Third Generation of Unary Calphad Descriptions and the Avoidance of Re-Stabilized Solid Phases and Unexpected Large Heat Capacity

• Published in: Journal of phase equilibria and diffusion 2022, Vol.43 (3), p.304-316
• Main Author: Schmid-Fetzer, Rainer
• Format: Article
• Language: English
• Subjects: Ceramics; Composites; Crystallography and Scattering Methods; Engineering Thermodynamics; Entropy; Glass; Heat and Mass Transfer; High temperature; Mathematical analysis; Metallic Materials; Natural Materials; Physics; Physics and Astronomy; Software; Solid phases; Thermodynamics
• ISSN: 1547-7037; 1863-7345; 1934-7243
• DOI: 10.1007/s11669-022-00976-3

The problem of solid phase re-stabilization at high temperature in the latest version of 3rd generation data for pure solid elements is studied and the remedy to apply new Calphad software with the Equal-Entropy Criterion (EEC) implemented is critically examined. The concept of Equal-Entropy Temperature (EET) as very top limit for crystal existence is used to develop amendments by a second temperature range without introducing any adjustable parameter to an existing set of 3rd generation unary functions of solid and liquid. This is shown to safely prevent by definition of the mathematical Gibbs energy function the violation of the EEC axiom, namely that no solid phase should be more stable than the liquid at high temperature. The comprehensive amendment also solves the problem of unexpected large C p contributions at high temperature. Detailed analyses are provided for the examples of pure Al and for binary Al-Ir, Al-Cr and Al-C systems. Serious risks of unforeseen pitfalls and artifacts are exemplified for some cases where an automatic exclusion of an stable phase by new “EEC-software” may occur. It is argued that this is not acceptable at least for applied Calphad simulations.