1. Introduction to the Phase Diagrams MME 293: Lecture 05
Department of MME
BUET, Dhaka

2. Introduction to Phase Diagrams
 We have seen how the properties of materials reflect their microstructures. The microstructure are controlled by the composition of the material and how it is processed.
 So it is important that we must know
 know about the structure of a material that has been developed during its manufacture, and
 learn how to control (and/or modify) the structure to enhance its properties.
 Phase diagram is an important tool for materials scientists that tells which phases are stable in a system under specified conditions (e.g. of temperature, overall composition, pressure)

3. Typical phase diagram for one component system
 Phase diagram is basically a map that presents the domains of stability of phases and the limits of stability of phases in a graphical form.
Pressure
Temperature
solid
liquid
gas
liquid-gas equilibria
Typical phase diagram for one component system
 Reading the map will tell you, at the
state when it comes to equilibrium,
  1. what phases are present,
2. the state of those phases, and
3. the relative quantities of each phase.
 Reading a phase diagram will also tell what phase transformations we can expect when we change one or more parameters of the system (T, P, X).

4. Definitions and Basic Concepts
Components
 Chemically recognisable species that are mixed to form the alloy.
 In Brass: Cu, Zn (element)
 In steels: Fe, C (element)
 In ceramics: SiO2, Al2O3 (compound)
 Binary alloy contains 2 components, ternary 3, etc.
Phase
 A phase is a homogenous, physically distinct and mechanically separable portion of the material with a given chemical composition and structure.

5. What and how many phases materials possess?
 Solid, liquid, or gas, (and plasma)?
 Is it possible to have more than one solid phases?
Iron, being an allotropic material, has more than one solid phases:
 When iron freezes at 1538 C from its liquid state, the first solid formed has BCC structure (which is known as d-iron)
 As it cools down to 1401 C, it changes to FCC structure (which is known as g-iron)
 Upon further cooling down to 910 C, it changes again to BCC structure (which is known as a-iron)

6. Microstructure
 The properties of an alloy depend not only on proportions of the phases but also on how they are arranged structurally at the microscopic level.
 Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space.
b phase (lighter)
a phase (darker)
Microstructure of Al-Cu Alloy
pearlite (finger print)
graphite (grey)
Microstructure of Cast Iron
 Phase diagrams will help us to understand and predict the microstructures like the one shown in this page.

7. Equilibrium state and Metastable state
 A system is in equilibrium if at constant T, P and X, the system is stable, not changing with time.
 The equilibrium state always has the minimum free energy.
 Equilibrium state requires sufficient time to achieve. When this time is too long (due to slow kinetics), another state along the path to the equilibrium may appear to be stable. This is called a metastable state.
 A system at a metastable state is trapped in a local minimum of free energy, which is not the global one.

8. Advantages of Using Phase Diagrams
 Selection of alloys having enhanced properties for a specific set of applications.
 Manipulation of phase transformations of materials to control their properties.
Limitations of Using Phase Diagrams
 Phase diagrams are also known as the equilibrium diagrams; structures produced by non-equilibrium cooling cannot be explained.
 Rate of phase transformations is missing; TTT (Temperature- Time-Transformation) diagrams are more useful in this regard.

9. Classification of Phase Diagrams
One component (unary) phase diagrams
Unary phase diagram of water
 Also known as P-T diagrams.
 The simple case is Water.
 How many single-phase regions?
 How many two-phase regions?
 Is there any three-, or more- phase regions?
Gibb’s Phase Rule:
F = C–P+2
F = # variables
C = # components
P = # phases

10. Binary (two-component) phase diagrams
Type 1 (COMPLETELY SOLUBLE) Binary Phase Diagrams
Ni-Cu phase diagram

11. Type 2 (COMPLETELY INSOLUBLE) Binary Phase Diagrams
Au-Si phase diagram

12. Type 3 (PARTIALLY SOLUBLE) Binary Phase Diagrams
Pb-Sn phase diagram

13. Type 4 (INERMEDIATE COMPOUND FORMING)
Binary Phase Diagrams
Al-Sb phase diagram

14. Construction of Phase Diagrams
Theoretical Construction
 By applying thermodynamic principles
 Use of software like Thermocalc
Experimental Methods
 Thermal analysis
 Generation of cooling curves (temperature vs. time) for a number of alloys of the alloy system to obtain arrest points (temperatures where a change in slope is observed)
 Solid-state phase changes are difficult to obtained in this method
 Metallographic method
 Heating samples of an alloy to different temperatures, and quench them after equilibrium to retain the high-temperature structure
 Observe the structure microscopically
 Rapidly cooled samples do not always retain high-temperature structures; considerable skill is required to interpret the microstructure correctly