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Transition Metals and their Carbides, Nitrides and Carbonitrides

from a CALPHAD Perspective

Time: Mon 2021-12-20 10.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm

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Language: English

Subject area: Materials Science and Engineering

Doctoral student: Fredrik Haglöf , Skolan för industriell teknik och management (ITM), Enheten strukturer

Opponent: Associate Professor Mauro Palumbo, University of Turin, Department of Chemistry

Supervisor: Professor Malin Selleby, Materialvetenskap, Strukturer; Docent Andreas Blomqvist, Materialvetenskap; Docent Susanne Norgren, Sandvik Coromant R&D

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The CALPHAD (CALculation of PHAse Diagrams) method consists of three cornerstones, i.e. experiments, computational methods (e.g. Density Functional Theory (DFT)) and thermodynamic models, which together are used to build CALPHAD descriptions. In this thesis the CALPHAD method, including all three parts, is used to study transition metals and their carbides, nitrides and carbonitrides. The focus is mainly on transition metals important for the steel and cemented carbide industries. Some of the weaknesses or uncertainties in existing thermodynamic descriptions of these transition metals are identified and investigated with either experiments or computational methods whereupon new descriptions are presented.

A method for calculation of the molar Gibbs energy as a function of temperature (Gm(T)) for metastable magnetic components is developed. The method uses DFT in combination with the Debye-Grüneisen model and the empirical magnetic model often used in CALPHAD. The developed method is also able to estimate the magnetic transition temperature (TC or TN). Furthermore, two new approaches for how to treat calculations on dynamically unstable allotropes are presented. This is important since this often is the reason why DFT calculations on metastable components fail. The method is developed and verified on bcc, fcc and hcp allotropes of Fe, Co and Ni, and is then used to calculate Gm(T) descriptions for fcc-TiC, fcc-CrC, fcc-CrN, hcp-Cr2C and hcp-Cr2N. Furthermore, the same method is also used to calculate the excess Gibbs energy at 0 K for the interactions CrC-TiC (fcc), CrC-CrN (fcc) and Cr2C-Cr2N (hcp), as well as TC and TN as a function of composition for the same composition ranges.

Experiments are performed at 1673 and 1773 K to determine the solubility of Cr in MC (M = Ti, Cr) and of Ti in M3C2 and M7C3, in the equilibria MC-M3C2-M7C3 and MC-M3C2-graphite of the C-Cr-Ti system. The solubilities are measured with EDS/WDS and verified with XRD/Rietveld refinement.

The experimental and computational results for the C-Cr-Ti system are used together with literature data to perform a reassessment of this system. Compared to the previous CALPHAD description, the new description also includes updated descriptions for two of the three constituent binary systems (C-Ti and Cr-Ti) and reproduces the experimental results in the TiC-Cr3C2 isoplethal section found in literature.

In summary, the outcome of this thesis is an updated CALPHAD description of the C-Cr-Ti system and calculated Gm(T) descriptions, including magnetic properties, for allotropes of Fe, Co, Ni, and several parameters in the C-Cr-N system. The developed method for calculation of Gm(T) for metastable magnetic compounds is also a useful tool for calculation of Gm(T) for many other transition metals and their carbides, nitrides, and carbonitrides.