MATS_15301: Microstructures

Question 1

What phase transformation occurs upon cooling at
Eutectic point?

Answer 1

One liquid phase transforms to two different solid phases, α and β

Where α and β are 2 diff solid solutions

Transformation is spontaneous

Question 2

What phase transformation occurs upon cooling at
Eutectoid point?

Answer 2

One solid phase turns into two other solid phases

Transformation is spontaneous

Question 3

What phase transformation occurs upon cooling at
Peritectic point?

Answer 3

A liquid and a solid phase, α, transform to another solid phase, β

Transformation is spontaneous

Question 4

What phase transformation occurs upon cooling at
Peritectoid point?

Answer 4

Two solid phases, transform to another solid phase

Question 5

What phase transformation occurs upon cooling at
Monotectic point?

Answer 5

The first liquid phase L₁ transforms into another liquid phase L₂ and a solid phase

Note the difference between these and the pairs we see in other flashcards

Question 6

What is the equation for a ternary eutectic reaction?

Answer 6

L → α+β+γ

where L is the liquid phase and α,β,γ are the solid phases

Question 7

True or False:

The melting temperature T_m, of a eutectic system point is lower than the T_m of the metals that form it?

Answer 7

True

Question 8

What is the product of a eutectic reaxn called?

Answer 8

Eutectic mixture

Question 9

What is te degree

Question 10

What are the phases present in the microstructure of a cooled hypoeutectic alloy?

Answer 10

α and β

Question 11

What are the microconstituents of the microstructure of a cooled hypoeutectic alloy?

Answer 11

Proeutectic α and eutectic micture (α+β)

Question 12

What is another name for proeutectic α?

Answer 12

Primary α

Question 13

In binary eutectic phase diagrams:

What is primary α?

aka ‘proeutectic α’

Answer 13

The alpha phase that is formed during cooling of a hypoeutectic alloy before the eutectic isotherm T_e is crossed.

When we say ‘before’, it should be visualised as ‘up until that point’.

Question 14

What is a solid solution?

Answer 14

A homogenous mixture of two or more elements with a specific composition of each

Note the keyword

Question 15

What is the mathematical representation of Fick’s first law of Diffusion

Answer 15

J=-D * (dC/dx)

Where J is the diffusion flux, D is the diffusion coefficient dC is the difference in conc btw the two positions, and dx is the distance btw the two positions

Question 16

What are intrinsic lattice defects?

Answer 16

They exist in semi-equilibrium with the rest of the lattic structure and* their concentration varies with temperature*

Question 17

What is an example of an intrinsic lattice defect?

Answer 17

Vacany in lattice structure

Question 18

True or False:

Alloy elements and dislocations are deliberate lattice defects

Answer 18

True

Question 19

What are the two main models of atomic diffusion within a crystal lattice

Answer 19

Interstitial and Vacancy Diffusion

Question 20

True or False:

At elevated temperatures there is a higher concentration of vacany in a crystal structure

Answer 20

True

Question 21

What is the driving force for nucleation?

Answer 21

Gibbs free energy difference between solid and liquid phases

Question 22

What is the driving force for nucleation?

Answer 22

Energy required for surface to form, surface energy of cluster formed at nucleation site.

Question 23

In relation to critical size, r^*, when does nucleation occur?

Answer 23

When the radius of the cluster is greater than the critical size.

Question 24

True or False:

Critical volumes are the same for homogenous and heterogenous nucleation

Answer 24

False

The critical volumes for homogenous nucleation is higher than that for heterogenous nucleation

Question 25

# True or False: Critical radii are the same for homogenous and heterogenous nucleation

Answer 25

True

Question 26

# True or False: Diffusion occurs much easier over shorter distances than longer ones.

Answer 26

True

Question 27

What is the most widely used engineering material?

Answer 27

Steel

Question 28

What is a **phase**?

Answer 28

A phase is a **physically separable** region in a system **whose thermodynamic properties** (e.g. enthalpy, entropy...) ***are distinguishable*** from other regions in the system.

Question 29

What is a **phase diagram**?

Answer 29

This is a grsaphical representation of the **most stable phase at equilibrium** under a given set of thermodynamic conditions. ## Footnote A phase is often a **part** of a thermodynamic system, however, the entire system can also be one phase

Question 30

# True or False: Enthalpy is *mainly determined* by bonding energy in materials

Answer 30

**True** ## Footnote **The higher the bond energy, the lower the enthalpy**

Question 31

What is metastable equilibrium?

Answer 31

A **long-lived** (chemical) state that is *not the equilibrium state* but *will require a lot of energy* to be converted to the equilibrium state

Question 32

# For a binary isomorphous system: The weight fraction of what phase increases upon continued heating | **Heating from a solid**

Answer 32

**The Liquid phase**

Question 33

How is a binary isomorphous system different from a binary eutectic system?

Answer 33

In a **binary isomorphous system**, the two components, A and B are fully soluble in each other in both solid and liquid phases at all temperatures

Question 34

What is the consequence of **non-equilibrium cooling** in a binary isomorphous system?

Answer 34

**Coring**

Question 35

A binary isomorphous sytem of components A and B has been subject to non-equilibrium cooling conditions. **B has a higher melting point than A.** As cooling occurs, which of the two would you expect to have a higher concentration in the centre of the grain?

Answer 35

Component B ## Footnote **The element with the higher melting point has a higher concentration in the centre of the grain**. This reduces as the grain grows outwards (as coring progresses)

Question 36

What is the resultant structure of a eutectic reaction on cooling

Answer 36

A **lamellar structure** of *alternating layers of solid solutions* α and β ## Footnote α and β are solid solutions (mixtures) **rich** in a particular element. **They are not the pure form of that element!**

Question 37

# For a hyper/hypo-eutectic alloy: What is the difference between primary α/β and eutectic α/β

Answer 37

Primary α/β is formed on cooling **before** the liquid reaches eutectic % composition. ## Footnote If the alloy was always at eutectic % composition, the eutectic reaction occurs with no primary α/β phase formed upon cooling

Question 38

# Which **type of diffusion** occurs faster in metal alloys? Vacancy diffusion or Interstitial Diffusion

Answer 38

**Interstitial Diffusion** ## Footnote The atoms are smaller and there are more interstitial positions than vacancies so this occurs faster than the other