phase diagrams Flashcards
COMPONENT:
COMPONENT: Pure Metal or Compound
SYSTEM
SYSTEM: Can be ladle of molten steel, or the whole range of alloys possible
PHASE
PHASE: Homogeneous part of system that has Uniform physical + chemical
characteristics (not necessarily the same compositions).
Composition
Composition: concentration of each component in an alloy
Solubility Limit
Maximum amount of solute that can go into solution in solvent
Max
concentration
for which only a
single phase
solution occurs.
Effect of Temperature (T) & Composition (C)
Changing either or both can change # of phases
go back and see lever rule
if densities are the same, the volume fractions
- Note; if densities are the same, the volume fractions will be equal to the
weight fractions.
slow cooling
Continually changing composition during slow cooling requires Solid State
Diffusion!
Solid state diffusion : takes time and energy (temperature)
* If allow very long time for cooling then diffusion can take place
rapid cooling
If cool faster, insufficient time/energy for complete diffusion
then Cored Microstructure Segregation
at eutectic
Immediate freezing of liquid (like pure metal but into 2 phases rather
than one)
At Eutectic line:
At Eutectic line: 3 phases in equilibrium; Liq, a, b
Eutectic composition temperature
- Eutectic composition solidifies at a lower temperature than other
compositions.
hyper eutectic alloy
In a hyper eutectic alloy, the composition of the ally contains more of the alloying elements then the eutectic Composition. This means What when the alloy solidifies, the primary phase that forms first will have a composition richer the alloying elements than the eutectic composition.
b’ + (a + b)
right of the eutectic composition
hypoeutectic alloy
A hypoeutectic alloys contains less of the alloying elements than the eutectic composition. this means that the primary phase that forms Ist during solidification will have a composition richer in alloying elements compared to the eutectic composition
a’ + (a + b)
left of the eutectic composition
refer back to Micro-constituents formulasc
compunds in phase diagrams
Sometimes Intermetallic Compounds are formed at particular ratios of
two metals or a metal and a non-metal.
These compounds appear as Vertical Lines on Phase Diagram
Eutectic
Eutectic - liquid in equilibrium with two solids
liquid freezes to two solids
Eutectoid
Eutectoid - solid phase in equilibrium with two other solid phases
S2 becomes S1+S3
Peritectic
Peritectic - liquid + solid 1 à solid 2
S1 + L S2
PEARLITE
(a + Fe3C)
formed by the EUTECTOID REACTION
Lamellar structure is the easiest.
* Once formed no significant changes on further cooling
If a steel is cooled at faster rates then other things may happen:
phase changes may occur at different temperatures or may vary
microstructure (fine & coarse pearlite)
- may get phases at room temperature which are not on the phase
diagram, i.e. non equilibrium phases.
When other elements (in addition to carbon) are added to steels they
can affect the Fe-Fe3C diagram.
- Some move eutectoid temperature up
- Some move eutectoid temperature down
- Some move eutectoid to higher carbon concentrations
- Some move eutectoid to lower carbon concentrations
