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Materials > Lecture 03/10 > Flashcards

Lecture 03/10 Flashcards

1
Q

What is the transformation rate?

A

Dependence of reaction progress on time

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2
Q

What are the classifications of transformations?

A
  • Simple diffusion dependent
  • Diffusion- dependent
  • Diffusionless
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3
Q

Do most transformations occur instantaneously?

A

No

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4
Q

How do phase transformations work?

A

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

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5
Q

What are the two stages of phase transformation?

A

Nucleation and growth

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6
Q

What are the types of nucleation?

A

Homogenous and heterogenous

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7
Q

Describe how nuclei form during homogeneous nucleation

A

Nuclei of the new phase form uniformly throughout the parent phase

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8
Q

Describe how nuclei form during heterogeneous nucleation

A

Nuclei form at structural inhomogeneities such as grain boundaries

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9
Q

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

A
  • Free energy difference between the solid and liquid phases

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

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10
Q

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

A

Positive

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11
Q

Describe the total free energy change curve

A
  • 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
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12
Q

What is a subcritical particle called?

A

Embryo

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13
Q

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

A

Nucleus

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14
Q

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

A

Solidification occurs more readily

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15
Q

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

A

Magnitude of n* increases

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16
Q

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

A

At surfaces and interfaces, heterogeneous nucleation

17
Q

During growth, the product phase particles depend on?

A

Transformation temperature

18
Q

During growth, transformations that occur near Tm correspond to?

A

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

19
Q

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

A

It’s possible that nonequilibrium phase structures are formed

20
Q

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

A

Inversely proportional

21
Q

What is the kinetics of a transformation?

A

Time dependence of rate

22
Q

Simple diffusion-dependent transformation have no change in?

A

Number or composition of phases

23
Q

Diffusionless transformation occurs when?

A

A metastable phase is produced

24
Q

Nucleation involves?

A

Appearance of nuclei

25
Q

Growth involves?

A

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

26
Q

Transformation reaches completion if?

A

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

27
Q

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

A

Negative

28
Q

What is a subcritical particle?

A

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

29
Q

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

A

Both decrease

30
Q

What is supercooling?

A

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

31
Q

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

A

S shaped curve

32
Q

The time dependence of transformation is represented by the?

A

Avrami equation

Materials (12 decks)

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