The Gibbs Energy Of Transition Metal Compounds

Abstract

The “Calphad approach” refers to a method for assessment of thermodynamic information which combines an analysis of experimental data using phenomenological models for the Gibbs energy G = H - TS and calculations of phase diagrams.¹ The models contain parameters (e.g., interaction energies) which are determined by searching for the best fit to the thermodynamic information. The analysis is carried out iteratively, using computerized optimization techniques which allow the simultaneous treatment of various types of experimental data. In the optimization, each measured value is given a certain weight, which reflects the probable accuracy, and the extent to which that piece of information is allowed to influence the optimum parameter values. The weights are chosen by personal judgement at the start of the work, but they are changed systematically during the assessment until most of the reliable information is well reproduced by the model parameters. In addition to the assessment techniques for measured values, the Calphad activity has led to the development of extrapolation procedures for thermodynamic properties of alloys that can be used to treat some metastable compounds and phases that are poorly known from experiments, e.g., in higher-order systems.