Experimental and ab initio data in the CALPHAD method: the Fe-Cr-Mo-C and Nb-Ni-Si systems.

Author(s)

Luiz Tadeu Fernandes Eleno

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 2012-04-18

Examining board members

Cláudio Geraldo Schön; Gilberto Carvalho Coelho; Ivan Gilberto Sandoval Falleiros; Helena Maria Petrilli; Silvio Roberto de Azevedo Salinas

• Advisor: Cláudio Geraldo Schön

Abstract

The aim of this project is the combination of advanced experimental and theoretical approaches for the development of thermodynamic databases dedicated to modelling steels and high performance alloys. Examples of materials are centrifugally-cast superalloys designed for use in reforming and pyrolisis furnaces, as well as intermetallic-reinforced tool steels. The theoretical methods are the combination of electronic structure calculations and thermodynamic modeling at finite temperatures using the CALPHAD method. This methodology has been used by different scientific groups, both in Brazil and around the world. Using experimental data in the Fe-Cr-Mo-C sytem, recently determined in our laboratory, and first principles calculations in the Nb-Ni-Si system, together with other experimental results from the literature, we improved the existing thermodynamic databases for these two systems, minimizing discrepancies regarding the experimental evidence about phase stability fields and phase equilibria. In the Fe-Cr-Mo-C system, we employed experimental data for a reoptimization of the thermodynamic description. We adopted new descriptions for the binary Cr-Fe, C-Cr, and C-Fe systems, with new models for cementite in the C-Fe system, and sigma in the Cr-Fe system. Because of these alterations, a reevaluation of the ternary descriptions was necessary, reassessing them when required (C-Cr-Fe) or just revalidating existing models (C-Cr-Mo). After that, we re-optimized the quaternary system, arriving at satisfactory results, in comparison with experimental data. The thermodynamic properties of the Ni-Nb-Si system is almost completely unknown. For that reason, there are not enough data in the literature to perform a complete assessment of the system. With that in mind, we decided to perform first-principles electronic structure calculations, in order to determine the formation energies of the ternary compounds. The binary systems, on the other hand, are very well-known, each one of them with several published thermodynamic assessments during the last few years. For this reason, we adopted the most recent thermodynamic descriptions of the binaries as a starting point for the modelling of the ternary system. The result of the modelling is very satisfactory, in comparison with the few experimental information available. (AU)

• FAPESP's process: 09/14532-9 - Thermodynamic modelling of the quaternary systems Fe-Cr-Mo-C and Ni-Nb-Si-C
• Grantee: Luiz Tadeu Fernandes Eleno
• Support Opportunities: Scholarships in Brazil - Doctorate