Lecture 03/10

Question 1

What is the transformation rate?

Answer 1

Dependence of reaction progress on time

Question 2

What are the classifications of transformations?

Answer 2

• Simple diffusion dependent
• Diffusion- dependent
• Diffusionless

Question 3

Do most transformations occur instantaneously?

Answer 3

No

Question 4

How do phase transformations work?

Answer 4

Formation of numerous small particles of the new phase which increase in size until the transformation is complete

Question 5

What are the two stages of phase transformation?

Answer 5

Nucleation and growth

Question 6

What are the types of nucleation?

Answer 6

Homogenous and heterogenous

Question 7

Describe how nuclei form during homogeneous nucleation

Answer 7

Nuclei of the new phase form uniformly throughout the parent phase

Question 8

Describe how nuclei form during heterogeneous nucleation

Answer 8

Nuclei form at structural inhomogeneities such as grain boundaries

Question 9

What are the contributions to the total free energy change that accompanies a solidification transformation during homogeneous nucleation?

Answer 9

• Free energy difference between the solid and liquid phases

- Energy from the formation of the solid-liquid phase boundary

Question 10

Is the energy from the formation of the solid-liquid phase boundary in homogeneous nucleation positive or negative?

Answer 10

Positive

Question 11

Describe the total free energy change curve

Answer 11

• As the solid particle begins to form, the free energy increases
• If the cluster reaches a size r*, the growth will continue with a decrease in free energy
• If the cluster is less than r*, it will shrink and redisolve

Question 12

What is a subcritical particle called?

Answer 12

Embryo

Question 13

What is a particle called if the radius is greater than r* ?

Answer 13

Nucleus

Question 14

During homogeneous nucleation, the lowering of temperatures below the equilibrium solidification temperature means?

Answer 14

Solidification occurs more readily

Question 15

During homogeneous nucleation, what happens for the number of stable nuclei when the temperature is lowered below Tm?

Answer 15

Magnitude of n* increases

Question 16

When is it easier for nucleation to occur? What is this called?

Answer 16

At surfaces and interfaces, heterogeneous nucleation

Question 17

During growth, the product phase particles depend on?

Answer 17

Transformation temperature

Question 18

During growth, transformations that occur near Tm correspond to?

Answer 18

Low nucleation and high growth rates leading to few but large particles

Question 19

During growth, if the material is supercooled to a low temperature where the transformation is very low what happens?

Answer 19

It’s possible that nonequilibrium phase structures are formed

Question 20

During growth, the rate of transformation and the time required for the transformation to proceed are?

Answer 20

Inversely proportional

Question 21

What is the kinetics of a transformation?

Answer 21

Time dependence of rate

Question 22

Simple diffusion-dependent transformation have no change in?

Answer 22

Number or composition of phases

Question 23

Diffusionless transformation occurs when?

Answer 23

A metastable phase is produced

Question 24

Nucleation involves?

Answer 24

Appearance of nuclei

Question 25

Growth involves?

Answer 25

Nuclei increasing in size, resulting in the disappearance of some of the parent phase

Question 26

Transformation reaches completion if?

Answer 26

Growth of the nuclei is allowed to proceed until the equilibrium fraction is attained

Question 27

If the temperature is below equilibrium solidification, is the volume free energy positive or negative?

Answer 27

Negative

Question 28

What is a subcritical particle?

Answer 28

The cluster of atoms have a radius less than the critical value

Question 29

As temperature decreases, what happens to the critical radius and activation free energy?

Answer 29

Both decrease

Question 30

What is supercooling?

Answer 30

Solidification will begin after the temperature has been lowered to below the melting temperature

Question 31

For solid transformations, a plot fraction transformation vs log time yields?

Answer 31

S shaped curve

Question 32

The time dependence of transformation is represented by the?

Answer 32

Avrami equation