Thermochemical Study of the Formation of Silicides, Borides, and Carbides in an Fe-Ni-Cr-Cu-Si-B-C Alloy

Calculation methods using combined GGA and GGA + U schemes (semiempirically tuned generalized gradient approximations) are applied to determine the thermochemical characteristics (enthalpy, molar heat capacity, Gibbs energy of formation) of silicides, borides and carbides in an alloy of the specified composition 40Fe-31Ni-16Cr-5Cu-5Si-2B-1C. Three modules of the HSC Chemistry 6.0 software package (Metso Outotec, version 6.0, Espoo, Finland) are used. First, the Reaction Equation module (Reaction Equations for the calculation of thermodynamic functions in the temperature range for individual substances or chemical reactions) is used to calculate a change in the free Gibbs energy at different temperatures. Second, the Equilibrium Composition module (Equilibrium Compositions for the calculation of equilibrium compositions of phases in the presence of reversible chemical reactions) is used to calculate the composition of each chemical substance in the equilibrium state. Third, the H, S, C, and G diagrams module (Graphs of Thermodynamic Functions aimed at plotting thermodynamic functions) is used to determine the relative phase stability of the compounds depending on temperature in the form of Ellingham diagrams. The thermochemical modeling results show that the heat capacities of formation of hardening compounds in the alloy increase with the temperature. The thermodynamic calculations of the enthalpies of the hardening phases in the alloy show that silicides, borides, and carbides form at temperatures >1400 degrees C. An increase in the Gibbs energies and a tendency toward stability with increasing temperature are observed when considering Delta G(T) of silicides. Strong heat absorption and an increase in the Gibbs energy in the temperature range under study are observed for the formation of borides in the alloy. The calculation results for the Gibbs energy as a function of temperature demonstrate the formation of carbides Ni3C, Fe3C, SiC, B4C, Cr3C2, Cr4C, and Cr7C3. Aggregate and polymorphic transformations occur with a decrease in the Gibbs energy down to a temperature of similar to 1500 degrees C. The absorption of thermal energy associated with a high temperature of carbide structure ordering is revealed with a further increase in temperature. Thus, the formation of silicides, borides, and carbides in the 40Fe-31Ni-16Cr-5Cu-5Si-2B-1C alloy is substantiated by the thermochemical study.