Eutectic material is a mixture of 2 or more different phases. The material consists of small (usually) grains or crystals, which have different composition (or different concentration of components) and different crystal structure (different unit cells, at least). 

Solid solution consists of the grains or crystals, which in ideal case are of the same crystal structure and composition.

The best way to distinguish them is X-ray phase analysis. In the case of solution we obtain set of reflexes which could be related to the one structure. The reflexes could be relatively wide if the material - solid solution is not homogeny enough inside of homogeneity region of the solution. In the case of eutectic we obtain XRD diffraction reflexes which cannot be indexed as a single phase.

Electron and optical microscopy can help to distinguish those materials too. 

A problem can arise in the case when an eutectic material consists of phases which in their turn contain the same components (but in different concentrations), have similar structure and each of them has solubility region broad enough (thus they are solid solutions).  In such case a researcher applies several different techniques to prove their results. Different phases those make up the eutectic material should manifest itself in different physical or chemical properties.

Dear Kumar,

In the case of simple binary phase diagram, with limited solubility of A and B components, usually you can observe a nice "eutectic" structure. It consist of fine lamellas  of A phase and B phase. This can be observed with Scanning Electron Microscopy using BSD detector that is very sensitive to different concentrations of components. This structure is characteristic for eutectic because as liquid solidifies it first creates lamellas of A phase and of course the liquid phase becomes poor with A component and then B phase starts to form leading to formation of B lamellas. If you already did DSC experiments on your samples, do a cross section polishing and take look at microstructure of the samples. Usually, you will observe eutectic lamellar structure in direction of highest thermal gradient (normal to interface with the DSC crucible). However, this method is not so good when you have ternary phases. In this case it possible to use extension of Tamman's triangle method where you need to do several DSC runs around the ternary eutectic and plot isothermal time of the solidification (corresponds to time of melting peak in DSC measurements) together with molar fractions of the samples measured. This gives 3D plot where isothermal time is perpendicular to molar fractions (ternary phase diagram) of your samples creating a kind of the pyramid where  apex of the pyramid is highest point representing longest isothermal time which defines the eutectic point. Projection of the apex to ternary phase diagram gives you the position of ternary eutectic point (see TammanTriangle.pdf).

I think Gregory nicely explained differences between solid solutions and eutectic material. In general is easier to obtain solid solutions materials then completely eutectic materials exactly because eutectic forms only in small compositional regions. 

Best regards,

Toni

Dear Toni,

I have a question concerning the construction of the pyramid to determine a ternary eutectic. Does it have a tetrahedral shape? Can you perhaps tell me a publication or a book where this procedure is explained in more detail?

Thank you very much for your answer!

Best regards,

Christoph

Are there conditions at room temperature where a liquid eutectic fluid can be solidified at room temperature? Perhaps by adding more components?