Phase Diagrams
Phase: A homogeneous portion of a system that have uniform
physical and chemical characteristics.
Single phase
Two
phases
For example at 20 oC the maximum solubility of sugar in water is
65wt%, note that the solubility limit increases with rising
temperature.
COMPONENTS AND PHASES
• Components:
The elements or compounds which are mixed initially
(e.g., Al and Cu)
• Phases:
The physically and chemically distinct material regions
that result (e.g., a and b).
AluminumCopper
Alloy
Adapted from
Fig. 9.0,
Callister 3e.
Temperature
Equilibrium phase diagrams represent the relationship between
temperature and the compositions and the quantities of phases
present at equilibrium. It can be considered as a map that determine
the phases depending on composition and temperature.
100% A
Map of
phases
present
10 20
Wt% of B
B 100%
Solid solution: It exist when different atoms take part in
building a crystal lattice (single phase).
Type of solid solutions:
A. Substitutional solid solution
B. Interstitial soild soltion
In both cases solute atoms completely dissolve (no limit to
solubility) in the host metal forming a single phase.
Temperature
Cooling curve of pure metals
L
Tm
S
Time
Pure metals
Pure metals solidifies at a constant temperature which is
known as the melting temperature
L
Ts
L+S
Tf
S
Time
Binary alloys
Temperature
Temperature
Cooling curve of binary alloys:
Range of
solidification
temperature
L
Ts
L+S
Tf
S
Time
Binary alloys
Binary alloys solidifies over a range of
temperatures.
Solid solution phase diagram:
How to construct
a phase diagram
from cooling
curves?
Draw the cooling
curves of
different
composition
alloys, the upper
inflection point
form a loci for
the liquidus line,
the lower
inflection point
form a loci for
the solidus line.
Melting
temp. of
Ni
Solid
solution
phase
Melting temp.
of Cu
Some notes about the phase diagram:
•The liquid L is a homogeneous liquid solution of both copper
and nickel.
•The a phase is a substitutional solid solution consisting of both
copper and nickel atoms, and having FCC crystal structure.
•Below 1080 C there is complete solubility between copper and
nickel (same crystal structure FCC, nearly equal atomic radii,
and electronegativity and similar valency).
•Melting temperatures for pure nickel is 1085 C and for pure
nickel is 1453C.
•Copper nickel system is termed isomorphous because of this
complete liquid and solid solubility.
•For each particular composition and temperature the number
and type of phases can be pointed out.
PHASE DIAGRAMS: number and types of phases
• Rule 1: If we know T and Co, then we know:
--the # and types of phases present.
• Examples:
Cu-Ni
phase
diagram
Adapted from Fig. 9.2(a), Callister 6e.
(Fig. 9.2(a) is adapted from Phase
Diagrams of Binary Nickel Alloys, P.
Nash (Ed.), ASM International,
Materials Park, OH, 1991).
PHASE DIAGRAMS: composition of phases
a. Construct a tie line across the two phase region
b. Intersection of the tie line and the phase boundaries on
either sides are noted.
c. Perpendiculars are dropped from these intersection to
the horizontal composition axis
For an alloy of
composition 35%Ni,
65%Cu, determine the
phase composition at
1250 C
Ca
If only one phase is present, for example a composition of 60% Ni40 %Cu at 1100C (point A), only the a phase is present. The
composition of this phase is the same as the composition of the
alloy, which mean that the composition of a is 60% Ni-40 %Cu .