Peritectic reaction temperature

The phase diagram of carbon-silicon is shown in Fig. 7.6.0 10 1 This diagram does not attempt to distinguish between aSiC and pSiC. pSiC is thought to be more stable than aSiC at any temperature below a peritectic reaction temperature of 2545 40°C. 

The region of solid solutions based on LajSej was observed in the LajSe,-Cu2Se system (Julien-Pouzol et al. 1972, Julien-Pouzol and Guittard 1972). The compound LaCuSe2 is formed only in case when the components ratio is 1 1 and decomposes according to a peritectic reaction at 1240°C. Another compound, LajCuijSe, decomposes at peritectic reaction temperature of 1000°C. The eutectic is formed at 910 C and 90 mol. % Cu2Se. 

Praseodymium di-iodide, Prl2, can essentially be made in the same way. If sufficient care is taken to exclude air and moisture, oxidic impurities can be avoided. To avoid the formation of Pr2ls, praseodymium metal is used in excess as chunks to easily remove the unreacted metal after the reaction is completed. The pure compound Prl2 is thus obtained, with a reaction temperature well above the peritectic temperature, around 800 °C. Reaction times seem not to matter much, a few days are usually sufficient, perhaps even less. The cooling procedure, however, is crucial as it determines the phases (I through V) that are formed and their relative quantities. Section 4.3 will deal with this issue. 

In the Au-Bi system the compound Au2Bi is stable in a restricted range of temperature only it is formed by a peritectic reaction (371°C) and, at a lower temperature (116°C), it is decomposed according to the eutectoidal reaction Au2Bi — (Au) + (Bi). In the Zn-Te system, finally, we have the congruently melting compound ZnTe. In this system a miscibility gap in the liquid state may also be noticed. 

This kind of reaction is called a peritectic reaction and the point p is called the peritectic point. The temperature TP is called the peritectic temperature. 

This reaction is called the peritectic reaction, and the point p is known as the peritectic point and Tp the peritectic temperature. 

In figure 6.4, the abnormal behaviour shown by steel number 305, containing 0,04% C, 13% Cr, 5,5% Ni, reflects the depression of both liquidus and solidus temperatures by nickel. At the 13% Cr- level the mean effect was roughly 5 and 10 degrees per weight percent of nickel for the liquidus and solidus respectively. In spite of the very low carbon content, steel number 305 had a peritectic reaction. This is an effect of nickel, as steel number 306 with a higher carbon content solidified completely to ferrite. 

Preparation of a peritectic compound requires a solid state (solid-solid) or solid-liquid reaetion. A solid state reaction requires transport of matter (diffusion) in the direetion of a chemical potential gradient (i.e., a chemical composition gradient). Thus solid state reactions are diffusion controlled, with diffusion fastest across grain boundaries. Within a crystal, diffusion is enhanced by defects. Clearly the preparation of a peritectic alloy should be carried out at the highest possible temperature just below the peritectic decomposition temperature. The speciality of powder metallurgy exploits the optimum conditions for solid state reactions. 

When the system attains the peritectic temperature the peritectic reaction A + L = A4B starts and crystals of the compound A4B appear. Since now there are three phases in the system (component A, compound A4B, and melt L), the system has no degree of freedom ( = 3 - 1 = 2, / = 3, V = 0), which means that its cooling due to the evolution of the reaction heat of the peritectic reaction must halt, even when the surrounding cools further. The system stays at the peritectic temperature until the melt disappear and the system solidifies. Below the peritectic temperature, there is again a mechanical mixture of the crystals of component A and the crystals of compound A4B. 

IrSb3 is formed by peritectic reaction at a temperature of 1373K as shown in Fig.l[3], so it is difficult to obtain the stoichometric IrSbs directly from the molten state. 

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. 

Phase diagrams are the roadmaps from which the number of phases, their compositions, and their fractions can determined as a function of temperature. In general, binary-phase diagrams can be characterized as exhibiting complete or partial solid solubility between the end members. In case of the latter, they will contain one or both of the following reactions depending on the species present. The first is the eutectic reaction is which a liquid becomes saturated with respect to the end members such that at the eutectic temperature two solids precipitate out of the liquid simultaneously. The second reaction is known as the peritectic reaction in which a solid dissociates into a liquid and a second solid of a different composition at the peritectic temperature. The eutectic and peritectic transformations also have their solid state analogues, which are called eutectoid and peritectoid reactions, respectively. 

Cooling also depends on the phase diagram as well as the intended use of the alloy. If there is no danger of separation of mixed ciystals (with subsequent alteration of the composition of the alloy) and no peritectic reactions are expected, or if the composition achieved at the high temperature is the one desired in the solid, the material is quickly cooled in air. Materials in metal or quartz vessels may also be quenched in water or oil. 

In the course of cooling of melts with 30-50 wt% of PI (type III) below the liquids line (below Ti), the melt releases a solid oil solution in PE (/ -phase) with up to 30% oil content. Further cooling to the peritectic temperature (Tm) brings about the third a-phase (PE solid solution in oil). When the liquid phase L disappears fully upon the peritectic reaction L a, the system transfers into a biphasic field a and (3. As the temper-... 

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