Dental Alloys: Types, Properties, Applications

Dental Alloys: Types, Properties, Applications

Peritectic Alloy:

• Peritectic alloys are alloys involving phase reaction and transformation upon solidification because of the partial solubility of constituent metals.
• Like the eutectic transformation, the peritectic reaction is an invariant reaction that occurs at a particular composition and temperature.
• In peritectic transformation, a liquid and solid phase of fixed proportion reacts at a fixed temperature to yield a single solid phase.
• Peritectic reaction during the cooling phase can be written as:
• Liquid + β solid solution ––––––→ α solid solution
• They are susceptible to coring during rapid cooling. The cored structure is more brittle and has less corrosion resistance than a homogenous solid solution.

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Peritectic Alloys In Dentistry

Examples of Peritectic Alloy:

• The Ag-Sn system which is the basis for the original dental amalgam alloys is a peritectic system.
• A peritectic phase transformation also occurs in the Ag- Pt system which is associated with many gold-casting alloys.
• Also, the Pd -Ru alloy system has a peritectic reaction at 16.5%. Ruthenium (Ru) is an important grain refining element for palladium casting alloys.

Phase Diagram of Silver—Platinum Alloy System for Peritectic Alloy:

As per the phase diagram, there is silver—a rich α phase and platinum-rich β phase with limited solid solubility in between.

The limited solubility of silver in platinum is less than approximately 12% at 700 °C while the equilibrium solid solubility of platinum in silver is approximately 56% at 700 °C. Peritectic transformation occurs at a point where the solid solution, i.e. composition at β and platinum β phase transforms in silver rich α phase.

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Peritectic Alloys In Dentistry

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Here in the phase diagram: Peritectic alloy

• α + β phase is an alloy combination of silver and platinum with limited solid solubility.
• α is the limited solid solubility of platinum in silver (56%)
• β is the limited solid solubility of silver in platinum (12%)
• 961.8°C is the melting point of silver
• 1768 °C is the melting point of platinum
• T is the peritectic transformation point.