Thermodynamics of Liquid Solutions of Nitrogen in Chromium

Abstract

A simple theory is proposed for the thermodynamic properties of nitrogen solutions in Fe–Cr melts. The theory is based on a lattice model of the solution. An fcc model lattice is adopted. Iron and chromium atoms are distributed at the lattice sites. Nitrogen atoms are located in octahedral interstices. The nitrogen atoms only interact with metal atoms at neighboring lattice sites. The energy of this interaction is assumed to depend neither on the composition nor on the temperature. The liquid Fe–Cr solutions are assumed perfect. Within this framework, the constant in the Sieverts law governing the solubility of nitrogen in liquid chromium may be expressed in terms of its value for the solubility of nitrogen in liquid iron and the value of the Wagner parameter for N–Cr interaction in liquid iron alloys. In addition, the partial enthalpy of solution of nitrogen in liquid chromium at infinite dilution is expressed in terms of the corresponding enthalpy for the solution of nitrogen in liquid iron and the Wagner parameter for N–Cr interaction in liquid iron alloys. A relation is also established between the Wagner parameter for N–Fe interaction in liquid chromium alloys and the Wagner parameter for N–Cr interaction in liquid iron alloys. On that basis, the constant in the Sieverts law governing the solubility of nitrogen in liquid chromium is calculated, along with enthalpy of solution of nitrogen in liquid chromium at infinite dilution and the Wagner parameter for N‒Fe interaction in liquid chromium alloys at 1873 K. The calculation results are compared with experimental data regarding the solubility of nitrogen in liquid chromium and Cr–Fe alloys obtained by various methods. The theoretical data are in best agreement with experimental data obtained by quenching. The values of the Wagner parameter for N–N interaction in liquid chromium alloys and iron alloys are discussed.