The document describes slow cooling and rapid cooling of an isomorphous alloy using a Cu-Ni system. For slow cooling, the alloy solidifies completely with a uniform composition along the solidus line. For rapid cooling, solidification takes longer and is incomplete, resulting in segregation of grains with non-uniform composition. The document also describes a binary eutectic system using Cu-Ag, which has three single-phase regions (α, β, liquid), limited solid solubility, and a eutectic reaction that occurs at a single temperature and composition.
1. CHAPTER 2 (CONTD.)
Slow Cooling of an Isomorphous Alloy
Rapid Cooling of an Isomorphous Alloy
Binary Eutectic System
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2. SLOW COOLING OF AN ISOMORPHOUS ALLOY
Also, called as Equilibrium Cooling.
Let us consider Cu-Ni system 35 wt% Ni–65 wt%
Cu.
At 1300° C; point a, the alloy is completely liquid.
As cooling begins, no microstructural or
compositional changes will be realized until we
reach the liquidus line.
At 1260°C; point b, the first solid α begins to form,
which has a composition dictated by the tie line
drawn at this temperature i.e., 46 wt% Ni–54 wt%
Cu.
With continued cooling, both compositions and
relative amounts of each of the phases will change.
The fraction of α phase will increase.MTE/III SEMESTER/MSE/MTE 2101 2
Pic Courtesy: Material Science and Engineering, Callister.
3. At 1250°C, point c, the compositions of the liquid
and α phases are 32 wt% Ni–68 wt% Cu [L(32 Ni)]
and 43 wt% Ni–57 wt% Cu [ (43 Ni)].
The solidification process is virtually complete
at1220°C about point d; the composition of the
solid is approximately 35 wt% Ni–65 wt% Cu.
Upon crossing the solidus line, this remaining
liquid solidifies; the final product then is a
polycrystalline -phase solid solution that has a
uniform 35 wt% Ni–65 wt% Cu composition at
point e.
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Pic Courtesy: Material Science and Engineering, Callister.
4. RAPID COOLING OF AN ISOMORPHOUS ALLOY
Also, called as Non Equilibrium Cooling.
Let us consider Cu-Ni system 35 wt% Ni–65
wt% Cu.
At 1300° C; point a´, the alloy is completely
liquid.
As cooling begins, no microstructural or
compositional changes will be realized until we
reach the liquidus line.
At point b´, (approximately 1260°C), α-phase
particles begin to form, which, from the tie line
constructed, have a composition of 46 wt% Ni–
54 wt% Cu [ (46 Ni)].
MTE/III SEMESTER/MSE/MTE 2101 4Pic Courtesy: Material Science and Engineering, Callister.
5. Upon further cooling to point c´ (about 1260°C
), the liquid composition has shifted to 29 wt%
Ni–71 wt% Cu; furthermore, at this temperature
the composition of the αphase that solidified is
40 wt% Ni–60 wt% Cu [ (40 Ni)].
Because of the rapid cooling, the solidus line
shifted (indicated in dashed line) and solid
composition has increased.
At point d´ (temperature 1220°C), the
solidification should complete but there is still
little amount of liquid is left.
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Pic Courtesy: Material Science and Engineering, Callister.
6. The solidification should be completed at point
e´ in comparison to slow cooling but since there
is a shift in the solidus line, solidification process
takes longer time than usual.
It completes at point e´ (temperature 1205°C).
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Pic Courtesy: Material Science and Engineering, Callister.
7. PHENOMENA OF SEGREGATION TAKES PLACE WHICH IS NOTHING BUT NON-
UNIFORM DISTRIBUTION OF GRAINS.
Slow Cooling Rapid Cooling
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8. BINARY EUTECTIC
SYSTEMS
Another type of common
and relatively simple phase
diagram found for binary
alloys for the copper–silver
system; this is known as a
binary eutectic phase
diagram.
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Pic Courtesy: Material Science and Engineering, Callister
9. Features of Phase Diagram
Three single-phase regions are found on the
diagram: α,β and liquid.
α phase: Cu rich solid solution, solute is Ag,
FCC structure.
β phase: Ag rich solid solution, solute is Cu,
FCC structure.
Pure copper and pure silver are also
to be α and β phases.
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10. Features of Phase Diagram (Contd.)
The solubility in each of these solid phases is
limited, in that at any temperature below line
BEG only a limited concentration of silver will
dissolve in copper (for the α phase) similarly
copper in silver (for the β phase).
The solubility limit for the α phase
to the boundary line, labelled CBA, between
the α/(α+β) and α/(α+L) phase regions; it
increases with temperature to a maximum [8.0
wt% Ag at 779°C] at point B and decreases
back to zero at the melting temperature of
pure copper, point A 1085°C.
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11. Features of Phase Diagram (Contd.)
The solid solubility limit line separating the α
and (α+β) phase regions is termed a solvus
line; the boundary AB between the α and
(α+L) fields is the solidus line.
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12. Features of Phase Diagram (Contd.)
There are also three two-phase regions found for
the copper–silver system: α + L, β+L, α+β.
As silver is added to copper, the temperature at
which the alloys become totally liquid decreases
along the liquidus line, line AE; thus, the melting
temperature of copper is lowered by silver
additions.
The same can be said for silver, as copper is added
to silver, the temperature of copper reduces
complete melting along the other liquidus line, FE..
These liquidus lines meet at the point E on the
phase diagram, through which also passes the
horizontal isotherm line BEG.
Point E is called an invariant point.
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13. Features of Phase Diagram (Contd.)
An important reaction occurs for an alloy of
composition CE as it changes temperature TE; in
passing through this reaction may be written as
follows:
The reaction occurring at Point E along line BEG is
called as “Eutectic Reaction”.
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