NATURE OF ALLOYS
Thermodynamic rules are used to get rules for forming alloys.
Pure metals have definite freezing point. When some element ( impurity ) is added to a pure metal, an alloy is formed, but it will solidify either at a higher or lower temperature as compared to the pure metal. In some cases, alloy will solidify over a range of temperature.
Most of the metals form completely homogeneous solution in liquid state.
When such alloys are cooled from liquid to solid state, there are 3 possibilities with regard to solubility into each other in solid state :
- Two substances may have infinite solubility
- Two substances may have limited solubility
- Two substances may have no solubility
Solubility : The maximum amount of a solute that can dissolve in a solvent before solute ceases to dissolve and remains in a different phase.
Solid solutions are formed under first 2 possibilities mentioned above.
1) Infinite solid solubility
When two substances ( metals ) show complete solid solubility, they form an isomorphous system as shown in figure given above.
Isomorphous : Substances of same crystal structure
Here,
A and B are two chemical substances,
α is substitutional solid solution of A and B,
L is liquid solution of A and B
α + L is mixture of α and L
Complete/Infinite solid solubility means both of the substances can be soluble in each other to any extent.
Eg. In Cu-Ni system, suppose we are considering only Cu.
here Cu is a pure metal. Now we can solute Ni in Cu to any weight % ( upto 99% since 100% is pure Ni ). In same way if we consider Ni then we can solute Cu to any weight %.
In order to achieve complete solid solubility, the atomic size of both substances should be same. This conclude that here the solid solution will be of substitutional type only. Atomic radii of Cu is 128 nm and atomic radii of Ni is 125 nm.
Since solid solution is a homogeneous phase, its microstructure would resemble that of a pure metal.
Eg. Both Cu and Ni have BCC crystal structure and the alloy they form, bronze, also have BCC structure.
2) Limited Solid Solubility
Those substances which have less difference in size factors show complete solubility in all proportions ( as seen in Cu-Ni system ). As size factor increases, the solid solubility limits are changed.
Eg. Cu-Ag system exhibits partial solubility.
Let's consider a Cu-Ag system which is a eutectic system as shown in figure.
Here ( eutectic system ) ,
A and B are two different substances
α is A rich solid solution,
β is B rich solid solution,
α + β is a mixture of both α and β solid solutions,
L is liquid solution,
α + L is mixture of α and liquid,
β + L is mixture of β and liquid.
As shown in figure,
solid solubility of B in A is α which is limited to start of hypoeutectic region; solid solubility of A in B is β which is limited to end of hypereutectic region.
In central region ( hypoeutectic + hypereutectic region ) there exists mixture of both α and β solid solutions. If we observe the microstructure of the α + β mixture we can clearly differentiate between both of them.
3) No solid solubility
When a solid solution is not formed, the formation of compounds may take place. The phases which formed in the intermetallic regions of the phase diagrams may give rise to the formation of compounds.
This situation will occur when the elevtomotive values of both components have large difference between them, as discussed in Hume-Rothery rules.
Solid solutions are formed under first 2 possibilities mentioned above.
1) Infinite solid solubility
Isomorphous : Substances of same crystal structure
Here,
A and B are two chemical substances,
α is substitutional solid solution of A and B,
L is liquid solution of A and B
α + L is mixture of α and L
Complete/Infinite solid solubility means both of the substances can be soluble in each other to any extent.
Eg. In Cu-Ni system, suppose we are considering only Cu.
here Cu is a pure metal. Now we can solute Ni in Cu to any weight % ( upto 99% since 100% is pure Ni ). In same way if we consider Ni then we can solute Cu to any weight %.
In order to achieve complete solid solubility, the atomic size of both substances should be same. This conclude that here the solid solution will be of substitutional type only. Atomic radii of Cu is 128 nm and atomic radii of Ni is 125 nm.
Since solid solution is a homogeneous phase, its microstructure would resemble that of a pure metal.
Eg. Both Cu and Ni have BCC crystal structure and the alloy they form, bronze, also have BCC structure.
2) Limited Solid Solubility
Those substances which have less difference in size factors show complete solubility in all proportions ( as seen in Cu-Ni system ). As size factor increases, the solid solubility limits are changed.
Eg. Cu-Ag system exhibits partial solubility.
Here ( eutectic system ) ,
A and B are two different substances
α is A rich solid solution,
β is B rich solid solution,
α + β is a mixture of both α and β solid solutions,
L is liquid solution,
α + L is mixture of α and liquid,
β + L is mixture of β and liquid.
As shown in figure,
solid solubility of B in A is α which is limited to start of hypoeutectic region; solid solubility of A in B is β which is limited to end of hypereutectic region.
In central region ( hypoeutectic + hypereutectic region ) there exists mixture of both α and β solid solutions. If we observe the microstructure of the α + β mixture we can clearly differentiate between both of them.
3) No solid solubility
When a solid solution is not formed, the formation of compounds may take place. The phases which formed in the intermetallic regions of the phase diagrams may give rise to the formation of compounds.
This situation will occur when the elevtomotive values of both components have large difference between them, as discussed in Hume-Rothery rules.