Solidus line

Fig. 4.5. Schematic of top left corner of the "silicon-impurity" phase diagram. To make things simple, we assume that the liquidus and solidus lines ore straight. The impurity concentration in the solid is then always less than that in the liquid by the factor k (called the distribution coefficient).

DEF. The phase boundary which limits the bottom of the liquid field is called the liquidus line. The other boundary of the two-phase liquid-solid field is called the solidus line. 

From 245°C to 183°C. The liquidus is reached at 245°C, and solid (a lead-rich solid solution) first appears. The composition of the liquid moves along the liquidus line, that of the solid along the solidus line. This regime ends when the temperature reaches 183°C. Note that the alloy composition in weight % (64) is roughly half way between that of the solid (81 wt%) and liquid (38 wt%) so the alloy is about half liquid, half solid, by weight. 

If an 80 at% Pb alloy is cooled, the first solid appears at 305°C, and is primary (Pb) with a composition of about 90% Pb (see Fig. A1.35). From 305 to 255°C the amount of primary (Pb) increases, and its composition, which (at equilibrium) follows the solidus line, changes it becomes richer in tin. This means that lead must diffuse out of the solid (Pb), and tin must diffuse in. 

C, is the concentration of impurity or minor component in the solid phase, and Cj is the impurity concentration in the liquid phase. The distribution coefficient generally varies with composition. The value of k is greater than 1 when the solute raises the melting point and less than 1 when the melting point is depressed. In the regions near pure A or B the liquidus and solidus lines become linear i.e., the distribution coefficient becomes constant. This is the basis for the common assumption of constant k in many mathematical treatments of fractional solidification in which ultrapure materials are obtained. 

In these phase diagrams, the liquidus line represents the temperature at which one of the components crystallizes, while, below the solidus line, the whole system solidifies. Between the solidus and liquidus lines are the regions where solid and liquid coexist. Since there is no solid phase above the liquidus lines and the liquid is thermodynamically stable. Ding et al. suggested that the liquidus temperatures should be adopted as the lower boundary of the liquid phase, instead of the solidus temperatures. The patterns of these phase diagrams are... 

Typically, the liquidus lines of a binary system curve down and intersect with the solidus line at the eutectic point, where a liquid coexists with the solid phases of both components. In this sense, the mixture of two solvents should have an expanded liquid range with a lower melting temperature than that of either solvent individually. As Figure 4 shows, the most popular solvent combination used for lithium ion technology, LiPFe/EC/DMC, has liquidus lines below the mp of either EC or DMC, and the eutectic point lies at —7.6 °C with molar fractions of - 0.30 EC and "-"0.70 DMC. This composition corresponds to volume fractions of 0.24 EC and 0.76 DMC or weight fractions of 0.28 EC and 0.71 DMC. Due to the high mp of both EC (36 X) and DMC (4.6 X), this low-temperature limit is rather high and needs improvement if applications in cold environments are to be considered. 

Figure 6.10. Comparison of the extiqralated liquidus/solidus lines relating to the f.c.c. phase in Ni-Cr alloys, derived by (a) using the van Laar method and (b) the trajectory obtained using the modified FP qiproach used by Chang et al. (l S).

For solidification described by the lever rule and assuming linear liquidus and solidus lines, the composition of the solid C, as a function of the fraction solid transformed (/,) is given by the equation... 

Fig. 22. Liquidus and solidus lines in the HgTe-CdTe pseudobinary section. Circles are from Szofran and Lehoczky (1981), squares on the solidus from Tung et al. (1981b), squares on the liquidus from Blair and Newnham (1961), diamonds from Steininger (1976), and triangles from Harman (1967).

Calculated Partial Pressures in Atmospheres Along the Solidus Line of Hg Cd Tefs)... 

Fig. 5.11. Feasible hardness-composition-temperature realtion on crossing the solidus line.

A second technique involves heating the cemented tungsten carbide to the solidus—liquidus temperature region of the binder phase in a decarburizing atmosphere, such as C02 gas (93). Decarburization occurs at the surface whereby the carbon concentration at the surface is reduced to reach the solidus line of the binder phase, and the liquid phase solidifies. As a result, the liquid phase is supplied to the inner portion, and this also reaches near the surface where it is decarburized to reach the solidus line and this again solidifies. This procedure is repeated until Co is enriched in the zone near the rake face. 

The formation of glass is a kinetically controlled phenomenon and is directly associated with atomic or ioiric diffusion within the melt. This controls the formation and growth of microcrystals when the temperature of the liquid reaches the solidus line (i.e. the crystallization temperature). 

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