Phase transformations

Question 1

What does the mean melting temperature tell us about Gibbs free energy?

Answer 1

Above the mean melting temperature, the liquid has a lower free energy.
For temperatures below the mean melting temperature, the Gibbs free energy of a solid is higher than the Gibbs free energy for a liquid.

Question 2

What is the process for solidification in real alloys?

Answer 2

Nucleation and growth of solid particles

Question 3

What two contributions are associated with the change in free enery?

Answer 3

-Formation of a volume of solid
-Formation of interface separating the solid and liquid phase

Question 4

How does a large particle radius affect the nucleation energy?

Answer 4

Decreases it

Question 5

What needs to be done by a system with a lower radius compared to critical radius?

Answer 5

Lower its energy by dissolution of the particle

Question 6

What happens to a system with a radius is higher than the critical radius?

Answer 6

Free energy decreases by growth of the particle

Question 7

How can the critical radius and free energy in terms of undercooling an latent heat of fusion be decreased?

Answer 7

Increasing the undercooling

Question 8

How does particle growth of a nucleated phase occur?

Answer 8

Via diffusion of atoms

Question 9

What parameter determined the growth rate of particles?

Answer 9

Diffusion coefficient of atomic species making up the phase of the nuclei

Question 10

What equation (name) can be used to describe the growth rate of particles?

Answer 10

Arrhenius equation

Question 11

Describe the system of particles at a temperature close to the melting temperature.

Answer 11

Nucleation rate is slow with high growth rates, the resulting microstructure will contain relatively few but coarse particles

Question 12

Describe a system of transforming particles at lower temperatures (below melting).

Answer 12

Nucleation rate is high and growth rates are slow, resulting in a fine distribution of particles

Question 13

What three factors induce phase transitions?

Answer 13

-Change in temperature
-Change in composition
-Change in applied pressure

Question 14

How is Martensite formed?

Answer 14

Steels are rapidly cooled from the austenite phase field to low temperatures

Question 15

What is the change in crystal structure that takes place when Martensite is formed?

Answer 15

FCC-BCT (Body centered tetragonal)