Phase diagrams

Question 1

What information does a phase diagram deliver?

Answer 1

It gives information about the state of the substance but NOT about the way the state can be reached.

Question 2

What is a solubility limit?

Answer 2

Maximum concentration for which only a single phase exists.

Question 3

Name the most important lines in a phase diagram.

Answer 3

Liquidus - between liquid and solid (the 2 phases coexist with each other)
Solidus - between solid and liquid
Solvus - between 2 solids
Tie line - extends between the maximum solubility of the 2 solids and goes through the eutectic or eutectoid point (depending on the diagram) - more phases can be in equilibrium at this line

Question 4

What is a phase?

Answer 4

Region of a material with uniform physical and chemical characteristics.
DIFFERENCE IN MICROSTRUCTURE LEADS TO DIFFERENT PHASES.

Question 5

What is an equilibrium phase?

Answer 5

When states (liquid, solid, gas) do not change with time.

Question 6

What does isomorphous mean?

Answer 6

Complete solubility of one substance into the other.

Question 7

How can phase weight fractions be determined?

Answer 7

With the lever rule.

Question 8

What is the eutectic reaction?

Answer 8

From liquid to 2 solids under equilibrium conditions.

Question 9

What is the eutectoid reaction?

Answer 9

From one solid phase to 2 solid phases.

Question 10

What does perlite look like and why?

Answer 10

Perlite has a lamellar microstructure and consists of alternating cementite and ferrite layers. It is achieved through slow cooling (controlled diffusion) from austenite into the ferrite + cementite region.
Perlite can be coarse or fine depending on the cooling rate. Finer pearlite provides better strength.

Question 11

Review microstructure development in the eutectic system.

Answer 11

ok :))

Question 12

Indicate the regions (materials based on carbon content) in the eutectoid phase diagram.

Answer 12

0.02% - 2.15% steel
2.1% - 6.7% cast iron
after 6.7% cementite

Question 13

What are the phases of an eutectoid phase diagram?

Answer 13

Alpha ferrite, gamma austenite, delta ferrite and cementite.

Question 14

What are the crystal structures of the phases in the eutectoid phase diagram?

Answer 14

Ferrite (alpha + delta) - BCC
Gamma austenite - FCC
Martensite - BCT

Question 15

What is a specific property of iron? Explain it.

Answer 15

Allotropy - the ability to form multiple crystal structures.

Question 16

What is the difference between iron, steel, and cast iron?

Answer 16

Iron is a pure metallic element (negligible amount of C). Steel has less than 2% C and cast iron has carbon content between 2-6%.

Question 17

How does perlite from?

Answer 17

The rate of perlite is controlled by diffusion of carbon through the austenite ahead of the advancing perlite front.
Carbon also diffuses from ferrite to cementite within the perlite structure.

Question 18

What is the difference between hypereutectoid and hypoeutectoid?

Answer 18

Hypereutectoid is formed after the eutectoid point while hypoeutectoid forms before.
Hypereutectoid forms proetectoid cementite along grain boundaries.
Hypoeutectoid forms proeutcetoid austenite along grain boundaries.