Define phase diagram, cooling curve, liquidity and solidification.
Connection between cooling curve and phase diagram.
Dissolvable alloy system.
Pure metals,Gibb’s order, lever order ,dual system and eutectic point.
Many of the engineering materials possess mixtures of phases, e.g. steel, paints, and composites.
The mixture of two or more phases may permit interaction between different phases, and results in properties usually are different from the properties of individual phases.
Define Phase Diagram
An equilibrium state of solid system can be reflected in terms of characteristics of the microstructure, phases present and their compositions, relative phase amounts and their spatial arrangement or distribution.
PHASE DIAGRAMS THEORY AND APPLICATIONS
Some basic concepts
A homogeneous region with distinct structure and physical properties
In principle, can be isolated
Can be solid, liquid or gas
Representation of phases present under a set of conditions (P, T, Composition etc.)
Change from one phase to another
E.g. L S, S S etc.
Occurs because energy change is negative/goes from high to low energy state
Boundary between phases in a phase diagram
A simple phase diagram Triple point (Invariant point) Solid Liquid Vapor Pressure Temperature Phase boundary System: H 2 O
Gibb’s Phase Rule P + F = C + 2 P=number of phases C=number of components F=number of degrees of freedom (number of independent variables) Modified Gibbs Phase Rule (for incompressible systems) P + F = C + 1 Pressure is a constant variable F = C - P + 2 F = C - P + 1
Application of the phase rule At triple point, P=3, C=1, F=0 i.e. this is an invariant point At phase boundary, P=2, C=1, F=1 In each phase, P=1, C=1, F=2
Solidification(cooling) curves L S L S Pure metal Alloy T m L S L + S Solidification complete Soldification begins T L T S
Construction of a simple phase diagram
Conduct an experiment
Take 10 metal samples(pure Cu, Cu-10%Ni, Cu-20%Ni, Cu-30%Ni………, pure Ni)
Melt each sample and then let it solidify
Record the cooling curves
Note temperatures at which phase transformations occur
Results L S L S t T L L + S T L T S S L L + S T L T S Pure Cu Cu-10%Ni Cu-20%Ni L L S Pure Ni S T Ni T Cu
Binary isomorphous phase diagram x x x x x x x x x x x x x x x x x x x x L+S L S Composition Temp T Cu T Ni 10 60 70 80 90 30 40 50 20 0 100 Cu Ni %Ni Cu Ni
Microstructural changes during solidification L S L S T t T m L S Pure metal S
Microstructural changes during solidification L S Alloy L + S T L T S L S T t
L Binary isomorphous phase diagram L+S L S Composition T 10 60 70 80 90 30 40 50 20 0 100 A B %B L L S T 1 T 2 T 3 T 4 C L C 0 C S
This is an equilibrium phase diagram (slow cooling)
The phase boundary which separates the L from the L+S region is called LIQUIDUS
The phase boundary which separates the S from the L+S region is called SOLIDUS
The horizontal (isothermal) line drawn at a specific temperature is called the TIE LINE
The tie line can be meaningfully drawn only in a two-phase region
The average composition of the alloy is C O
The intersection of the tie line with the liquidus gives the composition of the liquid, C L
The intersection of the tie line with the solidus gives the composition of the solid, C S
By simple mass balance,
C O = f S C S + f L C L
and f S + f L = 1
C O = f S C S + (1- f S ) C L
In our diagram at T 3 , C O = A-40%B,
C S =A-90%B and C L =A-11%B
Therefore, f S =29/79 or 37% and
f L =50/79 or 63%
If we take an initial amount of alloy =100 g,
amt. of solid=37 g (3.7 g of A and 33.4 g of B) and amt. of liquid=63 g (56.07 g of A and 6.93 g of B)
The Eutectic Phase Diagram T A B Wt%B L +L +L + E T E C E Liquidus Solidus Solvus L + T E , C L =C E
T A B Wt%B L +L +L + E T E C E Pure A or B Other alloys between A and B C E L S L + L + L L L+ L + +
T A B Wt%B L +L +L + E T E C E Solidification for alloy of eutectic composition L S + +
Eutectic microstructure Lamellar structure
T A B Wt%B L +L +L + T E C E L Proeutectic + +
L T A B Wt%B L +L +L + T E C E L particles
The Eutectoid Phase Diagram T A B Wt%B + + + E T E C E + T E , C =C E
T A B Wt%B + + + E T E C E Cooling of an alloy of eutectoid composition S + +
T A B Wt%B + + + E T E Cooling of an alloy of hypoeutectoid composition S + + Pro-eutectiod Pro-eutectiod
The Peritectic Phase Diagram T A B Wt%B L + L + L + P T P C P L at T P N M Note: and L will react only in a certain proportion= NP:PM L L
T A B Wt%B L + L + L + P T P C P L at T P N M Note: and L will react only in a certain proportion= NP:PM 10 25 85 NP:PM=1:2 60 L L L
T A B Wt%B L + L + L + P T P C P L at T P N M Note: and L will react only in a certain proportion= NP:PM L L L