PHASE RULE
To refresh your memory the phase relates the number of phases present in the system, the the number of components which define the system and the degrees of freedom displayed the by the system at equilibrium.
Phase Rule P + F = C + 1
With referernce to the hypothetical ABC system:
- At Point 1
Point 1 lies in the C + L field
- P = 2 - Solid C and Liquid
- C = 3 - three components A, B and C
- F = 2 - two degrees of freedom, giving a divariant surface
- To maintain equilbrium, i.e. to keep Solid C and Liquid as the stable phases, temperature and composition may both vary independently of each other .
- At Point 2
Point 2 lies on the Boundary Curve which separates the field of Solid A + LIquid from the field of Solid C + Liquid.
- P = 3 - Solid A, Solid C and Liquid
- C = 3 - three components, A, B and C
- F = 1 - One degree of freedom, giving a univariant line.
- To maintain equilibrium between the three phases i.e. to stay on the boundary curve, only temperature or pressure may be changed independently, fixing one automatically fixes the other.
- At Point E
Point E represents the ternary eutectic, where the three fields A + L, B + L and C + L meet.
- P = 4 - Solid A, Solid B, Solid C and Liquid
- C = 3 - three components, A, B and C
- F = 0 - No degrees of freedom, yielding an invariant point.
- To maintain equilibrium, i.e. to have all four phases stable and in euilibrium the position of the eutectic is fixed in terms of temperature and composition, changing either will result in a shift of the system away from the eutectic.
