Ternary eutectic point

At q, the second phase B appears, and the liquid composition moves along the curve qe. Until it reaches the ternary eutectic point e, both A and B crystallise. 

The ternary invariant points (e.g., e and /in the above diagrams) that appear in a system without solid solution are either ternary eutectics or ternary peritectics. Whether it is eutectic or peritectic is determined by the directions of falling temperatures along the boundary curves. 

The phase diagram of a ternary system in which the three species do not form solid solutions with each other and the constituent binary systems form eutectics, is shown in Figure 4(b). The temperatures A B and Tc correspond to the melting points of A, B, and C, respectively. The vertical faces of the prism represent the temperature-concentration behavior of the three binaries. Note that the behavior of each binary system is that shown in Figure 2d. The solidus lines are not shown for the sake of clarity. Points E g, E j. are the eutectic points of the three binary... 

At any fixed temperature along the liquidus curve, we may simultaneously solve equations (17) and (18) for the unknown compositions Xg, x, and Xy (=1—Xg—Xc). The location of the ternary eutectic point (Tp, Xg, x ) requires the simultaneous solution of three equations such as equation (17) written for each of the three components. 

Ternary eutectics are also possible. The binary eutectic points of three mixtures are as follows for aminophenazone-phenacetin 82°C for aminophenazone-caffeine 103.5°C and for phenacetin-caffeine IZS C. The ternary eutectic temperature of aminophenazone-phenacetin-caffeine is 81°C. In this mixture the presence of aminophenazone and phenacetin can be detected by the mixed melting point... 

The line Kk corresponds to the three phase system solution + solid A + sohd B this is a divariant system but when p is fixed the representative points for the solution lie on a line. The three lines k K, k K and k K meet at K which is the ternary eutectic point at which the four phases, liquid, solid A, solid B and solid C are in equilibrium. This system is monovariant, but at a given pressure there is only one point representing this state, namely K. 

The phase diagram of the ternary system KF—KCl—KBF4 was measured by Patarak and Danek (1992). The system is simple eutectic with the coordinates of the eutectic point of 19.2 mole % KF, 18.4 mole % KCl, 61.4 mole % KBF4, and the temperature of eutectic crystallization of 422°C. 

The boundary curves, coming out from the individual binary eutectic points, represent curves of the common crystallization of both components of the respective binary subsystems. All boundary curves meet at the ternary eutectic point, which is the lowest temperature where there is the liquid phase in this ternary system. 

On continued cooling of the system both the components, A and C, will crystallize simultaneously and the composition of the melt will move on the boundary line ei —et until the ternary eutectic point et is attained, where also the component B begins to crystallize. At the eutectic temperature the system has no degree of freedom (k = 3, / = 4, V = 0), which means that its cooling will stop. The system maintains the eutectic temperature until its whole solidification. Completely analogical is the crystallization process of mixtures lying in the crystallization fields of the components B and C. 

An analogical situation happens also in the case of the mixture, the composition of which is given by the figurative point X3. However, it depends on the position of X3, in which of the two ternary eutectic points, the system will solidify. Since the summit of the boundary line etj-et2 is the eutectic point of the binary system AB-C, the boundary line falls down from its summit towards both ternary eutectic points. Thus mixtures, the composition of which lies in the triangle A-AB-C will solidify in the eutectic point etj, while mixtures with composition lying in the triangle AB-B-C will solidify in the eutectic point et2-... 

If the mixture has the composition placed in the triangle AjB-Pt-a (point Xj in Figure 3.32) or A4B-Pb-Pt (point X"), their crystallization paths, according to the triangle rule, must end in the mechanical mixture of solid A4B, B, and C (apexes of the triangle A4B-B-C) in the ternary eutectic point Ct. The crystallization paths, however, will in both cases proceed in a different way. 

On cooling the mixture with the composition X2, which lies in the crystallization field of the compound A4B, the primary crystallization of A4B proceeds. The composition of the melt moves on the straight line A4B—X2 from point X2 up to the point A, where also component B starts to crystallize. The system has only one degree of freedom and the composition of the melt thus moves on the boundary line ei — Ct to the ternary eutectic point Ct, where the whole system solidifies under fhe formation of a mechanical mixfure of crystals A4B, B, and C. 

In the case of a mixture shown on Figure 3.32 by the figurative point X3, the situation is similar as in a simple eutectic ternary system. Starting with the crystallization of component C, the composition of the melt moves up to the point 5, where the component A4B begins to crystallize. At the ensuing cooling, the composition of the melt moves on the boundary line Pt—Ct up to the ternary eutectic point, where also component B crystallizes until the whole system solidifies. 

Below the temperature of the ternary eutectic point the system formed is a mechanical mixture of the solid phases A, A4B, and C if the composition of the system lies in the triangle A-A4B-C, or A4B, B, and C if the composition of the system lies in the other part of the concentration triangle. 

The vertical projection of the phase diagram of this system is shown in Figure 3.34. The phase diagram has four planes pc(A), pc(B), pc(C), and pc(ABC), which are the projections of the planes of primary crystallization of the compounds A, B, C, and ABC, respectively. The figurative point of the ternary compound ABC lies outside the plane of its primary crystallization. In contrary to the previous case, the phase diagram of this type has only one ternary eutectic point Ct and two singular points, which are the ternary peritectic points Ptj and Ptj. The boundary lines Ptj-et and Ptj-et represent the simultaneous crystallization of components B and ABC, and C and ABC, respectively. 

If we have a mixture AX-AY-BY, the composition of which is in Figure 3.35 shown by the figurative point Xi, the crystallization path at its cooling is completely similar as in the case of a simple ternary eutectic system. The component BY begins to crystallize first, the composition of the melt moves towards point Mi, where also component AX starts to crystallize. At the ensuing cooling, both the components fall out from the melt simultaneously and the composition of the melt moves on the boundary line etj — et2 from point Mi up to the eutectic point etj, where also component AY starts to crystallize and where also the whole system will solidify. 

It depends thus on the composition of the mixture in which both the ternary eutectic points of the system will solidify. The boundary line etj-et2 falls down from its summit S towards both the eutectic points. This summit is simultaneously the eutectic point of the pseudo-binary system AX-BY. 

After all the BY disappears, the system moves up to the ternary eutectic point, where the whole system solidifies. 

The binary compound BCX3 divides the AX-BX-CX2 and DX2-BX-CX2 ternary systems into four simple ternary subsystems. The crystallization paths of ternary mixtures end in one of the six ternary eutectic points E , where the ternary mixtures solidify. The crystallization path of any quaternary mixture follows the dotted boundary lines inside the concentration tetrahedron and ends in one of the two quaternary eutectic points Eq,. 

The ternary system KCI-KF-KBF4 is a simple eutectic with the coordinates of the eutectic point... 

In the ternary system KF-K2T1F6-KBF4, the intermediate compound KsTiFy divides the system into two simple eutectic systems. The calculated coordinates of the two ternary eutectic points are ... 

A. The Ternary Eutectic Point.—In passing to the consideration of those ternary systems in which one or more solid phases can exist together with one liquid phase, we shall first discuss not the solubility curves, but the simpler relationships met with at the freezing-point. That is, we shall first of all examine the freezing-point curves of ternary systems. 

---