THERMOCHEMICAL STUDY OF THE FORMATION OF SILICIDES, BORIDES, CARBIDES IN Fe–Ni–Cr–Cu–Si–B–C ALLOY

Abstract

To determine thermochemical characteristics: enthalpy, molar heat capacity and Gibbs energy of formation of silicides, borides and carbides in an alloy of a given composition (40Fe–31Ni–16Cr–5Cu–5Si–2B–1C) calculation methods were used using mixed GGA and GGA + U schemes (semi-empirically tuned generalized gradient approximations). Three modules of the HSC Chemistry 6.0 software package (Metso Outotec, version 6.0, Espoo, Finland) were used in the study. First, the “Reaction Equation” module was used to calculate the change in Gibbs free energy at different temperatures. Secondly, to calculate the composition of each chemical in the equilibrium state, the module “Equilibrium Composition” was used (“Equilibrium compositions” – calculation of equilibrium compositions of phases in the presence of reversible chemical reactions). Thirdly, the module “H, S, C and G diagrams” (“Graphs of thermodynamic functions” – plotting thermodynamic functions) was used to determine the relative phase stability of compounds depending on temperature in the form of Ellingham diagrams. The results of thermochemical modeling showed that the temperature dependences of the heat capacity of the formation of hardening compounds in the alloy increase with increasing temperature. Thermodynamic calculations of the enthalpies of the hardening phases in the alloy showed that at temperatures 1400°C, silicides, borides, and carbides are formed. ∆G(T) of silicides, there is an increase in the values of the Gibbs energy and a tendency towards stability with increasing temperature. During the formation of borides in the alloy, one can see a strong absorption of heat, an increase in the Gibbs energy in the studied temperature range. The results of calculating the Gibbs energy as a function of temperature showed the formation of carbides Ni3C, Fe3C, SiC, B4C, Cr3C2, Cr4C, Cr7C3. The formation of phases occurs with a decrease in the values of the Gibbs energy to a temperature of ~1500°C. A further increase in temperature indicates the absorption of heat, which is associated with a high ordering temperature of the carbide structures. Thus, the thermochemical study justified the formation of silicides, borides, carbides in the alloy 40Fe–31Ni–16Cr–5Cu–5Si–2B–1C.