Phase Diagram – A phase in a material in terms of its micro-structure is a region that differs in structure and composition from another region. Phase diagrams are graphical representations of what phases are present in a materials system at various temperatures, pressures and compositions.
Necessity of a Phase Diagram- Following points shows you the why it is necessary to use phase diagram.
1. To show what phases are present at different compositions and temperatures under slow cooling conditions.
2. To indicate equilibrium solid solubility of one element in another.
3. To indicate temperature at which an alloy cooled under equilibrium conditions starts to solidify and the temperature range over which solidification occurs.
4. To indicate the temperature at which different phases start to melt.
In single component systems, there is no composition variable and the only other variables present are temperature and pressure. Such as Water can exist in solid, liquid or vapour phases depending on temperature and pressure.
Gibbs Phase Rule
J.W. Gibbs derived an equation from thermodynamic consideration that enables the number of phases that can coexist in equilibrium in chosen system to be computed.
P + F = C + 2
where, P is the number of phases which coexist in a chosen system; C is the number of components in the system and F is the degrees of freedom.
The degrees of freedom are the number of variables which can be changed independently without changing the state of the phase or phases in equilibrium in a chosen system.
Binary Phase Diagrams
Two component systems are usually presented in binary phase diagrams. Therefore, three-dimensional diagram is needed to plot the variations in temperature, pressure and composition. In order to simplify the presentation of phase relationships on paper binary phase diagrams are usually drawn at constant (atmospheric) pressure.
F = C – P + 1
Application of Phase Diagrams
Three kind of information are available, namely, i) the phases that are present, ii) the compositions of these phases, iii) the percentage or fraction of the phases.
Phases Present – One just locates the temperature-composition point on the diagram and note the phase with which the corresponding phase field is labelled.
Determination of Phase Compositions – The first step in the determination of phase composition is to locte the temperature-composition point on the phase diagram.
In some binary alloy systems, the two elements are completely soluble in each other in both the liquid and the solid states. In these systems, only a single type of crystal structure exists over all compositions range of components and therefore, is called the isomorphous system.
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