peritectic

A peritectic reaction is a type of invariant reaction in phase diagrams, where during cooling a liquid and a solid phase react to form a different solid phase at a fixed temperature, known as the peritectic temperature. This reaction defines a specific point on a multi-component phase diagram in which the liquid and one solid phase coexist with a second solid phase.

In a binary system, the reaction is typically written as L + α → β, and the reverse β → L + α occurs on heating. At the peritectic point, the compositions of the liquid, the original solid, and the product solid lie on a tie line, defining the unique equilibrium relationship among the three phases.

A common example occurs in the iron–carbon system. At about 1495°C, liquid and delta ferrite react to form austenite, described as L + δ → γ, with roughly 0.2 weight percent carbon. The exact temperature and composition depend on the overall alloy composition and other alloying elements.

The microstructure resulting from a peritectic reaction can be distinctive, often featuring newly formed solid phase that evolves away from the reaction front, producing fine lamellae or network-like structures. This can lead to non-uniformities if cooling is not carefully controlled during solidification or heat treatment.

Peritectic reactions influence solidification behavior, heat treatment responses, and the mechanical properties of alloys. They are distinct from eutectic reactions, where a liquid simultaneously transforms into two solids, and from peritectoids, where two solids transform into a different solid without a liquid involved.