Calculation of Phase Boundaries by Thermochemical Methods in Contrast to Thermal Analysis

Abstract

The calculation of individual phase boundaries and whole equilibrium diagrams from thermochemical data is an important recent development. The method, being based on thermodynamic measurements carried out at suitably high temperatures, is much less likely to incur the type of error caused by diffusional delays in approaching equilibrium as is sometimes the case in conventional phase boundary determinations. It has been found that, in particular, solidus curves and solid solubilities obtained by thermal analysis may be subject to substantial errors. In favourable instances, the thermochemical method has been shown to allow the calculation of phase boundaries in multicomponent systems when only the thermodynamic properties of the binaries are known.

The present paper describes the thermochemical approach and compares the phase boundaries in individual systems with those obtained by conventional methods. Examples are taken partly from calculations on metallic systems, but the method is equally applicable to oxide-oxide systems, though accurate thermodynamic data for the latter are as yet much less complete — and such experimental work is therefore to be encouraged.