Mapping the Phases of Alloys
A binary phase diagram is the roadmap of alloy metallurgy. For any combination of two elements, it tells you exactly which phases are stable at a given temperature and composition. The diagram is constructed from cooling curve experiments and thermodynamic calculations, and it underpins every decision in alloy design, casting, and heat treatment.
The Isomorphous System
The simplest binary system is isomorphous — the two components are completely soluble in each other in both liquid and solid states (like Cu-Ni). The diagram has just two curves: the liquidus (above which everything is liquid) and the solidus (below which everything is solid). Between them, liquid and solid solutions coexist. The compositions of these coexisting phases are read from the endpoints of a horizontal tie line.
The Lever Rule
When you're in a two-phase region, the lever rule tells you how much of each phase is present. Think of the tie line as a seesaw balanced at the overall composition. The fraction of each phase is proportional to the opposite arm of the lever. This simple geometric construction is one of the most powerful tools in metallurgical engineering — it connects the phase diagram directly to microstructure.
Engineering Applications
Phase diagrams guide real engineering decisions. They tell foundry engineers the pouring temperature for a casting. They tell metallurgists which heat treatment will produce the desired microstructure. They predict whether an alloy will have a narrow or wide freezing range (important for avoiding casting defects). The Fe-C phase diagram alone is the foundation of the entire steel industry.