the peritectic reaction
/ pehr-ih-TEK-tik /
The peritectic is the eutectic's awkward cousin. Instead of a liquid splitting into two solids, here an existing solid and the liquid react together on cooling to form a different single solid. Peri means around, and the new solid tends to form as a shell wrapping around the old one, which then chokes the reaction.
More precisely, the peritectic reaction is solid alpha plus Liquid to solid beta on cooling, at an invariant temperature where three phases coexist (F = 0). One existing solid consumes the surrounding liquid and turns into a new phase. Because the product beta forms a layer around alpha and separates it from the liquid, the reaction often cannot finish, since atoms must diffuse through the growing beta shell, and real castings end up with leftover cored alpha. It appears at the top of the iron-carbon diagram (delta ferrite plus L to austenite near 1493 degrees C) and in systems like Cu-Zn and Pt-Ag.
Peritectics matter in steel solidification and in many technical alloys, but their sluggish, incomplete nature makes non-equilibrium structures common. You recognize one on a diagram as a horizontal line where the composition of the product phase lies between the other two, unlike the eutectic, where instead the reacting liquid sits between its two products.
In Fe-C: delta ferrite + L to austenite near 1493 degrees C, at the very top of the diagram.
The product's composition lies between the two reactants, unlike a eutectic.
Peritectic reactions are notoriously incomplete because the product forms an enveloping shell that blocks diffusion, so full equilibrium is rarely reached in practice.