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Developing the third generation Calphad models and descriptions: a journey from unary to higher-order systems

Time: Thu 2022-05-19 10.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm

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Language: English

Subject area: Materials Science and Engineering

Doctoral student: Zhangting He , Strukturer

Opponent: Associate Professor Gabriele Cacciamani, Department of Chemistry and Industrial Chemistry, University of Genova, Italy

Supervisor: Professor Malin Selleby, Strukturer; Docent Huahai Mao, Materialvetenskap; Adjunct professor Andreas Blomqvist, Materialvetenskap

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The third generation Calphad descriptions aim to provide more accurate computational thermodynamics for an extended temperature range, i.e. from 0 K to high temperatures. In order to achieve such a goal, new Calphad models are needed. This thesis work has been performed to comprehensively study the existing third generation Calphad models and also to develop new ones.

To be able to move forward towards higher-order systems, one issue addressed in this work is how to improve the extrapolation treatment for solid phases from that used in the second generation Calphad descriptions. Thus, the first outcome of this work is that a new extrapolation treatment has been proposed.

For higher-order systems, focus was first given to developing the third generation Calphad models for compounds and end-members of solution phases. A so-called "hybrid'' model has been developed in this work to model compounds and end-members obeying the third law of thermodynamics. Furthermore, how to model the magnetic and chemical ordering effects has been studied. The revised IHJ (Inden-Hillert-Jarl) model has been successfully used in the present work to describe the magnetic ordering effects of solution phases with complex magnetic behaviour. It has also been demonstrated through the study of the Fe-Ni system that a reliable magnetic description is critical for obtaining an accurate thermodynamic description. The partitioning model has been applied to model chemical ordering effects. However, some issues remain for modelling the chemical ordering effect which have been discussed in this work. To extrapolate the binary descriptions into a ternary system, the EBEF (Effective Bond Energy Formalism) has been used to describe the bcc, fcc and sigma phases in the Cr-Fe-Ni system. The extrapolation results show that more reliable data are needed in order to further verify the usefulness of this formalism.

The model parameters of the third generation Calphad models have been extensively analysed in this work. Accordingly, some methods have been proposed to estimate the model parameters, e.g. the Einstein temperature and the electronic heat capacity coefficient. Some of these methods are based on DFT calculations which demonstrates one way to make use of DFT-calculated data for Calphad modelling.