Ternary peritectic

As with the pnictates, a quasi-ternary phase diagram can be developed to map out possible compounds in this composition phase space using key chalcotetre-late building blocks. We have begun to make use of the peritectic nature of the starting materials, as this has facilitated reactions between phases. 

Alternatively, we can consider other schemes for structure composition as there is no precedence for the scheme in Eq. (5). We can prepare ternary compounds that peritectically decompose, a feature that may facihtate their reactivity at low temperatures ... 

Let us examine the crystallization process of an initial composition Cl the first crystals to form are those of the pure component 2 (y", T = 1175 °C). Crystallization of y" drives the residual liquid radially away from corner 2 along direction 2-Cl, until peritectic curve PP is reached. Here, crystals y" react with the liquid and are transformed into intermediate compound C . Displacement along the peritectic curve terminates when all phase y" is completely resorbed. This occurs geometrically when trajectory C-Cl encounters peritectic curve PP. Crystallization of Cj then proceeds, while the residual liquid moves radially away from C . The liquid path then reaches cotectic curve E -Em and crystals y begin to precipitate in conjunction with Cj. Crystallization continues along the cotectic line and, at ternary eutectic Em, crystals y " precipitate together with C and y until the residual liquid is exhausted. 

No peritectic point was noticed with lithium chloride and potassium or sodium chlorides. T. W. Richards and W. B. Meldrum (1917) have also studied ternary mixtures of lithium and sodium chlorides with potassium, rubidium, or caesium... 

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. 

In the previous diagrams, points e and / for the congruently-melting compound are both ternary eutectic. On the other hand, for the incongruently-melting compound die point e is the ternary eutectic, whereas the point/is the ternary peritectic. 

At point m, a ternary peritectic reaction occurs isothermally. 

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. 

There are five ternary eutectic and one peritectic points. The crystallization path of all mixtures, figurative points of which lay inside the triangles BY-A3B2X3Y2-BX2 and A3B2X3Y2-P-BX2, starts with the primary crystallization of BY, A3B2X3Y2, or BX2, followed by the crystallization of the neighboring salt till the peritectic point P is reached. 

The fusion diagram of the ternary PbCl2-UCl4-ThCl4 system has been studied by differential thermal analysis and the components found to form one peritectic and two eutectics. ... 

The vertical section of the Nd-Fe-B ternary system which passes through the Fe comer and the phase Nd2Fe14B is shown in fig. 4a. Schneider et al. emphasize that this is not a pseudo-binary section. The dominant feature of this vertical section is the peritectic reaction L + Fe —> at 1180° C. It also follows from the results shown in fig. 4a that cooling of a liquid whose composition corresponds to leads to the formation of primary crystallized Fe. The concentration limit beyond which no primary Fe crystals are formed is at 77 at.% Fe. This is very close to the overal composition of commercial magnets, as will be discussed in more detail in section 3.2. Schneider et al. note that the vertical section of fig. 4a represents the stable situation which applies only to melts that were kept near the liquidus temperatures for a sufficiently long time. For superheated alloys the vertical section is quite different and corresponds to a metastable situation (fig. 4b). A comparison of the two vertical sections reveals that the liquidus temperatures, and the temperature at which the univariant reaction L - + tj begins, are unaltered, but that the temperature at which the 4> phase forms is lower in fig. 4b than in fig. 4a. Furthermore, one notices a new phase in fig. 4b (x) which is formed peritectically at 1130 °C. The latter temperature is below the temperature of the stable reaction L + Fe - 4> (1180 °C). Schneider et al. note that the primary crystallization of is suppressed in the metastable sequence (fig. 4b), in favour of Fe. In the microstructure one now observes that primary Fe is surrounded by a shell of Fe + which is the decomposition product of x- The x phase was identified by Grieb et al. (1987), as a compound of the 2 17 structure type. 

This reaction is known as a peritectic reaction and is quite common in ceramic systems. Other examples of incongruently melting ternary compounds are 2Na20 Si02 (Fig. 8.7a) and 3Li20 B2O3 (Fig. 8.9). 

Alloys with low r value (i.e., high concentration of calcium and low-tin content) have fine-grain structure. Figure 4.38 shows the correlation between corrosion rate and calcium content for Pb—Ca—Sn alloys with 0.5 or 1.5 wt% Sn content [18]. With increase of calcium content in the alloy, the r value decreases and the corrosion rate increases. When the Sn content is increased from 0.5 to 1.5 wt%, the r value increases from 7.1 to 21.4 at the peritectic phase composition of the ternary alloy and the corrosion rate decreases. 

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