Eutectic formation

Despite their improved resistance to general corrosion by sulphur, the dilute alloys with many elements are sensitive to intergranular attack and embrittlement, and, at temperatures above 635°C, to eutectic formation. 

Melting. Loss of crystallinity of a solid reactant through melting, or eutectic formation, or dissolution in a product often results in an enhanced rate of decomposition as a consequence of the relaxation of the bonding forces responsible for lattice stabilization. The appearance of a... 

The reaction products, MC103 and MCI, reduce the melting point of the alkali perchlorate reactant by eutectic formation and, at high a, the accumulation of MCI results in resolidification. Since reaction proceeds... 

Early discussions, by Tammann and by Hedvall, considered the possible existence of a common characteristic (approximate) temperature for a solid at which chemical interactions with other reactants became detectable. For example [111], such a characteristic temperature for CaO, measured in various reactions with CuS04, Co3(P04)2, MgC03, and MnSi03, was found to be 788—838 K. Similarly, the onset of reaction of BaO with the sulphates of Mg, Ca, Sr, Co, Cu, and Zn occurred between 601 and 645 K. In the latter example, it has been shown that the fusion of Ba(OH)2 (an impurity not easily excluded from BaO) could contribute to the initiation of reaction. Eutectic formation during the reactions of BaCl2 with alkali metal sulphates... 

The concept of a characteristic reaction temperature must, therefore, be accepted with considerable reservation and as being of doubtful value since the reactivity of a crystalline material cannot readily be related to other properties of the solid. Such behaviour may at best point towards the possible occurrence of common controlling factors in the reaction, perhaps related to the onset of mobility, e.g. melting of one component or eutectic formation, onset of surface migration or commencement of bulk migration in a barrier phase. These possibilities should be investigated in detail before a mechanism can be formulated for any particular chemical change. 

It should be noted that liquefaction at times occurs because of eutectic formation (e.g., often with caffeine combinations) and that this may not necessarily be associated with decomposition. On the other hand, discoloration (e.g., amines and sugars) usually is. 

Figure 15.5. Eutectic formation in the ternary system o-, m- and p-mtrophenol 3 a) Temperature-concentration space model b) Projection on a triangular diagram. (Numerical values represent temperatures in K)...

In one patented process (19) a m-p-xylene fraction is produced from xylene mixtures by distillation, and is subsequently cooled to about —70° F. to produce p-xylene crystals, which are removed in high purity by filtering or centrifuging. The yield of p-xylene is limited by eutectic formation with m-xylene. As the mixture behaves as an ideal solution, the yield and temperature level can be calculated from the thermodynamic properties of xylenes, which were reported by Kravchenko (12). 

In another process (14) ethylbenzene is included with the crystallizer charge to inhibit eutectic formation, thereby increasing the yield of p-xylene. 

Other data are also available (Refs 42 54) Eutectic Formation. TNT forms binary and ternary eutectics with various types of nitro compds and nitrate esters. Many of these are listed by Urbanski (Ref 29) others have been reported more recently (Ref 25). Since the di-nitrotoluenes and the unsymmetrical TNT isomers form such eutectics with 2,4,6-TNT which melt just above room temp, their removal is necessary to prevent exudation from TNT-based expls. One procedure for purifying TNT takes advantage of the fact that the eutectic containing the undesired isomers migrates to the crystal surface (see discussion under Exudation and Purification )... 

Industrially, milk fat is commonly added to cocoa butter to produce milk chocolate. However, there is a thermodynamic incompatibihty between the miUc fat and cocoa butter solids which results in eutectic phase behavior. This eutectic formation leads to a chocolate with a decreased hardness (Timms 1980 Bama et al. 1992 Bystrom and Hartel 1994 Reddy et al. 1996). Extensive work has been carried out on the addition of milk fat to cocoa butter because of its abihty to reduce the incidence of bloom formation in chocolate (Hartel 1996 Kleinert 1961 Dimick et al. 1993). Chocolate bloom is evident as a white-gray layer on top of the chocolate surface and eventually the chocolate will acquire a crumbly texture. 

Temperature of MC- and M2C-austenite eutectic formation, °C Solidus temperature, °C ... 

Steel Number Type Analyses Average Cooling Rate, °C/s Primary Phase Liquidus Start of Formation of Austenite Start of Formation of MC-Austenite Eutectic Formation of Carbide -Austenite Eutectic Solidus... 

Fig. 1 Simple binary-phase diagram with eutectic formation. Ta is melting point of pure A Th is melting point of pure B and E is eutectic point.

Polyethylene glycols (PEG) have been intensively characterized by thermal analytic methods. The melting points of PEG increase with the molecular weight and decreases with the content of water as a result of eutectic formation. " Corrigan studied the different DSC peaks of PEG Once folded chain crystals and extended chain crystals are present in PEG 6000, which results in two DSC peaks. In PEG of higher molecular mass, only folded chain crystals are present. Lower molecular mass PEG contain only extended chain crystals. Craig reviewed the thermal studies of PEG, including properties in aqueous solutions. " " ... 

The thermal stabilities of the three silver salts decrease [14] in the sequence AgCC04 > AgCCOj > AgC 02, which is also the order of decreasing ionic character. The fusion of AgC 04 well below its melting point is attributed to eutectic formation with AgC during an autocatalytic reaction. The chlorate decomposed at about 80 K above the melting point through disproportionation accompanied by some decomposition. In contrast, the disproportionation of silver chlorite ... 

SrV20 -SrNb,0 BaVjOg-BaNbjO, Eutectic formation, d.t.a., X 9... 

The typical dependence of mechanical properties of Ti-Si binary alloys in as-cast and as-deformed states is shown in Fig. 3. It is seen that in as-cast state room temperature plasticity decreases with increase of silicon content, reducing practically to zero at silicon content of 2-3 wt. % then it raises a little at eutectic formation. Unfortunately, our attempts to increase room temperature plasticity in as-cast condition more or less substantially via employing various kinds of heat treatment procedures were not successful. At the same time, using thermo-mechanical treatment (forging at temperature 1050 °C), it was possible to increase plasticity of deformed alloys essentially. 

Design based on heat transfer. Limited by eutectic formation. Multistaging possible but usually <10 stages. 

Figure 6.2 Eutectic formation coconut oil/palm oil blends, from author s data. Key —30°C ...

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