EQUILIBRIUM CRYSTALLIZATION 2
EQUILIBRIUM CRYSTALLIZATION OF Z
Composition Z lies on the En-An join, therefore the final solid must be a mixture of En and An in the proportions En:An = 70:30 - NO Fo or Si in the final solid.
For equilibrium crystallization of Z, the final liquid is situated at P and it behaves like composition X previously looked at.
At P, Fo is being resorbed and En and An are crystallizing from a liquid with a composition of the peritectic.
This resorbtion and crystallization contiunes until the last grain of Fo is resorbed into the last droplet of liquid which crystallizes into the last grains of En and An.
EQUILIBRIUM CRYSTALLIZATION OF A
Composition A lies in the En-Si-An triangle, so it's last liquid must lie at E, the eutectic, where En + Si +an are in equilibrium.
As this composition is initially in the Fo + L field, once it reaches the liquidus, it will begin to crystallize Fo and move directly away from Fo towards the En-Fo boundary curve. At the boundary curve the Fo will be resorbed and the En will crystallize, with the liquid composition moving down the boundary curve towards P.
The proportion of Fo to be resorbed is very small, so at some point along the boundary curve all the Fo will be removed. This point can be located by drawing a line through A from En to intersect the boundary curve ( Point R). At this point all the Fo is resorbed, but En continues to crystallize, so the liquid now moves off the boundary curve, directly away from En into the En + L field.
Crystallization of En continues until the boundary curve between the fields of En + L and An + L is reached (Point S), where An begins to crystallize along with the En. The liquid moves down the boundary curve from S to E.
At E En and An continue to crystallize and are joined by Si. The liquid composition stays at E until all the liquid is used up and the final solid that results is a mixture of En-An-Si.
EQUILIBRIUM CRYSTALLIZATION OF W
Composition W lies in the En + L field, so at the liquidus temperature En crystallizes and the liquid moves directly away from En to the boundary curve between En + L and Si +L (Point V), where Si joins En in crystallizing.
The liquid moves down the boundary curve from V to E, where An joins En and Si in crystallizing. At E all three solids phases crystallize until all the liquid is used up. The location of Composition W in the small triangle En-An-Si tells us that the final solid must be a mixture of these three phases.
It should be noted that on the phase diagram that nowhere do Fo and Si coexist together.