Congruent phase transformations

It need not be assumed that phase diagrams exist only for metal-metal systems in fact, phase diagrams that are very useful in the design and processing of ceramic systems have been experimentally determined for many of these materials. Ceramic phase diagrams are discussed in Section 12.7. 

Phase diagrams have also been determined for metallic (as well as ceramic) systems containing more than two components however, their representation and interpretation may be exceedingly complex. For example, a ternary, or three-component, composition-temperature phase diagram in its entirety is depicted by a three-dimensional model. Portrayal of features of the diagram or model in two dimensions is possible, but somewhat difficult. 

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Congruent Phase Transformations Eutectoid and Peritectic Reactions... 

Figure 9.37 is a portion of the copper-aluminum phase diagram for which only single-phase regions are labeled. Specify temperature-composition points at which all eutectics, eutectoids, peritectics, and congruent phase transformations occur. Also, for each, write the reaction upon cooling. 

Besides these thermotropic phase transitions, a variety of pressure-induced phase transformations can be observed," and it has been demonstrated that temperature and pressure have non-congruent effects on the structural and phase behaviour of these systems. 

What is the principal difference between congruent and incongruent phase transformations ... 

For those substances which melt congruently and do not suffer reconstructive phase transformations between the melting point and the temperature of interest the flame fusion technique has been successfully applied. The H2 + O2 flame originally used is sometimes replaced by an argon plasma made oxidizing or... 

The yellow phosphides M3P7 differ from all other phosphides by a remarkable property they sublime congruently dissociative (cf. NHi Cl). In the gas phase only M,g and Pi,g appear, whereas under similar conditions the phosphides M3P11 dissociate into M3P7 and Pi ( ). These compounds can also be obtained as amorphous products by sublimation under suitable conditions. It has been shown that these amorphous products are extraordinarily reactive, therefore they are the most suitable starting materials for the transformations discussed above. 

C. Equilibrium between Solid and Liquid Phases only. I. The Components are Completely Miscible in the Liquid State. a) The pure components only occur as solid phases. Polymorphism of components. Determination of the equilibrium curve. Example, b) Compounds are formed with a congruent meltings point. The indifferent point. Determination of the composition of a compound by thermal analysis. Examples, (c) Compounds are formed with an %ncongruent melting-point. Determination of the composition of the coinpound by thermal analysis. Example. (d) Solid solutions or " mixed crystals are formed, i) The two components can form an unbroken series of solid solutions. Examples. Melting-point curve. Example. Fractional crystallisation of solid solutions, h) The two components do not form a continuous series of solid solutions. Examples. Changes in solid solutions with the temperature. II. The Components are not Completely Miscible in the Liquid State. Suspended transformation. 

Congruent transformation - A phase transition (melting, vaporization, etc.) in which the substance preserves its exact chemical composition. 

Figure 8.22 shows an Mn0-Al203 phase diagram. The equimolar composition melts congruently, i.e., there is a direct transformation between the solid and liquid states without a change in the number of phases (in this case X = 1). Ba0-Ti02 is another example of a two-component... 

The dissolntion of the CA j layer and the formation of saturated solution in relation to this snrface layer is the next stage of reaction [6]. The internal layers of CA are exposed and their hydroxylation occnrs immediately. Simultaneously, some amount of AH3 gel is precipitated and therefore the C/A molar ratio in the solution increases insignificantly to the valne abont 1.1. According to Bertrandie [123], this corresponds to the intersection of CA j solnbility cnrve with minimum instability curve (m in Fig. 3.46). This minimnm instability cnrve, from the chemical point of view, corresponds to the transformation of AH3 gel into the C2AHg phase [6, 124]. The CA j dissolntion is congruent and accompanied by the following reaction ... 

The equilibrium phase diagram for the cerium-yttrium system is presented in fig. 40. This diagram does not show a peritectoid reaction to form the Sm phase as did that of Lundin and Klodt. Instead, a congruent transformation is presented, based on the results of the investigations into the nature of the formation of the 5 phase. The melting points and transformation temperatures of the pure metals have been adjusted to conform with the accepted data listed in table 1. 

The phase diagram shown in fig. 63 for this system has been modified to avoid these defects. Based on Lundin s (1966) study of the nature of the formation of Sm-type structure, the S phase is shown here to be formed by a congruent transformation (see sections 2.1.3. and 2.1.4.). The upper portion of the diagram has been revised to bring melting temperatures and the aNd /3Nd transition temperature into accord with table 1 while utilizing as many of the data points listed by Kobzenko et al. as possible. 

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