Phase Diagrams I Flashcards
Define phase
A homogeneous portion of a system that has uniform
physical and chemical characteristics
What are examples of phases?
Solids of a similar structure, liquid, gas
What is an allotrope?
Solids with same composition, but different structural
forms (eg. FCC, BCC, HCP) are considered different phases
What can phase diagrams be used to study for?
- Alloying
- Heat treatments and microstructure
How do phase diagrams depict equilibrium conditions?
- Represents the system after a long period of time has elapsed
What is a binary system?
A system consisting of two elements
What does isomorphous mean?
Liquid and
solid regions each contain
only one phase
What is an example of a binary isomorphous system and what similar characteristics do they have?
Cu-Ni is an example, where
both copper and nickel
have FCC crystal structure,
similar atomic radii and
electronegativity values,
and valence structures
What is the liquidus line in a phase diagram for a binary isomorphous system?
Boundary
between 100% liquid
and the liquid/solid two
phase region
What is a solidus line?
Boundary
between 100% solid and
the liquid/solid two
phase region
What information can be obtained from phase diagrams?
- Phases at given temperature and composition
- Composition of each phase.
- Mass or volume fraction and amount of each phase is present
What is a lever rule?
A commonly used mathematical model to predict
fractions of a particular phase at a given temperature T
What is the lever rule based on?
The assumption of infinite diffusion in the solid and
liquid and therefore mimics the phase diagram (ignores mass and heat transfer).
What is an implication of the assumption of infinite diffusion for the level rule?
Mass is
redistributed instantaneously
What does non-equilibrium cooling assume?
- No or very little diffusion in solid,
but instantaneous diffusion in liquid - No or little compositional
changes in solid over time
What are cored structures?
Structures with a center of solid
particles may have higher
concentration than outer regions
of the solid
What gives rise to cored structures?
Non-uniform composition in the solid.
What is the difference between most manufacturing processes and phase diagrams in depicting the existence of equilibrium?
- Phase diagrams depict equilibrium conditions:
- In most manufacturing processes, equilibrium is not
achieved (ie. Kinetics is important)
What is a tie line?
A horizontal line at
temperature of interest
through the point.
How do you find the phase composition of a two-phase region?
- 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.
Is mass
redistributed instantaneously in real life in diffusion?
not true in
reality but is a useful first approximation for the assumption of infinite diffusion.
