# Talk:Eutectic system

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## List of problems

There is a host of problems with this article, especially with the introduction. First, I would think that the naming of the article is not optimal, in contrast to the opinion below. I think that the eutectic reaction is the main aspect. A eutectic system is just a system displaying such a reaction. That's just as if you would discuss magnetism under the heading of magnetic system. And further, there are more complex cases: think of Al-Ru, having a eutectic reaction in the high-Ru region at a higher temperature than the melting temperature of pure Al, and a complicated liquidus line. Is this a eutectic system? For sure it has a eutectic reaction.

Further: In the very first sentence (joint superlattice, unique atomic ratio) -- this sounds as if you are talking about an intermediate phase. No, in the simplest case we do not have a superlattice below the eutectic point, but a solid solution. Further, as said above, the eutectic temperature does not need to be the lowest melting temperature. In the next paragraph, it is not 'any other mixture ratio', but only those over which the invariant temperature line extends. And note that due to the lever rule also a large part of the other solid phase has to melt to give a correct composition of the liquid phase. And each component does not solidify at a distinct temperature at non-eutectic temperatures, but during slow cooling (equilibrium) the volume of the solid part continuously increases, in line with composition changes both of solid and liquid phase.

And lastly, the issue with the valence electrons is beside the point. It seems that this is again written (as the first sentence with the superlattice) from the point of view that you need intermediate phases for a eutectic reaction. No, you get a eutectic reaction if and only if you have phase separation in the solid state (either by a miscibility gap, due to intermediate phases, or due to unequal crystal lattices of the pure components) and a minimum in the liquidus line (which is often the case when there is an unmixing tendency, which will typically be more pronounced in the solid than in the liquid) and the (extrapolated) region with phase separation in the solid overlaps with the liquidus line. The given example of Ag-Au is a counterexample to the first condition, as Ag and Au have the same lattice, have no intermediate phases, and do not show phase separation. For those of you who want to edit but are not so firm with phase diagrams, please look into http://www1.asminternational.org/asmenterprise/apd/help/Intro.aspx. Seattle Jörg (talk) 08:27, 20 January 2016 (UTC)

## thermodynamics of eutectics

This article needs some major expansion in this area; eutectics form because of limited solubility of one component in another and thus the favourability of separating into a mixture of two phases of distinct compositions, but the article doesn't really comment on this. John Riemann Soong (talk) 12:47, 12 November 2009 (UTC)

## Renaming to Eutectic system

I don't like how this article is named "eutectic point" because that is just part of the whole system. My Foundations of material science and engineering book calls it a binary eutectic alloy system, but I feel that's too complex, so I propose a rename to eutectic system. Then we can easily incorporate lots of other terms (some of which are already in the article), like eutectic composition, eutectic composition, eutectic reaction, etc. Wizard191 (talk) 00:46, 19 November 2009 (UTC)

## Excellent article

I would just like to say that I was very impressed with this article. It explains a complicated concept clearly and concisely.

Thank you. Jnnnnn (talk) 03:55, 4 January 2010 (UTC)

Thanks for the kind words! Wizard191 (talk) 15:43, 4 January 2010 (UTC)

## There is no Theory Explained

I agree it describes the phenomenon very well. I would like to understand why the melting point of a mixture of two substances would be lower than that of either substance individually. —Preceding unsigned comment added by 130.76.96.21 (talk) 18:05, 4 May 2010 (UTC)

You might want to read this (through page 53). It appears that scientists don't know. Wizard191 (talk) 16:39, 6 May 2010 (UTC)

## Influence of time

It might be interesting to also explain the influence of time in this process. Yes, Eutectic reaction takes place if cooling is sufficiently slow. Then the thermodynamics and free energy drive the process. Okay. But crystallization of two solid phases from a liquid is a kinetic reaction and takes time. If you cool very rapidly and get to a sufficiently low temperature while remaining in a non-crystallized structure like the liquid, then you form a metallic glass. These metallic glasses are amorphous materials with no long range crystal structure (though short range clustering is certain to occur). If the local minima of the free energy is sufficiently deep, then the amorphous structure can be stable over long periods of time at room temperature. In all cases, the discussion here leads to a new link to amorphous materials which are an interesting class of materials. —Preceding unsigned comment added by 178.198.133.52 (talk) 21:21, 12 October 2010 (UTC)

Feel free to add the above to the article. Wizard191 (talk) 22:25, 13 October 2010 (UTC)

## Definitions of variables

In figure 1, L, alpha, and beta are not defined. In the equation, alpha and beta are not defined. Hence it is difficult for the non-expert to glean knowledge from this article. —Preceding unsigned comment added by 128.171.160.40 (talk) 03:37, 21 March 2011 (UTC)

I defined the variables in the image. Wizard191 (talk) 16:40, 24 March 2011 (UTC)
if Wizard191 defined the variables in the image someone must have removed the definitions because alpha and beta are not currently defined either in the image or anywhere on the entire page. This is a major gap that needs to be corrected. I presume there is probably some relationship between alpha and A and between beta and B but it needs to be explicitly stated withing the article. Without a a clear definition of all variables the reader is left with guesswork and the article is only useful to someone already familiar with the topic. 68.37.208.42 (talk) 01:34, 9 May 2013 (UTC) Paul Carver 2013-05-08

but I'm still confused. I need something to guide me thru that phase diagram.

