eutectoid

Eutectoid describes a specific type of invariant phase transformation in solid alloys, occurring at a defined temperature and composition. At the eutectoid temperature, a single solid phase becomes unstable and transforms into two distinct solid phases in a single, cooperative reaction. The term denotes both the temperature and the composition at which this transformation occurs and is a key feature of phase diagrams for alloy systems.

In the iron–carbon system, the eutectoid transformation happens at about 0.76–0.77% carbon and 727°C. The austenite (gamma, γ-Fe) transforms upon cooling into a fine, lamellar mixture of ferrite (alpha, α-Fe) and cementite (Fe3C), known as pearlite. Pearlite offers a balance of strength and ductility in steel and is a central consideration in heat treatment. In hypo- and hyper-eutectoid steels, proeutectoid ferrite or proeutectoid cementite forms before the eutectoid transformation.

The concept of a eutectoid reaction generalizes to other alloy systems beyond iron–carbon. In those systems, a single solid phase at the eutectoid composition and temperature decomposes into two different solid phases, illustrating the solid-state counterpart to the eutectic reaction, where a liquid transforms into two solids at the eutectic composition. The eutectoid point is thus a defining feature of phase diagrams, marking where a single phase becomes two distinct phases upon cooling.

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