Phase Diagrams II

Question 1

What is a eutectic system?

Answer 1

A mixture of chemical compounds or elements that have
a single chemical composition that solidifies at a lower temperature than any
of its constituents.

Question 2

What is the eutectic point?

Answer 2

*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

Question 3

Define hypoeutectic

Answer 3

Concentrations lower than the
eutectic is hypoeutectic

Question 4

Define hypereutectic

Answer 4

Concentrations higher than the
eutectic is hypereutectic

Question 5

What are the single phase fields in a binary eutectic system?

Answer 5

α – 1st rich solid solution
* β – 2nd rich solid solution
* Liquid

Question 6

What are the two-phase fields in a binary eutectic system?

Answer 6

α + liquid
* β + liquid
* α + β

Question 7

What is the solvus?

Answer 7

The solid solubility limit
between α and α + β

Question 8

What is the liquidus?

Answer 8

Boundary between the
Liquid region and the α +
L

Question 9

What is the solidus?

Answer 9

The boundary
between the α and α
+ L

Question 10

Write the following reaction for a metal alloy with eutectic composition at the eutectic temperature

Answer 10

Check Phase Diagrams II annotated slides (slide 10)

Question 11

Conventional solders for plumbing and electronics used 60
wt% Sn, 40 wt% Pb. Why is solder an alloy of near-eutectic
composition?

Answer 11

Solidification occurs over a short temperature range, such
that it occurs “instantaneously” and due to the guaranteed low melting point.

Question 12

What is the lever rule used for?

Answer 12

It is used to determine the mass fraction of 2
phases in a two phase region under equilibrium conditions

Question 13

Which type of cooling is cored structures formed from?

Answer 13

cored structures are formed from non-equilibrium
cooling

Question 14

What is the c1 region for a eutectic alloy?

Answer 14

Compositions remaining
entirely in α-phase upon cooling

Question 15

Why does the lamellar microstructure form? (Written Response)

Answer 15

• 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

Question 16

How does the lamellar microstructure change
with further cooling below the eutectic temperature?

Answer 16

• Minimal changes in microstructure (remains
  lamellar)
• Mass fraction of beta phase increases ever
  so slightly, with corresponding slight
  decrease in alpha solid

Question 17

When is solidification complete in C1 and what is the result in the eutectic system?

Answer 17

At the solidus. Result is a polycrystalline alloy
of uniform C1 composition
(point c), a Pb rich alloy with 0-
2 wt% Sn

Question 18

After the polycrystalline alloy is formed, what happens when it is further cooled?

Answer 18

No compositional changes will
occur when further cooled

Question 19

When does α first form in C1 in the eutectic system??

Answer 19

In the narrow α+L region

Question 20

What happens during the cooling of the microstructure composition in C1?

Answer 20

Slow cooling from the liquid region; Remains liquid until it passes
through liquidus line

Question 21

Define what c2 is for a eutectic system

Answer 21

Compositions ranging from room
temperature solid solubility to maximum
solid solubility (at eutectic temperature) –
crosses the solvus line

Question 22

Where does c2 start in the eutectic system and what does it eventually cross?

Answer 22

In the liquid-region and crosses the solvus line

Question 23

Describe the cooling process for the alloy during c2 for the eutectic system?

Answer 23

Alloy of composition C2 slowly cooled from
liquid region to α+L region to the α region where the Solvus line is reached.

Question 24

What happens when the solvus line is reached for c2 in a eutectic system?

Answer 24

Upon crossing the Solvus line, the α
solid solubility is exceeded

Question 25

In the α + β phase for c2, what types of particles are formed and why?

Answer 25

Small β particles form and will grow on
cooling because the mass fraction
increases slightly as the temperature
decreases

Question 26

What type of composition is c3?

Answer 26

Eutectic composition

Question 27

What is lamellae?

Answer 27

Resultant microstructure
consists of fine alternating
layers of α and β phases

Question 28

How does redistribution of metal atoms in a eutectic system such as Pb-Sn occur?

Answer 28

Redistribution of Pb and Sn
occurs by atomic diffusion

Question 29

What happens when the eutectic isotherm is crossed in c3 in a eutectic system?

Answer 29

Upon crossing the eutectic
isotherm the liquid is
transformed to α and β phases

Question 30

Up until what point does the alloy remain liquid in the eutectic system at c3?

Answer 30

Remains liquid until the
eutectic temperature is reached.

Question 31

What is the c4 composition in a eutectic system?

Answer 31

: Any composition other than
the eutectic which passes the
eutectic isotherm or crosses the α+L or β+L region

Question 32

How do we calculate the mass
fraction of each phase (primary
α, eutectic α and β)?

Answer 32

• 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

Question 33

When does some α phase form in c4 of the eutectic system?

Answer 33

When cooled through the
liquidus into the α+L region

Question 34

What phases form and what is transformed below the eutectic temperature at c4 in a eutectic system?

Answer 34

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.

Question 35

What is primary α?

Answer 35

α’ particles formed during
cooling through α+L region at c4 composition.

Question 36

What is eutectic α?

Answer 36

the
α phase in the eutectic
structure