Say I have a chunk of metal in my crucible. It's an alloy that's 90% alpha and 10% beta (pretending the diagram is to scale and everything). We start near the bottom of the diagram and we're going straight up. It's cold cuz I just turned on the heat. So it's 'alpha + beta' - does that mean a solid solution of alpha and beta, or a nonsolution, some nonsolution composition with lumps of beta floating in alpha or something?

then it warms up, getting close to around the eutectic temperature and it magically becomes all alpha? that's what the diagram seems to say. what happened to the 10% beta? or do you mean it separates into two parts, one part is all alpha (so it follows a path more to the left) and the other part is a mix that follows a path more to the right?

Warming up more, it turns into a mixture of

• solid chunks of pure alpha
• a liquid, whatever solution of alpha and beta makes up the rest

This mixture is like a slurry? or more like honey or candle wax or chocolate? and as it warms, the viscosity decreases and finally:

The last few bits of alpha melt and it's entirely a liquid composed of 90% alpha and 10% beta.

Is this how it works? OsamaBinLogin (talk) 00:30, 26 December 2011 (UTC)

I think this is the correct answer, so bear with me if I'm wrong: Consider a 10% salt solution (L = brine), again assuming that the diagram is to scale and representative of the NaCl-water system. If you cool it down slowly, ice (alpha) begins to form, rejecting salt from its structure, and the salinity of the solution increases, depressing the freezing point. This means that over a range of temperatures, you have both L and alpha present. If you cool it down past zero Farenheit, all the brine will freeze, and if the ratios are right, it'll all resemble impure ice (alpha). If you manage to cool it right down, the salt ions/water molecules will begin to separate into ice (alpha) and salt (beta).
If your salt concentration was greater (60% as per the diagram), as you cooled the solution down the solubility of salt would decrease. Salt (beta) would precipitate from the brine, corresponding to L + beta. Cool it further, and you have a mixture of salt and ice. Hope that helps. Tomásdearg92 (talk) 13:39, 8 April 2012 (UTC)

## a single multiple chemical composition ??

A eutectic system is a mixture of chemical compounds or elements that has a single multiple chemical composition that solidifies...

This "single multiple" is hardly readable. What is it trying to say? Does anyone care to rephrase that? — Preceding unsigned comment added by 129.132.45.228 (talk) 13:58, 12 November 2012 (UTC)

Fixed it myself (by looking at the history), sorry for the spam! — Preceding unsigned comment added by 129.132.45.228 (talk) 14:02, 12 November 2012 (UTC)

## More detail needed on binary alloys without any eutectic point.

In the lead section, a eutectic system is defined as follows:

A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition made up of the same ingredients.

It then says:

Not all binary alloys have a eutectic point; for example, in the silver-gold system the melt temperature (liquidus) and freeze temperature (solidus) both increase monotonically as the mix changes from pure silver to pure gold.

The article could really use some expansion on this: why is there not a eutectic point at a composition of 100% silver and 0% gold? From the explanation and by looking at cited phase diagram, that composition seems to fit the definition of "solidifying at a lower temperature than any other composition made up of the same ingredients." Draconx (talk) 19:14, 28 November 2012 (UTC)

That's because a eutectic system is, as pointed out in your qouted text, a mixture- "a material system made up of two or more different substances". Tomásdearg92 (talk) 23:56, 6 December 2012 (UTC)

## Please Check the Logic Behind Wording

Dear Author(s),

Please revisit the opening phrase construct -- "a mixture of chemical compounds ... (with) ... a single chemical composition". Could you have possibly meant "a solid mixture of at least two crystalline lattice systems of one and the same element or chemical compound" -- with a unique, specific ratio, for which the melting of ALL lattice systems occurs at once ?

P.S. ON SECOND THOUGHT, I HAVE DECIDED AND FOLLOWED THE REQUEST OF MANY READERS AND PERFORMED MYSELF SOME MAJOR EDITING TO CLARIFY THIS CONFUSING BEGINNING OF AN ARTICLE. THANK YOU.

 — Preceding unsigned comment added by Waldemahr (talk • contribs) 16:09, 19 August 2014 (UTC)


## melting temperature = freezing temperature ?

So, a eutectic system is one that has a melting temperature equal to freezing temperature? If so, the first sentence can and should be simplified considerably. Danielx (talk) 17:29, 19 March 2015 (UTC)

That's not the definition of it. It's one of several properties that they have, but other substances have this property and this property here is only one of several that together result from the underlying definitions. DMacks (talk) 18:06, 19 March 2015 (UTC)

## Eutectic Reaction

The eutectic reaction is stated to be the following in the article:

${\displaystyle {\text{Liquid}}{\xrightarrow[{\text{cooling}}]{\text{eutectic temperature}}}\alpha \,\,{\text{solid solution}}+\beta \,\,{\text{solid solution}}}$

Shouldn't this statement make clear the reversible nature of the eutectic reaction about the eutectic temperature, i.e. be of the form:

${\displaystyle {\text{Liquid}}{\begin{matrix}{\scriptstyle {\text{cooling}}}\\\rightleftharpoons \\{\scriptstyle {\text{heating}}}\end{matrix}}\alpha \,\,{\text{solid solution}}+\beta \,\,{\text{solid solution}}}$

I'm no master at LaTeX, so I'm sure there could be a more legible way of rendering this. 38.129.203.74 (talk) 04:42, 12 May 2015 (UTC)

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