Eutectic Alloy Properties and Phase Diagrams

Study Notes

Factors Affecting Coring

• The centers or core composed of compositions with higher solidus temperature and the matrix containing compositions with a lower solidus. • The cored structure is not an equilibrium structure. • The greater the temperature range between the liquidus and the solidus, the greater is the tendency toward coring. • During slow cooling, there is greater atomic diffusion toward equilibrium in an attempt to avoid coring. • Rapid cooling does not allow for atomic diffusion and causes coring.

Homogenization

• A heat treatment procedure to relieve coring. • The alloy is held at a temperature near its solidus so that atomic diffusion can occur. • Little or no grain growth occurs. • Usually heating for 6 hours at high temperatures is required for homogenization. • The ductility of the alloy is usually greater after homogenization.

Eutectic Alloy

• Exhibits complete liquid solubility but limited solid solubility. • Characterized by having a melting point rather than a melting range like other types of alloys. • Properties of eutectic alloys: + Brittle because the presence of insoluble phases definitely inhibits slip and dislocation movement. + Strength and hardness exceed those of the constituent metals. + Poor tarnish and corrosion resistance due to the heterogeneous structure with different phases.

Intermetallic Compound

• Occurs when there is chemical affinity between metals. • On cooling of some liquid metal solution, the resulting solid phase has a fixed chemical composition which is formed at different temperatures.

Methods of Metal Strengthening

Control of Grain Size

• Grain size is known to be time and temperature controlled. • The smaller the grain size, the stronger are the mechanical properties. • Factors affecting grain size: + Rate of cooling from the liquid state + Rate of crystallization and rate of nucleation + Nucleating agents “Grain refiners”

Solid-Solution Strengthening

• Discussed previously

Strain Hardening or Cold Working

• Phenomenon by which a ductile metal becomes harder and stronger as it is plastically deformed at low temperatures relative to the absolute melting temperature of the metal. • Also called work hardening or cold working. • Most metals strain harden at room temperature.

Wrought Metals

• Metals that had been formed from cast structures by cold working to attain a microscopically fibrous structure. • Plastic deformation (hammering, rolling or drawing into a wire) transforms the cast structure into a fibrous structure.

Effect of Cold Working on Mechanical Properties

• During cold working, the stress induced in the metal higher than its yield strength permanent deformation occurs by movement of dislocation. • Dislocation motion is obstructed by the grain boundary, due to the difference in the orientation of the crystalline planes between the adjacent grains. • As more and more dislocations move in the same direction and accumulate near the same boundary, a “dislocation pile-up” is formed, leading to “back stress” that acts in an opposite direction and resists further dislocation movement.

Reversing Cold Working Effect

• The effect of cold working can be reversed simply by heating, termed heat treatment annealing. • The heating of a cold-worked metal may lead to the following three stages: 1. Stress-relief anneal or recovery 2. Recrystallization 3. Grain growth

Annealing Heat Treatment

• Recovery: Relief of the induced stresses during cold working. • Recrystallization: Growing new crystals from previously deformed crystals, resulting in softer material with high ductility and low strength. • Grain Growth: Increase in ductility and decrease in strength and proportional limit.