Solid Solutions

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 common types of solid solutions are

1)   Substitutional solid solution

2)      Interstitial solid solutions

Substitution solid solution

 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.

 Interstitial solid solutions:

 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.

 Phases

One-phase systems are homogeneous. Systems with two or more phases are heterogeneous, or mixtures. This is the case of most metallic alloys, but also happens in ceramics and polymers.

A two-component alloy is called binary. One with three components is called ternary.

 Microstructure

The properties of an alloy do not depend only on concentration of the phases but how they are arranged structurally at the microscopy level. Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space.

 A binary alloy may be

·         A single solid solution

·         Two separated essentially pure components. ØTwo separated solid solutions.

·         A chemical compound, together with a solid solution.

Phase diagram: 

A graph showing the phase or phases present for a given composition as a function of temperature.

 Poly phase material:

A material in which two or more phases are present.

 Eutectoid:

Transforming from a solid phase to two other solid phases upon cooling.

 Peritectoid:

Transforming from two solid phases to a third solid phase upon cooling.

 Peritectoid reaction:

A reaction in which a solid goes to a new solid plus a liquid on heating, and reverse occurs on cooling.

Iron-Iron Carbon diagram is essential to understand the basic differences among iron alloys and control of properties.

Iron is allotropic; at room temperature pure iron exists in the Body Centered Cubic crystal form but on heating transforms to a Face Centered Cubic crystal. The temperature that this first transformation takes place is known as a critical point and it occurs at 910 degrees Celsius. This change in crystal structure is accompanied by shrinkage in volume, sine the atoms in the face centered crystal are more densely packed together than in the body centered cubic crystal. At the second critical point the F.C.C crystal changes back to a B.C.C crystal and this change occurs at 1390 degrees Celsius. 

·         Iron above 1390 degrees is known as delta iron

·         Iron between 1390 and 910 degrees is known as gamma iron, Iron below 910 degrees is known as alpha iron.