Eutektoid

Eutektoid, or eutectoid, refers to a transformation in which a single solid phase decomposes into two distinct solid phases at a specific composition and temperature. This type of reaction occurs at a fixed point in a phase diagram and is analogous to a eutectic reaction, but it involves solid phases rather than a liquid and solids.

The most studied eutectoid system is iron–carbon. At approximately 0.76 percent carbon and about 727°C (the eutectoid point of the Fe–C diagram), austenite (gamma-iron) transforms into a finely layered mixture of ferrite (alpha-iron) and cementite (Fe3C). The resulting microstructure is known as pearlite. The lamellar spacing of pearlite depends on the cooling rate: slower cooling yields coarser lamellae, while faster cooling produces a finer pearlite with higher strength.

In steels and cast irons, the eutectoid composition divides materials into hypoeutectoid, eutectoid, and hypereutectoid categories, influencing heat-treatment responses and mechanical properties. Hypoeutectoid steels contain proeutectoid ferrite ahead of the pearlite front, while hypereutectoid steels form cementite networks before full pearlite reaction. The concept of eutectoid transformations helps explain how a single-phase austenite can yield a two-phase, composite microstructure that combines ferrite's ductility with cementite's hardness.

Beyond iron–carbon, eutectoid reactions also occur in other alloy systems, where a single solid phase decomposes into two solids at a fixed temperature and composition. The term “eutectoid” thus describes a general class of solid-state transformations with fixed stoichiometry and temperature.