Phase Diagrams II Flashcards
What is a eutectic system?
A mixture of chemical compounds or elements that have
a single chemical composition that solidifies at a lower temperature than any
of its constituents.
What is the eutectic point?
*Describes a
single composition at which liquid transforms to a solid phase
(or vice versa) at the eutectic temperature.
*Point at which the
LIQUIDUS lines meet
Define hypoeutectic
Concentrations lower than the
eutectic is hypoeutectic
Define hypereutectic
Concentrations higher than the
eutectic is hypereutectic
What are the single phase fields in a binary eutectic system?
α – 1st rich solid solution
* β – 2nd rich solid solution
* Liquid
What are the two-phase fields in a binary eutectic system?
α + liquid
* β + liquid
* α + β
What is the solvus?
The solid solubility limit
between α and α + β
What is the liquidus?
Boundary between the
Liquid region and the α +
L
What is the solidus?
The boundary
between the α and α
+ L
Write the following reaction for a metal alloy with eutectic composition at the eutectic temperature
Check Phase Diagrams II annotated slides (slide 10)
Conventional solders for plumbing and electronics used 60
wt% Sn, 40 wt% Pb. Why is solder an alloy of near-eutectic
composition?
Solidification occurs over a short temperature range, such
that it occurs “instantaneously” and due to the guaranteed low melting point.
What is the lever rule used for?
It is used to determine the mass fraction of 2
phases in a two phase region under equilibrium conditions
Which type of cooling is cored structures formed from?
cored structures are formed from non-equilibrium
cooling
What is the c1 region for a eutectic alloy?
Compositions remaining
entirely in α-phase upon cooling
Why does the lamellar microstructure form? (Written Response)
- At the eutectic point, liquid transforms into a
solid-solid phase instantaneously (single
temperature) - The two solid phases are formed by atomic
diffusion of lead and tin - Because the phase transformation has to
occur quickly, diffusion occurs over short
distances, resulting in a fine lamellar
microstructure
How does the lamellar microstructure change
with further cooling below the eutectic temperature?
- Minimal changes in microstructure (remains
lamellar) - Mass fraction of beta phase increases ever
so slightly, with corresponding slight
decrease in alpha solid
When is solidification complete in C1 and what is the result in the eutectic system?
At the solidus. Result is a polycrystalline alloy
of uniform C1 composition
(point c), a Pb rich alloy with 0-
2 wt% Sn
After the polycrystalline alloy is formed, what happens when it is further cooled?
No compositional changes will
occur when further cooled
When does α first form in C1 in the eutectic system??
In the narrow α+L region
What happens during the cooling of the microstructure composition in C1?
Slow cooling from the liquid region; Remains liquid until it passes
through liquidus line
Define what c2 is for a eutectic system
Compositions ranging from room
temperature solid solubility to maximum
solid solubility (at eutectic temperature) –
crosses the solvus line
Where does c2 start in the eutectic system and what does it eventually cross?
In the liquid-region and crosses the solvus line
Describe the cooling process for the alloy during c2 for the eutectic system?
Alloy of composition C2 slowly cooled from
liquid region to α+L region to the α region where the Solvus line is reached.
What happens when the solvus line is reached for c2 in a eutectic system?
Upon crossing the Solvus line, the α
solid solubility is exceeded
In the α + β phase for c2, what types of particles are formed and why?
Small β particles form and will grow on
cooling because the mass fraction
increases slightly as the temperature
decreases
What type of composition is c3?
Eutectic composition
What is lamellae?
Resultant microstructure
consists of fine alternating
layers of α and β phases
How does redistribution of metal atoms in a eutectic system such as Pb-Sn occur?
Redistribution of Pb and Sn
occurs by atomic diffusion
What happens when the eutectic isotherm is crossed in c3 in a eutectic system?
Upon crossing the eutectic
isotherm the liquid is
transformed to α and β phases
Up until what point does the alloy remain liquid in the eutectic system at c3?
Remains liquid until the
eutectic temperature is reached.
What is the c4 composition in a eutectic system?
: Any composition other than
the eutectic which passes the
eutectic isotherm or crosses the α+L or β+L region
How do we calculate the mass
fraction of each phase (primary
α, eutectic α and β)?
- Use lever rule to determine the
amount of primary α present in
the α + L solution just above the
eutectic isotherm, and assume
that the primary α stays constant
throughout cooling - The remaining liquid is at the
eutectic concentration. Use lever
rule again to determine its mass
fraction, and composition of
eutectic α and β
When does some α phase form in c4 of the eutectic system?
When cooled through the
liquidus into the α+L region
What phases form and what is transformed below the eutectic temperature at c4 in a eutectic system?
Just below the eutectic
temperature, any remaining
liquid will transform to α+β
as per the eutectic reaction.
Also, two types of α
phase forms.
What is primary α?
α’ particles formed during
cooling through α+L region at c4 composition.
What is eutectic α?
the
α phase in the eutectic
structure
