Solid Solution Alloys

What is a solid solution?

A solid solution occurs when we alloy two metals and they are completely soluble in each other. If a solid solution alloy is viewed under a microscope only one type of crystal can be seen  just like a pure metal. Solid solution alloys have similar properties to pure metals but with greater strength but are not as good as electrical conductors.

The usual forms of solid solution are.

The name of this solid solution tells you exactly what happens as atoms of the parent metal ( or solvent metal) are replaced or substituted by atoms of the alloying metal (solute metal) In this case, the atoms of the two metals in the alloy, are of similar size.
Here we see the brown atoms have been replaced or substituted by the blue atoms.
Click here to see this occurring.
 
 
 
 
 
 
 
 
 
 
 
  In interstitial solid solutions the atoms of the parent or solvent metal are bigger than the atoms of the alloying or solute metal. In this case, the smaller atoms fit into interstices i.e spaces between the larger atoms.
The purple atoms are small enough to fit into the spaces between the larger solvent atoms.
Click here to see this occurring
 
 
 
 
 
 
 
 
 
 
 
 

In both substitutional and interstitial solid solutions the overall atomic structure is virtually unchanged. Examples of solid solution alloys include Copper- Nickel, Gold- Silver all whom have an F.C.C structure. Molybdenum- Tungsten is an example of an solid solution with a B.C.C structure.
Thermal diagrams created using solid solution alloys are given the name binary alloys and examples of these diagrams are shown below.
 


 
 
 

To learn more about these diagrams click here
 

One final thing we must deal with before we move on to the next type of alloy combination is The lever rule.
 
 

The Lever Rule

The equilibrium diagram for a solid solution alloy that we have just been dealing with contains two distinct phases, liquid and solid solutions. Between the liquidus and solidus lines these two phases exist together in equilibrium and hence the area between the curves is known as the two phase region.
If a horizontal line is drawn through the two phase region, such a line is called a tie line.
We see a tie line drawn in this equilibrium diagram.

 
 
 
 

The lever rule may be introduced by considering the simple see - saw shown here. For the see- saw to be balanced, i.e in equilibrium, without movement up or down on either side, (weight W1) ( distance X1) = (weight W2) (distance X2).

This is the lever rule and in metallurgy the horiziontal constant temperature tie-line represents the see - saw with the fulcrum at the alloy composition under consideration.
Therefore if we take the diagram for the Copper -Nickel alloy as above and we take the composition of 60% copper and 40% Nickel the lever rule will apply like this.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Weight of solid solution of composition q   = bm
Weight of liquid of composition m                  qb

     Ratio = bm/pb
 
 
 

You are now ready to move on to the next type of alloy combination i.e
EUTECTIC ALLOYS