Eutectic point, definition

Silver Arsenides.—A study of the system Ag-As gives no evidence of the existence of any definite compound of the two elements.10 When the mixtures were heated in sealed tubes it was observed that the addition of silver lowered the melting point of arsenic to a eutectic point... 

By a similar usage in petrology, a euleeltc is a discrete mixture of two or more minerals, in definite proportions, which have simultaneously crystallized front the mutual solution of their constituents The eutectic point is the lowest temperature at any given pressure at which the above physical-chemical process may lake pluce, The eutectic ratio is the ratio hy weight of two minerals dial originate bv the above process. 

Fig. 1.13. Temperature as a function of the concentration of water-glycerine mixture at which phase transformations occur (Figure 14 from [1.10]) Definitions by Luyet AE, formation of small crystals or mo lecular groups E, eutectic point EB, formation of clusters R, eruptive recrystallization G, glass transition...

The point t is a eutectic point the martensite capable of existing as far as this temperature (T =690 ) has a well-defined composition (x= 0.0085). The least lowering of temperature disintegrates it and it then forms a eutectic of the same average composition as formed by small particles of ferrite (Fea) embedded in cementite. Arnold and Sarby, who took this eutectic for a definite compound, gave it the name perlite. 

In this case, as in the case of those systems where the pure components are deposited, a minimum freezing-point is obtained. In the latter case, however, there are two freezing-point curves which intersect at a eutectic point in the case where mixed crystals are formed there is only one continuous curve. On one side of the minimum point the liquid phase contains relatively more, on the other side relatively less, of the one component than does the solid phase while at the minimum point the composition of the two phases is the same. At this point, therefore, complete solidification and complete liquefaction will occur without change of temperature, and the solid solution will accordingly exhibit a definite melting-point. ... 

In this case the freezing-point curve is of the simple type (Fig. 33). Such curves have been obtained in the case of a number of pairs of metals, e.g, zinc—cadmium, zinc—aluminium, copper—silver (Heycock and Neville), tin— zinc, bismuth—lead (Gautier), and in other cases. From molten mixtures represented by one branch of the freezing-point curve one of the metals will be deposited while from mixtures represented by the other branch, the other metal will separate out. At the eutectic point the molten mass will solidify to a heterogeneous mixture of the two metals, forming what is known as the eutectic alloy, Such an alloy, therefore, will melt at a definite temperature lower than the melting-point of either of the pure metals. 

According to Gibbs condensed phase rule, at the ternary eutectic points four phases coexist (G = 3, P = 4, F = 0) three solid phases and one Hquid melt phase. For example, at the ternary eutecHc point Ei the two ternary phases X and Z, the binary phase A2C, and the melt (not shown in projection) coexist. By definition, this point is invariant (F = 0) to changes in temperature and/or composition, as each departure from its values will displace the system from point Ej. A composition anywhere along a phase boundary Hnes is subject to a univariant equUibrium (F = 1), since it is possible to alter either the temperature or the concentration of one of the two components without leaving the boundary line between two... 

Liquid mixtures freeze at lower temperatures than the pure components of which they are comprised. The lowest possible temperature in which we only have liquid is called the eutectic point. The presence of solid compounds of stoichiometry aj y increases the complexity of their phase diagrams. A compound with a definite melting point is termed congruent, while a compound that is not stable all the way up to a well-defined melting point is said to have an incongru-ent melting point. The state at which the latter case dissociates is called the peritectic point. 

As an example, liquefied mixtures are easily formed by using some amino acids protected with acetyl, Boc-, Fmoc- and Z-groups. Some authors have called the resulting systems eutectic mixtures , but this term should be reserved for the textbook definition as the composition of minimum melting point that freezes isothermally at the eutectic temperature. Hence we refer to them as eutectic melts . 

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