A reaction in which two solid phases crystallize simultaneously from the liquid on cooling, L → α + β. In a binary system it occurs at a fixed temperature and composition and produces eutectic structures such as lamellae.
Japanese name: 共晶反応
Eutectic reaction appears in 4 practice questions across 2 topics, including Phase diagrams and the lever rule, Materials processing and joining.
In a binary system with a eutectic reaction, the point where the liquidus lines descending from both sides meet in a V shape is the eutectic point, and liquid of that composition solidifies all at once at the lowest temperature in the system. The eutectic temperature is lower than the melting point of either component. Typical examples are the Pb–Sn system (183 °C, 61.9 wt% Sn), the Al–Si system (about 577 °C, about 12.6 wt% Si) and the Fe–C system (1147 °C, 4.3 wt% C), whose eutectic structure is called ledeburite. Three phases coexist at the eutectic temperature, and the number of degrees of freedom is zero.
In a eutectic reaction, the two solid phases grow side by side while exchanging solute through the liquid, producing fine lamellar or rod-like structures. The faster the solidification rate v, the smaller the lamellar spacing λ, and the relation that λ²v is nearly constant is well known. If one of the phases tends to form crystal facets, the structure becomes irregular; in Al–Si alloys, Si forms coarse plates and makes the alloy brittle. Adding small amounts of Na or Sr changes the Si into fine fibers (modification), raising elongation and toughness.
In alloys away from the eutectic composition, one solid phase first crystallizes as the primary phase when the liquidus is crossed, and only the liquid remaining when the eutectic temperature is reached becomes eutectic structure. The room-temperature microstructure is therefore "primary phase + eutectic structure", while in terms of phases both consist of the two phases α and β. The fractions of the microconstituents are found with the lever rule just above the eutectic temperature, and the fractions of the phases just below it. In real solidification, segregation can produce a small amount of eutectic in the last regions to solidify, even at compositions that would form no eutectic in equilibrium.
Alloys of eutectic composition solidify at a single temperature with no freezing range, so they have good fluidity and are less prone to cracking and segregation during solidification. Eutectic or near-eutectic compositions are therefore used for solders (such as the Sn–Pb eutectic 63Sn–37Pb and Sn–3.0Ag–0.5Cu, which melts at about 217–220 °C), die-casting Al–Si alloys and cast irons. Because eutectics lower the melting point, the eutectic concept is also essential to designing low-melting alloys and brazing filler metals.
Answer: A. About 0.27
The eutectic structure is what the liquid (61.9 wt% Sn) remaining just above the eutectic temperature turns into through the eutectic reaction. So apply the lever rule in the α + L region just above the eutectic temperature: W_eutectic = W_L = (30 − 18.3)/(61.9 − 18.3) = 11.7/43.6 ≈ 0.27. About 0.73 is the fraction of the remaining primary α, the mistake of taking the opposite distance. About 0.15 is the fraction of all β phase just below the eutectic temperature, (30 − 18.3)/(97.8 − 18.3). All the β is inside the eutectic structure, but the eutectic structure also contains α, so the two do not match. The key to this kind of problem is distinguishing microconstituents (primary and eutectic) from phases (α and β).
Answer: C. L + α → β
A peritectic reaction is one in which, on cooling, the liquid L reacts with a previously formed solid α to produce a different solid β: L + α → β. The name comes from the new β phase forming around the α grains as if wrapping them. In the Fe-C system δ + L → γ occurs at about 1495 °C, and peritectics are common in the Cu-Zn and Cu-Sn systems. L → α + β is the eutectic reaction, in which two solids form simultaneously from the liquid, and γ → α + β is the eutectoid reaction, in which two solids form from a solid (such as γ → α + Fe₃C in the Fe-C system). In a peritectic reaction, the β product coats the α and hinders diffusion, so in real solidification the reaction does not go to completion and α tends to remain.
On cooling, a reaction in which two solid phases form simultaneously from a liquid is eutectic (L → α + β), and one in which two solid phases form simultaneously from one solid phase is eutectoid (γ → α + β). A eutectoid reaction proceeds by diffusion in the solid, so its structure tends to be finer than a eutectic one.
Yes. The eutectic point is where the liquidus lines descending from both sides meet, so the eutectic temperature is lower than the melting point of either pure component. For example, the eutectic temperature of Pb (327 °C) and Sn (232 °C) is 183 °C.
Their liquidus and solidus temperatures coincide, so they solidify at a single temperature. With no temperature range in which solid and liquid coexist, the melt flows well and is less likely to crack or segregate during solidification.
The eutectic structure is what the liquid (61.9 wt% Sn) remaining just above the eutectic temperature turns into through the eutectic reaction.
In the Pb-Sn system, take the eutectic temperature as 183 °…L → α + β is the eutectic reaction, in which two solids form simultaneously from the liquid, and γ → α + β is the eutectoid reaction, in which two solids form from a solid (such as γ → α + Fe₃C in the Fe-C system).
Which equation represents a peritectic reaction? Write it i…The reaction at about 4.3 wt% C and 1147 °C is the eutectic reaction involving the liquid, and the structure it produces is ledeburite, not pearlite.
Which statement about the eutectoid point of the Fe-Fe₃C ph…In the WC–Co system, a eutectic reaction between WC and Co produces a liquid phase at a temperature below the melting point of Co (about 1495°C).
To make WC–Co cemented carbide, WC powder is mixed with Co …Lever rule Eutectoid reaction Pearlite Sintering Gibbs phase rule