Eutectic vs Eutectoid vs Peritectic Reactions: Complete Comparison

Invariant reactions govern how phases nucleate, disappear, and rearrange at fixed temperature and composition points on any binary phase diagram. Eutectic, eutectoid, and peritectic reactions look superficially similar — three lines converging at a horizontal isotherm — but they involve fundamentally different parent and product phases, diffusion mechanics, and resulting microstructures. This guide compares all three side by side using the iron-carbon system as the working example, and includes an interactive lever rule calculator for computing phase fractions at each reaction.

What Is an Invariant Reaction?

An invariant reaction occurs at a fixed temperature and fixed phase compositions, with zero degrees of freedom under the Gibbs phase rule for a binary system at constant pressure (F = C − P + 1 = 2 − 3 + 1 = 0 when three phases coexist). Because F = 0, the reaction proceeds at constant temperature until one of the parent phases is fully consumed — it cannot occur over a temperature range the way ordinary solidification or solid-state transformations do. Eutectic, eutectoid, and peritectic reactions are the three most common invariant reactions encountered in binary metallic systems; monotectic, syntectic, and metatectic reactions exist but are far less common in structural alloys.

General forms (on cooling):
Eutectic:    L        →  α + β
Eutectoid:   γ (solid) →  α + β
Peritectic:  L + α    →  β

Eutectic Reactions

In a eutectic reaction, a single liquid phase decomposes on cooling into two solid phases simultaneously, at one fixed composition and temperature — the eutectic point. This is the lowest-melting composition in the system, which is why eutectic alloys (solder, cast iron, Al-Si casting alloys) are chosen where low, sharp melting points are advantageous.

Fe-C Eutectic: Ledeburite Formation

In the iron-iron carbide system, the eutectic reaction occurs at 4.3 wt% C and 1147 °C: liquid transforms into austenite (2.14 wt% C) plus cementite (6.67 wt% C). The resulting fine mixture is called ledeburite. This reaction is only relevant to cast irons and high-carbon alloys — plain carbon and low-alloy steels never reach 4.3 wt% C and therefore never pass through the eutectic point.

Other Common Eutectic Systems

• Pb-Sn solder: eutectic at 61.9 wt% Sn, 183 °C
• Al-Si casting alloys: eutectic near 12.6 wt% Si, 577 °C
• Ag-Cu: eutectic at 71.9 wt% Ag, 779.4 °C

Eutectoid Reactions

A eutectoid reaction has the identical topology as a eutectic reaction, but the parent phase is solid, not liquid. One solid phase decomposes into two different solid phases at a fixed composition and temperature.

Fe-C Eutectoid: Pearlite Formation

The eutectoid point in the Fe-C system sits at 0.77 wt% C and 727 °C. On slow cooling through this temperature, austenite transforms into pearlite — alternating lamellae of ferrite (0.022 wt% C) and cementite (6.67 wt% C). Pearlite is a microstructural constituent, not a phase; it consists of two distinct phases arranged in a characteristic lamellar morphology that nucleates at austenite grain boundaries and grows as coupled diffusion fronts.

This single reaction underpins essentially all conventional heat treatment of steel: annealing produces coarse pearlite, normalizing produces finer pearlite, and suppressing this reaction entirely by rapid quenching is what produces martensite instead.

Peritectic Reactions

A peritectic reaction is structurally the reverse of the other two: a solid phase and a liquid phase, present together above the reaction temperature, combine on cooling into a single new solid phase. Unlike eutectic and eutectoid reactions, the peritectic reaction consumes two parent phases to produce only one product phase.

Fe-C Peritectic: Delta to Austenite

The peritectic reaction in the Fe-C system occurs near 1495 °C, where delta-ferrite (about 0.09 wt% C) reacts with liquid (about 0.53 wt% C) to form austenite (about 0.17 wt% C). This reaction only affects steels solidifying with bulk composition in roughly the 0.09-0.53 wt% C range — most structural and machinery steels fall in this window during solidification, even though the reaction has no bearing on their room-temperature microstructure.

Why Peritectic Reactions Rarely Go to Completion

Because the new gamma phase nucleates at the delta-liquid interface, it forms as a shell that separates the two remaining parent phases. Further reaction requires solid-state diffusion through this shell, which is far slower than liquid-phase diffusion. Consequently, peritectic reactions are notoriously incomplete even under slow equilibrium cooling, and cored, non-equilibrium microstructures are common — a practical concern in continuous casting of peritectic-range steels, where it contributes to surface cracking.

Side-by-Side Comparison

Applying the Lever Rule at Each Reaction

Just below (or, for peritectic, just above) the invariant temperature, the alloy sits inside a two-phase field bounded by a horizontal tie line. The lever rule computes the mass fraction of each phase from the three compositions on that tie line — the same mechanics used anywhere on the iron-carbon phase diagram.

Fraction of left-side phase  W_left  = (C_right - C0) / (C_right - C_left)
Fraction of right-side phase W_right = (C0 - C_left) / (C_right - C_left)

Worked example (eutectoid steel, C0 = 0.77 wt% C):
  C_left  (ferrite)   = 0.022 wt% C
  C_right (cementite) = 6.67  wt% C
  W_ferrite   = (6.67 - 0.77) / (6.67 - 0.022) = 5.90 / 6.648 = 0.888  (88.8 wt%)
  W_cementite = (0.77 - 0.022) / (6.67 - 0.022) = 0.748 / 6.648 = 0.112  (11.2 wt%)

For the peritectic reaction, the same arithmetic determines whether delta-ferrite or liquid is present in excess relative to the exact peritectic ratio, which governs whether the alloy ends up fully austenitic or retains untransformed delta-ferrite just below 1495 °C. Use the calculator above with the “Peritectic” preset to run this for any composition in the 0.09-0.53 wt% C window.

Distinguishing the Reactions on an Unlabeled Diagram

Look at What Sits Above the Isotherm

If a single phase field sits directly above the horizontal invariant line and two different phase fields sit below it, the reaction is eutectic (if the single field above is liquid) or eutectoid (if it is solid). If two different phase fields approach the isotherm from opposite sides above it, converging into one phase field below, the reaction is peritectic.

Check the Number of Phases Consumed

Eutectic and eutectoid reactions consume one phase and produce two. Peritectic reactions consume two phases and produce one. This is the fastest diagnostic once you have identified the phase fields immediately above and below the isotherm.

Industrial Significance

Every plain carbon and low-alloy steel passes through the peritectic region during solidification if its composition falls below about 0.53 wt% C, then cools through the austenite field, and finally passes through the eutectoid point during any slow cool, anneal, or normalizing treatment. Cast irons additionally pass through the eutectic point during solidification. Understanding which reaction governs a given processing step is fundamental to controlling as-cast segregation, hot cracking susceptibility during continuous casting, and the final annealed or normalized microstructure. It also underlies interpretation of weld HAZ microstructures, where rapid thermal cycles can suppress or truncate the eutectoid reaction locally.

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