Phase Diagrams I

Question 1

Define phase

Answer 1

A homogeneous portion of a system that has uniform
physical and chemical characteristics

Question 2

What are examples of phases?

Answer 2

Solids of a similar structure, liquid, gas

Question 3

What is an allotrope?

Answer 3

Solids with same composition, but different structural
forms (eg. FCC, BCC, HCP) are considered different phases

Question 4

What can phase diagrams be used to study for?

Answer 4

• Alloying
• Heat treatments and microstructure

Question 5

How do phase diagrams depict equilibrium conditions?

Answer 5

• Represents the system after a long period of time has elapsed

Question 6

What is a binary system?

Answer 6

A system consisting of two elements

Question 7

What does isomorphous mean?

Answer 7

Liquid and
solid regions each contain
only one phase

Question 8

What is an example of a binary isomorphous system and what similar characteristics do they have?

Answer 8

Cu-Ni is an example, where
both copper and nickel
have FCC crystal structure,
similar atomic radii and
electronegativity values,
and valence structures

Question 9

What is the liquidus line in a phase diagram for a binary isomorphous system?

Answer 9

Boundary
between 100% liquid
and the liquid/solid two
phase region

Question 10

What is a solidus line?

Answer 10

Boundary
between 100% solid and
the liquid/solid two
phase region

Question 11

What information can be obtained from phase diagrams?

Answer 11

1. Phases at given temperature and composition
2. Composition of each phase.
3. Mass or volume fraction and amount of each phase is present

Question 12

What is a lever rule?

Answer 12

A commonly used mathematical model to predict
fractions of a particular phase at a given temperature T

Question 13

What is the lever rule based on?

Answer 13

The assumption of infinite diffusion in the solid and
liquid and therefore mimics the phase diagram (ignores mass and heat transfer).

Question 14

What is an implication of the assumption of infinite diffusion for the level rule?

Answer 14

Mass is
redistributed instantaneously

Question 15

What does non-equilibrium cooling assume?

Answer 15

• No or very little diffusion in solid,
  but instantaneous diffusion in liquid
• No or little compositional
  changes in solid over time

Question 16

What are cored structures?

Answer 16

Structures with a center of solid
particles may have higher
concentration than outer regions
of the solid

Question 17

What gives rise to cored structures?

Answer 17

Non-uniform composition in the solid.

Question 18

What is the difference between most manufacturing processes and phase diagrams in depicting the existence of equilibrium?

Answer 18

• Phase diagrams depict equilibrium conditions:
• In most manufacturing processes, equilibrium is not
  achieved (ie. Kinetics is important)

Question 19

What is a tie line?

Answer 19

A horizontal line at
temperature of interest
through the point.

Question 20

How do you find the phase composition of a two-phase region?

Answer 20

• Draw a horizontal line at
  temperature of interest
  through the point.
• Intersection of the tie line
  and the phase boundaries
  gives the composition of
  respective phases.

Question 21

Is mass
redistributed instantaneously in real life in diffusion?

Answer 21

not true in
reality but is a useful first approximation for the assumption of infinite diffusion.