Some might be knowing that i have to present a seminar today in dental materials, and the topic is “Eutectic Alloys”.
Here is something in brief related to my topic:
Eutectic alloys are a type of alloy in which the component metals exhibit limited solid solubility. Two metals A & B, which are completely insoluble in each other, are considered as a eutectic alloy. Some grains are solely composed of metal A and some are solely of metal B.
Similarly in a brine solution, salt and water molecules intermingle with each other in liquid phase but are a mixture of salt crystals and ice crystals in a frozen state, and are independent of each other.
One of these types of eutectic alloys is the Silver-Copper system.
This system has 3 phases:
1. Liquid phase.
2. Alpha phase - Silver rich phase with small amounts of copper atoms.
3. Beta phase - Copper rich phase with small amounts of silver atoms.
In a phase diagram, these 2 phases are seen in the left and right side of the diagram, and so are called as “Terminal solid solutions”.
Composition:
A composition of 72% silver and 28% copper is called as eutectic composition or the Eutectic.
Characteristics:
1. Temperature at which the eutectic composition melts(779 C) is lower than the fusion temperature of silver or copper and this is the lowest temperature at which Ag-Cu alloy is completely liquid. Eutectic is also called ‘Lowest Melting’.
2. The liquid composition freezes at a constant temperature like the pure metals but solid has 2 phases alpha and beta.
3. Eutectic alloys are often used when a lower fusion temperature is required, such as in dental solders.
Solidification:
When binary eutectic solidifies, atoms segregate to form regions of nearly pure patent metals. A layered or lamellar structure is formed. For some alloys, rod shaped eutectic morphologies are seen and this is called “Eutectic Constituent”.
Eutectic Reaction:
Liquid -> Alpha solid solution and beta solid solution.
Reaction is also called Invariant Transformation, as it occurs in single temperature and composition.
Physical Properties:
- Mechanical properties of the solid solution alloys vary much higher with composition, than that of Pd-Ag system.
- Alloys in these systems with system with composition less than that of eutectic are called hypoeutectic alloys.
- Alloys in the systems with system with composition greater than eutectic are called hypereutectic alloys.
Primary crystals of hypoeutectic alloys in Ag-Cu systems consists of alpha solid solutions and primary crystals of hypereutectic alloys consists of beta solid solution.
- Hypoeutectic or hypereutectic alloys containing eutectic constituent in their microstructures are relatively brittle and those lacking eutectic constituent are ductile. This is due to permanent deformation of the alloys due to the alternating lamellae of both the phases.