Eutectic Alloys in Dentistry: Structure, Properties, and Applications

Structure And Properties Of Metals And Alloys

An alloy is defined as a mixture of two or more metals and non-metals.

Eutectic Alloys In Dentistry

Eutectic Alloy:

• Eutectic alloys are named on the basis of the dominant phase system present in the alloy.
• Eutectic is a term that describes two components that are completely soluble in each other in the liquid or molten state, but only partially soluble in the solid state.

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• Therefore, these alloys do not form solid solutions. Rather, eutectic alloys on cooling convert from liquids to intimately mixed solids.
• Alloys with a composition less than that of eutectic are hypoeutectic and alloys with a composition more than eutectic are called hypereutectic.

Eutectic Alloys with Example:

Silver–copper alloy is used in dentistry. Silver and copper are not soluble in each other.

They form different phases depending on the concentration of silver and copper. The following three distinct phases can be found, i.e.

1. A liquid phase (L) that can occur at temperatures lower than 400°C only with extremes of composition, i.e. <l% Ag or <l% Cu, since they are insoluble in each other
2. A silver-rich solid solution phase (α) containing a small amount of copper
3. A copper-rich solid solution phase (β) containing a small amount of silver

The α and β phases are termed terminal solid solutions.

• AED is Liquidus line
• ABEGD is solidus line

Solidus and liquidus lines meet at E rather than at pure metal composition

Eutectic Alloys In Dentistry

When the composition of the silver-copper alloy is 72% silver and 28% copper, the alloy melts at 779°C. This temperature is lower than the fusion temperatures of either silver or copper, hence the name “eutectic” is given which means “lowest melting”.

A eutectic alloy system occurs when two metals are soluble as liquids but nearly insoluble as solids. These alloys usually have a single composition (in the case of silver-copper alloy, 72% Ag and 28% Cu), and all the alloys will be in the eutectic phase at room temperature. If the composition varies other than this, then the alloy will have varied combinations of solid solutions and eutectic phases. The melting point of an alloy containing pure eutectic composition is lower than the melting point of either of the components.

Eutectic Alloys In Dentistry

Equilibrium Phase Diagram of Ag–Cu:

In the phase diagram, at three different concentrations, the material will be solid until it is heated to the melting point and then after adding the heat of fusion it becomes, liquid at that same temperature

1. Unalloyed extreme left
2. Unalloyed extreme right
3. Dip in the center E (Eutectic composition)

At other compositions, the material will enter a mushy or pasty phase till it warms to its melting temperature. The mixture at the dip point (E) of the diagram is called a eutectic alloy.

Features of Eutectic System:

• Eutectic composition melts at a temperature of 779 °C which is lower than either of the pure parent metals. This property is useful for soldiers.
• Solidus and liquidus lines meet at the Eutectic temperature.
• Thus, there is no solidification range at the eutectic temperature of 779 °C.

Applications of Eutectic Alloys:

• 50% lead and 50% tin alloy is a good example of eutectic alloy systems.
• In dentistry, Ag-Cu eutectic alloy is used in silver amalgam systems.
• Gold-iridium system is eutectic at an iridium concentration of 0.005%.