Heat Treatment of Gold Alloys

Softening Heat Treatment

• Alloy heated at 700°C for 10 minutes, followed by water quenching at room temperature.
• The heating causes atomic diffusion, creating a disordered solid solution; rapid cooling prevents atomic ordering.
• Results in increased tensile strength and hardness, while ductility decreases.
• Suitable for gold alloys intended for cold working.

Hardening Heat Treatment

• Begins with softening heat treatment to form a disordered solid solution.
• Alloy is cooled for 15-30 minutes at 250-450°C before quenching.
• Controlled cooling facilitates the formation of an ordered Au-Cu superlattice, transitioning from disordered to ordered solid solution.
• Increases tensile strength and hardness at the expense of ductility.
• Recommended for metallic removable partial dentures and dental crowns and bridges.

Coring in Metallurgy

• Defined as a structure where the core contains higher melting alloy constituents, while the matrix has lower melting components.
• The cored structure is undesirable due to its low corrosion resistance.
• Occurs under conditions of:
  • Wide liquidus to solidus line range.
  • Rapid cooling of molten alloy.
  • Differences in melting temperatures of alloying metals.

Homogenization

• A process aimed at eliminating coring by removing compositional differences.
• Achieved by heating the cored structure below its melting temperature to promote atomic diffusion.

Eutectic Alloys

• Characterized by the lowest melting temperature, these alloys are entirely soluble in their liquid state but can be insoluble or partially soluble in the solid state.
• Common examples include:
  • Lead and tin, primarily used in soldering applications but not in dentistry.
  • Silver and copper (Ag-Cu) utilized in dental soldering, targeting the removal of the β-phase, which is the weakest phase in amalgam.

Properties of Solid Solution Alloys

• Exhibit a melting range instead of a distinct melting point.
• Homogeneous with a "1 phase system," showing good thermal and corrosion resistance.
• Generally, they exhibit higher strength and hardness compared to their parent metals, though with lower ductility.
• Solid solution alloys typically demonstrate higher ductility yet lower strength and hardness than eutectic alloys.

Properties of Eutectic Alloys

• Identified by a melting point rather than a range.
• Heterogeneous, containing a "2 phases system," leading to diminished thermal and corrosion resistance.
• Possess greater strength and hardness than their parent metals but display low ductility.
• Prone to brittleness due to the presence of insoluble phases inhibiting dislocation movement.

Definition of Eutectic

• Refers to the alloy with the lowest melting point.

Types of Solid Solutions

• Substitutional Solid Solutions: Two different atom types occupy different positions in the same crystal lattice. Often more ductile.
• Interstitial Solid Solutions: Small atoms fit into the spaces between larger ones, exemplified by carbon in iron.

Conditions for Formation of Substitutional Solid Solutions

• Metals should share the same type of space lattice.
• Must possess similar valence.
• Atomic size difference should be less than 15%.
• Non-reactive to form intermetallic compounds.

Phase Diagram

• Graphical representation indicating temperature-composition relationships, derived from a collection of alloy cooling curves.
• Key uses:
  • Predict phase presence.
  • Determine chemical composition of each phase.
  • Identify appropriate melting temperature for casting.
• Casting temperature should be 100°C above the melting range to ensure fluidity and improve castability.
• Pure metals possess a distinct melting point, while alloys show a melting range.