Phase diagrams.ppt

Chapter 09: Phase Diagram 2
Introduction
Phase Diagrams are road maps

Component: Pure metals/compounds of which an alloy is
composed
System: Alloy system, e.g., Iron-Carbon alloy system,
copper-nickel alloy system
Solid solutions
- Substitutional
- Interstitial

Solubility Limit: Max concentration of solute atoms that
may dissolve in the solvent to form a solid solution.
Phases: Homogenous portion of a system that has uniform
physical and chemical properties

Source: William Callister 7th edition, chapter 09, page 254, figure 9.1

Each phase has different phase physical properties
Microstructure
•Characterized by number of phases, proportions and the
manner of distribution of phases.
•Depends on: Alloying elements, concentrations, heat
treatment (temp, heating/cooling rate etc.)

Phase equilibria
Free energy
Equilibrium
Phase equilibrium
Non-equilibrium state: State of equilibrium is never
reached since the rate of approach to equilibrium is very
slow.

Equilibrium phase diagram
•Represents the relationships between temperature and
compositions, and the quantities of phases in equilibrium
•Binary Alloy: two components

Binary Isomorphous Systems
Source: William Callister 7th edition, chapter 09, page 259, figure 9.3 a&b

Isomorphous: Complete solubility in both liquid and solid
states
Three kinds of info:
•Phases,
•compositions,
•percentages/fractions of phases

73%
0.73
32
43
35
43
WL





27%
0.27
32
43
32
35
W 





Where, WL: weight fraction of liquid
At 1250°C and Co=35% Ni

Inverse lever rule
1.Draw tie-line across the two phase region
2.Locate the overall composition (e.g., Co=35%Ni)
3.To compute the fraction of one phase, take the length of
the tie-line from the overall composition to the opposite
phase boundary and divide by the total tie line length
4.Repeat above procedure for the other phase.

------------------ (1)
------------------ (2)
Where, Cs and C both are same

Development of microstructures
Source: William
Callister 7th edition,
chapter 09, page
265, figure 9.4

Source: William
Callister 7th edition,
chapter 09, page 267,
figure 9.5
Development of microstructures
continue…

Development of Microstructures
The compositions readjust with changes in temperature.
These changes occur through the process of diffusion.
Because diffusion is time-dependent, much more time is
required, at each temperature for compositional
adjustments. Diffusion rates decrease with temperature. In
reality, cooling rates are so fast that there is little time to
enable equilibrium cooling.

•Because of fast cooling, there is a
non-uniform distribution of the two
elements for isomorphous alloys
“Segregation”
In the centre of each grain, the high
melting element solidifies first. At the
periphery, the low melting element
solidifies.
Non-Equilibrium Solidification

Coring
Cored Structure: Concentration contours
•Undesirable less than optimal properties due to
inhomogeneity.
•Coring is eliminated by a homogenization treatment. At a
temperature below the solidus point for the composition;
atomic diffusion causes homogenization.

Mechanical Properties of Isomorphous
Alloys

•Solid solution hardening or an increase in strength and
hardness by the addition of the other component
•At an intermediate composition, the curve (TS Vs Comp)
passes through a maximum
Mechanical Properties of Isomorphous
Alloys

Binary eutectic systems

Binary eutectic systems Continue …..
:Solid Solution of Ag in Cu-Rich solvent
:Solid solution of Cu in Ag-Rich solvent
Technically, : Pure Cu
: Pure Ag
Below BEG, only limited solid solubility takes place

CEA: (solid) solubility limit for Ag in – phase (Cu-Rich)
HGF: Solubility limit for Cu in -phase (Ag-Rich)
CBA is between /(+) and /(+L) phase regions
•Max. at 7.9% Ag (780°C)
•Decrease to zero at 1085°C (Melting point of pure Cu)

For –phase CB: solvus line: /(+)
AB: Solidus line: /(+L)

Prepare similar notes for -phase region
HGF is between
For -phase
HG: Solvus line and GF: Solidus line

Three two-phase regions: +L, +L and +
E: Eutectic invariant point
CE: 71.9 wt% Ag
Cu-Ag system: TE: 780°C

Binary Eutectic systems Continue …..
Eutectic Reaction:
Liquid Solid 1 + Solid 2
L  + 
L(CE)  (C E) +  (C E)
At E,

For Cu-Ag system,
L(71.9 wt% Ag)  (7.9 wt% Ag) +
 (91.2 wt% Ag)

Solidus line at 780°C: Eutectic isotherm
Phase volume fractions represent proportions seen in the
microstructure; so they can be estimated from
microstructures, and the mechanical properties can be
estimated as well.

Problem:
For Pb-Sn system at 150°C, calculate relative amounts of
each phase by (a) Mass fraction (b) Volume fraction
0.67
11
99
40
99
C
C
C
C
W
n
(a)Solutio
gm/cm
7.3
:
ρ
gm/cm
11.2
:
ρ
:
Given
α
β
1
β
α
3
β
3
α








Problem: Continue ….
3
3
3
3
α
β
α
1
β
cm
4.52
gm/cm
7.3
gm
33
)
(
v
cm
5.98
gm/cm
11.2
gm
67
)
(
v
Fraction
Volume
(b) Solution
0.33
11
99
11
40
C
C
C
C
W













1-0.67=0.33

0.43
4.52
5.98
4.52
v
v
v
V
0.57
4.52
5.98
5.98
v
v
v
V
Fraction
Volume
β
α
β
β
β
α
α
α










Problem: Continue ….

Phase diagrams.ppt

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Phase diagrams.ppt