Determination of solvus curves disappearing-phase method

Determination of solvus curves (disappearing-phase method) 377 

12-4 DETERMINATION OF SOLVUS CURVES (DISAPPEARING-PHASE METHOD) 

To return to the main subject of this chapter, we might now consider the methods used for determining the position of a solvus curve (solid solubility curve) on a phase diagram. Such a curve forms the boundary between a single-phase solid 

One method of locating such curves is based on the lever law. This law, with reference to Fig. 12-7 for example, states that the relative proportions of a and p in an alloy of composition z in equilibrium at temperature T, is given by the relative lengths of the lines zy and zx, or that 

Since the curve of Ipjl vs. weight percent B is not linear, high accuracy in the extrapolation depends on having several experimental points close to the phase boundary which is being determined. The accuracy of the disappearing-phase method is therefore governed by the sensitivity of the x-ray method in detecting small amounts of a second phase in a mixture, and this sensitivity varies widely 

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