Phase boundary solidus

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. 

So, for this binary solution of components A and B, which mix perfectly at all compositions, there is a two-phase region at which both solid and liquid phases can coexist. The uppermost boundary between the liquid and liquid + solid phase regions in Figure 2.3f is known as the liquidus, or the point at which solid first begins to form when a melt of constant composition is cooled under equilibrium conditions. Similarly, the lower phase boundary between the solid and liquid + solid phase regions is known as the solidus, or the point at which solidification is complete upon further equilibrium cooling at a fixed composition. 

Fig. 9. Pressure and temperature estimates for 114 nodules (see McKenzie 1989) from the Kaapvaal Craton calculated using (a) Brey Kohler s (1990) geothermometer and geobarometer, and (b) Bertrand Mercier s (1985) geothermometer and Finnerty Boyd s (1984) geobarometer. The solidus is from McKenzie Bickle (1988), and the diamond-graphite phase boundary from Kennedy Kennedy (1976).

Equation (6.84) may be arranged to give the phase boundaries of the solidus and liquidus curves as... 

Some comments are in order regarding nomenclature First, for metallic alloys, solid solutions are commonly designated by lowercase Greek letters (a, p, y, etc.). With regard to phase boundaries, the line separating the L and a + L phase fields is termed the liq-uidus line, as indicated in Figure 9.3a the hquid phase is present at all temperatures and compositions above this line. The solidus line is located between the a and a + L regions, below which only the solid a phase exists. 

The solubility limit at some temperature corresponds to the maxLmmn concentration of one component that will go into solution in a specific phase. For a binary eutectic system, solubility limits are to be foimd along solidus and solvus phase boundaries. 

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... 

During vaporization of non-stoichiometric refractory carbides each element vaporizes at a different rate which is dependent on surface composition or relevant activities at the surface. When the initial bulk composition is near C/M = 1, the vaporization of C is much greater than that of M. As a result, the surface C content decreases and eventually approaches a constant value, which we will call the steady-state CVC (ssCVC). At the ssCVC, the vapor composition is nearly equal to the initial bulk composition. As C diffusion to the vaporizing surface reduces the C content of the bulk material, the surface composition asymptotically approaches the equilibrium CVC (eCVC). The rate at which eCVC is approached depends on the relative magnitudes of C vaporization and diffusion. When the eCVC has been reached, the surface and bulk C/M ratios are equal to the vapor composition. The intersection of the solid eCVC map with the solidus boundary of the monocarbide phase determines where melting occurs under equilibrium conditions for a particular atmosphere. 

The preceding information may be assembled into an equilibrium phase diagram shown in part (f), which is typical for this type of system. The diagram shows the T-x regions in which the homogeneous solid or liquid is stable the two corresponding boundary lines are known as the solidus and liquidus the T-x... 

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