Solidliquid melting and solidIsolidII transformations

The direction and magnitude of the pressure effect on phase transformation equilibria are determined from the Clapyron equation. Inverting Eq (5.5) and regarding the temperature as the dependent property and total pressure as the adjustable variable, the variation of the transition point Tr with pressure is given by:

dp Ahjj.

where Ahtr is the enthalpy change of phase transformation and Avtr = vII - vI is the corresponding volume change. Table 5.1 summarizes applications of Eq (5.15) to melting of ice and iron and to the a - y solid-solid transition in iron. For most materials, AvM is positive - phase I (solid) is denser than phase II (liquid). Ice/water is the notable exception to this general rule because the liquid is denser than the solid; 13.5 MPa lowers the melting point by 1oC.

Table 5.1 Pressure effect on solid/liquid and solidI/solidII transitions


transitio n

Ttr, K

Ahtr, J/mole

Avtr, m3/molex106

dTtr/dp, k/MPa


ice ^ L

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