Alloy Systems
T-x Phase Diagram
Microstructure Evolution
Parameters
Composition Control
Temperature Control
Display Options
Lever Rule
Physical Principles
Phase Regions
Instructions
- Select different alloy systems (Pb-Sn, Cu-Ag) or create custom parameters
- Adjust composition and temperature to explore different phase regions
- Observe microstructure changes based on composition and temperature
- Use lever rule to calculate phase fractions in two-phase regions
- Enable cooling curve animation to see solidification process
What is an Alloy Phase Diagram?
An alloy phase diagram is a graphical representation showing the equilibrium relationships between phases in a two-component alloy system as a function of temperature and composition. The T-x diagram displays liquid, solid, and two-phase regions where liquid and solid coexist. Understanding phase diagrams is crucial for predicting microstructure and designing alloys with specific properties.
Eutectic Reaction
The eutectic reaction (L ⇌ α + β) occurs at a specific composition and temperature where liquid transforms simultaneously into two solid phases. This point represents the lowest melting temperature in the system. Eutectic alloys solidify at a constant temperature, similar to pure metals, and form characteristic lamellar or rod-like microstructures. Examples include Pb-62%Sn solder (melting point 183°C) and Al-12%Si casting alloys.
Lever Rule
The lever rule calculates the relative amounts of phases in two-phase regions. It treats the composition as a lever balance point: the fraction of each phase is proportional to the distance from the overall composition to the opposite phase boundary. For liquid (L) and solid (α) phases: fₗ = (x - xₐ)/(xₗ - xₐ), where x is overall composition, xₗ is liquid composition, and xₐ is solid composition. This principle is fundamental for predicting microstructure and material properties.
Microstructure Evolution
During cooling, alloys develop different microstructures depending on composition. Hypoeutectic alloys (composition < eutectic) form primary α phase first, then eutectic mixture. Hypereutectic alloys (composition > eutectic) form primary β phase first. The eutectic composition forms entirely as fine lamellar structure. The scale and morphology of these microstructures determine mechanical properties: fine eutectic structures increase strength, while primary phases affect ductility and toughness.
Cooling Curves
Cooling curves plot temperature vs. time during solidification. Pure metals and eutectic alloys show thermal arrest (plateau) at the melting point due to latent heat release. Off-eutectic compositions show two arrests: first when primary phase begins forming, second at eutectic temperature. The length of eutectic plateau increases with amount of eutectic mixture. Cooling curve analysis is used experimentally to determine phase diagram boundaries.
Applications
Alloy phase diagrams are essential in materials science and engineering: selecting solder alloys for electronics (Pb-Sn, Sn-Ag-Cu), designing casting alloys (Al-Si, Fe-C), developing heat-treated alloys (Al-Cu, Mg-Al), predicting welding behavior, controlling microstructure through heat treatment, and troubleshooting material failures. Common applications include soft solder (Pb-63%Sn, melts at 183°C), brazing alloys (Cu-Ag, melts at 780°C), and aluminum casting alloys (Al-Si system).