Phase diagrams and microstructures

In the liquid state, any nickel-copper alloy is homogeneous. Solid will begin to form as soon as 

during equilibrium cooling, the composition of the solid will mn down the solidus line, ri to S2 to S3, and so on, and the composition of the liquid in equilibrium with the solid runs down the liquidus from Zi to I2 to I3, and so on, as the liquid cools. The composition of the solid phase when all of the liquid has solidified will be equal to that of the original liquid phase. Not only does the composition of the solid and liquid phases change continuously as the temperature falls through the two-phase region, but the number of small crystals present also increases. When temperature T4 is reached, the microstructure of the solid consists of crystallites or grains 

During normal processing, cooling is usually rather fast, and solidification is rarely an equilibrium 

These nonequilibrium structures can be removed by heating the solid for an appropriate amount of time at a temperature below that of the solidus, a process called annealing. Annealing is effective only if the atoms can diffuse in the solid to correct the compositional differences generated during the cooling. 

The composition and microstructure of a solid formed in a system showing a eutectic point depends critically on the composition of the liquid with respect to the eutectic composition. The situation will be explained by using the lead-tin (Pb-Sn) phase diagram described in Section 4.2.3. 

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Figure 2.42. The Cu-Zn system phase diagram and microstructure scheme of the diffusion couple obtainable by maintaining Cu and Zn blocks in contact for several days at 400°C. Shading indicates subsequent layers, each one corresponding to a one-phase region. The two-phase regions are represented by the interfaces between the one-phase layers (adapted from Rhines 1956).

Ecjuilibrium Phase Diagram and Microstructure of the Lead—Antimony Alloy System... 

The interpretation of small SANS data from systems of type D20/NaCl- -decane/triolein-CioE4 showed that the order of the microstructure systematically decreases with increasing triolein content (Fig. 11.8(c) and Table 11.3). However, the value of the amphiphilicity factor [49, 50] /a = —0.65 indicates that the pure triolein microemulsion is still a microemulsion in the narrower sense. The bending constants k and i< obtained from phase diagrams and scattering curves furthermore verify that the rigidity of the amphiphilic film decreases with increasing triolein content (Fig. 11.9). 

Friberg was certainly the one who made the most important contributions to establish the thermodynamic stability of micro emulsions, providing key phase diagrams and being very active in refuting arguments of kinetic stability in the scientific literature and at conferences. He also at an early stage realised the problem of microstructure. This was... 

Fig. 12. Phase diagram and phase microstructures for block copolymers. The areas L, G, C, and S correspond to the structures sketched at the bottom Sep represents more complicated structures (112).

However, intriguingly all the aforementioned studies, except the one reported by Atkin and Warr [29], were performed at room temperature so that detailed knowledge about the phase diagram and the structure were not explored. To fill this gap, we carried out a thorough study of water-[bmim][PF ]-TX-100 microemulsions as a function of temperature, T, and surfactant mass fraction at different IL mass fractions. The results are described later where we illustrate the formation of 1, 2, and 3 phases and present the fishtail phase diagram as well as the location of the point of optimal efficiency, called X point. The coordinates of the X point are y and r, the fraction of surfactant and temperature, respectively. The reported construction is carried out by inspection of the samples with optical and polarizing microscopy, and the microstructure is studied by SANS. For the sake of completeness, the experimental evidences are compared with data on the phase behavior of water-[bmim] [PFJ ternary systems with two CiEj surfactants. 

By using a similar approach. Von Corswant and Thoren (23) have shown the influence of the solubilization of an active drug in lecithin-based microemulsions. By combining phase diagram determinations, NMR spectroscopic measurements and solubility determinations of the solute in aqueous and oil phases, they have determined the effect of the solute on the phase behaviour and microstructure of the microemulsion. Depending on the nature of the solute, the influence on the curvature varies. The first solute studied (felodip-ine) is water-insoluble and slightly soluble in the oil. The presence of this solute increases the polarity of the oil phase and turns the film towards the water, even if this solute has no affinity for the surfactant film. 

In this discussion of the microstructural development of iron-carbon alloys, it has been assumed that, upon cooling, conditions of metastable equilibrium have been continuously maintained that is, sufficient time has been allowed at each new temperature for any necessary adjustment in phase compositions and relative amounts as predicted from the Fe-FejC phase diagram. In most situations these cooling rates are impracti-cally slow and unnecessary in fact, on many occasions nonequilibrium conditions are desirable. Two nonequilibrium effects of practical importance are (1) the occurrence of phase changes or transformations at temperatures other than those predicted by phase boundary lines on the phase diagram, and (2) the existence at room temperature of nonequilibrium phases that do not appear on the phase diagram. Both are discussed in Chapter 10. 

In the final chapter. Chapter 11, we discuss phase diagrams and thermodynamic modelling, which are becoming increasingly important methods for understanding the phase compositions determined by microstructural analysis. 

Carbon steels as received "off the shelf" have been worked at high temperature (usually by rolling) and have then been cooled slowly to room temperature ("normalised"). The room-temperature microstructure should then be close to equilibrium and can be inferred from the Fe-C phase diagram (Fig. 11.1) which we have already come across in the Phase Diagrams course (p. 342). Table 11.1 lists the phases in the Fe-FejC system and Table 11.2 gives details of the composite eutectoid and eutectic structures that occur during slow cooling. 

Eutectics and eutectoids are important. They are common in engineering alloys, and allow the production of special, strong, microstructures. Peritectics are less important. But you should know what they are and what they look like, to avoid confusing them with other features of phase diagrams. 

Figure 5.29. Fe-rich region of the Fe C phase diagram. Stable Fe-C (graphite) diagram solid lines metastable Fe-Fe3C diagram dashed lines. The following current names are used ferrite (solid solution in aFe), austenite (solid solution in 7Fe) and cementite (Fe3C compound). Pearlite is the name given to the two-phase microstructure which originates from the eutectoid reaction ...

The effect of adding a surfactant, (NaDDS), was also investigated. One such case only is shown in Fig. 6 where BE is replaced by a 5 1 mixture of BE-NaDDS. The main effect of NaDDS is to increase the miscibility range of the oil in water. Various ratios of BE-NaDDS were used and, as a first approximation, the change in the phase diagram is directly proportional to the concentration of NaDDS. The addition of a surfactant probably stabilizes the microstructures which were already present in the ternary system BE-DEC-H O and decreases the quantity of BE needed to solubilize DEC. Therefore the presence of a surfactant is useful but not essential to the stability of microemulsions. 

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