phase
2. Atomic radii of the elements
3. Electronegativity of the different elements
4. Valency of each element

Question 18

How do you convert from weight percentage to atomic percentage? Derive the expression for this

Answer 18

C’1 = C1A2/(C1A2 + C2A1) * 100

Question 19

Describe a phase and multi phase material, how does it differ to a grain boundary?

Answer 19

Phase - a region that has uniform physical and chemical characteristics

Multi phase - A heterogenous system has two or more distinct phases separated by phase boundaries

GB - different orientations meet within the same phase

Question 20

What is the difference between equilibrium and metastable?

Answer 20

Equilibrium - a system is in this state if it does not change with time at a constant temperature, pressure and composition

Metastable - slow kinetics may result in a material behaving like it’s in equilibrium, despite not being in its lowest energy states
• local minimum of free energy
• Diamond metastable of Carbon

Question 21

What is isomorphous system?

Answer 21

Simplest binary system where components of A can completely accommodate the components of B (100% soluble with each other)

Question 22

How does a phase diagram look in non-equilibrium conditions?

Answer 22

This is when the atoms aren’t given infinite time to move back to their equilibrium points

Layers at each tie line remain at that composition after solidification, forming layered grains with a Ni (example) content in the core.

This means grain boundaries will melt first as they have lowest Ni (point of weakness)

Question 23

What is binary Eutectic system?

Answer 23

Next simplest diagram, where a binary alloys components aren’t 100% insoluble with each other.

Hence u have 3 phases: Liquid (L + a, L + b), alpha, beta

Question 24

What is the Eutectic (invariant) point? What is the Eutectic Isotherm?

Answer 24

A single point where the liquid phase (L) goes straight to the solid phase of the two components (alpha + beta)

Eutectic Isotherm - the temperature at which this occurs

Question 25

What is a Eutectic reaction (ER)? Why is it unique?

Answer 25

L —> alpha + beta

Due to completely different compositions of alpha and beta, ER drives but redistributions of Pb and Sn by atomic diffusion

A layered (or lamellar) structure forms called eutectic structure

Question 26

Draw 4 phase bubbles which transforms from: 1. Liquid, alpha + Liquid, alpha + beta

What is important to consider?

How do the names of these phases differ?

Answer 26

Must calculate alpha percentage in alpha + Liquid

Then calculate alpha in alpha + beta

alpha + L : primary alpha (denoted via ‘)

Alpha + beta : Eutectic alpha

Question 27

What is an intermetallic? How does it appear on a phase diagram?

Answer 27

Is a phase of a different crystal structure to the main phase and at a very specific composition

Show as vertical straight lines - only be 100% of the phases at one specific place on the composition graph

Tend to be brittle and problematic

Question 28

What is the Eutectoid reaction?

Answer 28

Reaction occurs from one solid phase to two NEW solid phases

Alpha —> beta + gamma

Question 29

What is a Peritectic reaction?

Answer 29

Solid + liquid form a new solid phase after cooling

Alpha + liquid —> beta

Reaction is slow, and becomes harder with time as beta is separating the Alpha and liquid

Question 30

What is a congruent transformation?

Answer 30

Involves no change in COMPOSITION

L —> gamma in Ti

Alpha —> gamma in Fe (most significant)