Title:	Atomic-scale simulation of finite temperature thermodynamic properties in transition metal – carbon compounds
DNr:	SNIC 2015/1-27
Project Type:	SNIC Medium Compute
Principal Investigator:	Malin Selleby <malin@kth.se>
Affiliation:	Kungliga Tekniska högskolan
Duration:	2015-01-29 – 2016-02-01
Classification:	20506 20502 10304
Keywords:

Abstract

Recent advances in ab initio molecular dynamics based atomic-scale simulation techniques have enabled efficient and very accurate calculations of finite temperature thermodynamic quantities, such as heat capacity and free energy, of transition metals in well-defined crystalline structures. These calculations have been extensively compared and benchmarked with other theoretical approaches as well as experimental measurements to verify the validity of the results. The task at hand is therefore to apply this treatment to compounds and, in the future, alloys. Compounds, such as transition metal carbides, are suitable for such a study since they are present in most carbon containing alloys as desired or undesired constituents of the microstructure. Knowledge and control of the microstructural evolution during heat treatment of e.g. hard metals and steels is therefore important to control their final properties. Thermodynamics govern the formation of carbides during processing with respect to e.g. what type of phases may form and in what ratio. Thus we are interested in computing the thermodynamic stability of transition metal carbides, which can then be used as a foundation for predicting the equilibrium state of multicomponent alloys by using the CALPHAD method. More specifically we are interested in Cr-based carbides due to the considerable improvement in the mechanical properties of Cr-containing WC-Co hard metal alloys, where Cr is added as a carbide powder during the production process, compared to unalloyed WC-Co materials. It is important to realize that although a multitude of experimental studies on the stability of transition metal carbides have been performed over the years many of the studies show conflicting results and/or the scatter in data is often substantial. This is especially true concerning the thermodynamic properties of Cr-based carbides. The knowledge gained from this project will also be relevant for the steel and superalloy industries as Cr and C are alloying elements of major importance in these types of materials as well. References: [1] B. Grabowski, L. Ismer, T. Hickel, and J. Neugebauer, “Ab initio up to the melting point: Anharmonicity and vacancies in aluminum,” Phys. Rev. B, vol. 79, no. 13, p. 134106, Apr. 2009. [2] H. Kleykamp, “Thermodynamic studies on chromium carbides by the electromotive force (emf) method,” J. Alloys Compd., vol. 321, pp. 138–145, May 2001. [3] H. L. Lukas, S. G. Fries, and B. Sundman, Computational Thermodynamics: The Calphad Method. Cambridge University Press, 2007. [4] J. Weidow and H.-O. Andrén, “Grain and phase boundary segregation in WC–Co with small V, Cr or Mn additions,” Acta Mater., vol. 58, no. 11, pp. 3888–3894, Jun. 2010.