Editing Eutectic system (section)

== Other critical points==
[[File:Iron carbon phase diagram.svg|thumb|upright=1.75|Iron–carbon phase diagram, showing the eutectoid transformation between austenite (γ) and pearlite.]]

===Eutectoid===
When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur.  For instance, in the iron-carbon system, the [[austenite]] phase can undergo a eutectoid transformation to produce [[Allotropes of iron|ferrite]] and [[cementite]], often in lamellar structures such as [[pearlite]] and [[bainite]].  This eutectoid point occurs at {{convert|723|C|abbr=on}} and 0.76 wt% carbon.<ref>{{cite web |first=Kim |last=Ballentine |date=28 April 1996 |url=https://nptel.ac.in/content/storage2/courses/112108150/pdf/PPTs/MTS_07_m.pdf|title=Iron-Iron Carbide Phase Diagram Example|archive-url=https://web.archive.org/web/20080216023642/http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/kimcon.html|archive-date=16 February 2008}}</ref>

===Peritectoid===
A ''peritectoid'' transformation is a type of [[isothermal]] [[reversible reaction]] that has two solid [[Phase (matter)|phase]]s reacting with each other upon cooling of a binary, ternary, ..., ''n''-ary [[alloy]] to create a completely different and single solid phase.<ref name="Gold Book PAC,1994,66,588">{{goldbookref|title=Peritectoid reaction|file=P04501}}</ref>  The reaction plays a key role in the order and [[decomposition]] of [[quasicrystalline]] phases in several alloy types.<ref>{{cite journal |last1=Das |first1=Amit |last2=Manna |first2=Indranil |last3=Pabi |first3=S. K. |title=A numerical model of peritectoid transformation |journal=Metallurgical and Materials Transactions A |date=October 1999 |volume=30 |issue=10 |pages=2563–2573 |doi=10.1007/s11661-999-0295-2 |publisher=[[The Minerals, Metals & Materials Society]], [[ASM International]]|bibcode=1999MMTA...30.2563D |s2cid=95279944 }}</ref> A similar structural transition is also predicted for [[Cylinder sphere packing#Columnar structures created by rapid rotations|rotating columnar crystals.]]

===Peritectic===
[[File:Phasendiagramm Gold-Aluminium.svg|upright=1.25|thumb|Gold–aluminium [[phase diagram]]]]
Peritectic transformations are also similar to eutectic reactions. Here, a liquid and solid phase of fixed proportions react at a fixed temperature to yield a single solid phase. Since the solid product forms at the interface between the two reactants, it can form a diffusion barrier and generally causes such reactions to proceed much more slowly than eutectic or eutectoid transformations. Because of this, when a peritectic composition solidifies it does not show the [[lamellar structure]] that is found with eutectic solidification.

Such a transformation exists in the iron-carbon system, as seen near the upper-left corner of the figure.  It resembles an inverted eutectic, with the δ phase combining with the liquid to produce pure [[austenite]] at {{convert|1495|C|abbr=on}} and 0.17% carbon.

At the peritectic decomposition temperature the compound, rather than melting, decomposes into another solid compound and a liquid. The proportion of each is determined by the [[lever rule]]. In the [[Gold-aluminium intermetallic|Al-Au]] phase diagram, for example, it can be seen that only two of the phases melt congruently, [[Gold-aluminium intermetallic|AuAl<sub>2</sub>]] and [[Gold-aluminium intermetallic|Au<sub>2</sub>Al]], while the rest peritectically decompose.

==="Bad solid solution"===
Not all minimum melting point systems are "eutectic". The alternative of "poor solid solution" can be illustrated by comparing the common precious metal systems Cu-Ag and Cu-Au.
Cu-Ag, source for example https://himikatus.ru/art/phase-diagr1/Ag-Cu.php, is a true eutectic system. The eutectic melting point is at 780 °C, with solid solubility limits at fineness 80 and 912 by weight, and eutectic at 719.
Since Cu-Ag is a true eutectic, any silver with fineness anywhere between 80 and 912 will reach solidus line, and therefore melt at least partly, at exactly 780 °C. The eutectic alloy with fineness exactly 719 will reach liquidus line, and therefore melt entirely, at that exact temperature without any further rise of temperature till all of the alloy has melted.
Any silver with fineness between 80 and 912 but not exactly 719 will also reach the solidus line at exactly 780 °C, but will melt partly. It will leave a solid residue with fineness of either exactly 912 or exactly 80, but never some of both. It will melt at constant temperature without further rise of temperature until the exact amount of eutectic (fineness 719) alloy has melted off to divide the alloy into eutectic melt and solid solution residue.
On further heating, the solid solution residue dissolves in the melt and changes its composition until the liquidus line is reached and the whole residue has dissolved away.
Cu-Au source for example https://himikatus.ru/art/phase-diagr1/Au-Cu.php does display a melting point minimum at 910 °C and given as 44 atom % Cu, which converts to about 20 weight percent Cu - about 800 fineness of gold. But this is not a true eutectic.
800 fine gold melts at 910 °C, to a melt of exact same composition, and the whole alloy will melt at exact same temperature. But the differences happen away from the minimum composition.
Unlike silver with fineness other than 719 (which melts partly at exactly 780 °C through a wide fineness range), gold with fineness other than 800 will reach solidus and start partial melting at a temperature different from and higher than 910 °C, depending on the alloy fineness. The partial melting does cause some composition changes - the liquid will be closer in fineness towards 800 than the remaining solid, but the liquid will not have fineness of exactly 800 and the fineness of the remaining solid will depend on the fineness of the liquid.
The underlying reason is that for an eutectic system like Cu-Ag, the solubility in liquid phase is good but solubility in solid phase is limited. Therefore when a silver-copper alloy is frozen, it actually separates into crystals of 912 fineness silver and 80 fineness silver - both are saturated and always have the same composition at the freezing point of 780 °C. Thus the alloy just below 780 °C consists of two types of crystals of exactly the same composition regardless of the total alloy composition, only the relative amount of each type of crystals differs. Therefore they always melt at 780 °C until one or other type of crystals, or both, will be exhausted.
In contrast, in Cu-Au system the components are miscible at the melting point in all compositions even in solid. There can be crystals of any composition, which will melt at different temperatures depending on composition.
However, Cu-Au system is a "poor" solid solution. There is a substantial misfit between the atoms in solid which, however, near the melting point is overcome by entropy of thermal motion mixing the atoms. That misfit, however, disfavours the Cu-Au solution relative to phases in which the atoms are better fitted, such as the melt, and causes the melting point to fall below the melting point of components.