Thermodynamic re-modelling of the ternary B–Fe–Mo system based on novel experimental data

Abstract

In the present paper, the complete ternary B–Fe–Mo system is thermodynamically re-modelled based on our own experimental results and novel information reported in literature. A few key experiments were performed with ternary B–Fe–Mo alloys in order to clarify the morphology and stability of the MoxFe3-xB phase and measure temperatures of invariant second-order equilibria caused by the ferro-to-paramagnetic transformation of Fe-rich phases. In samples Mo6Fe64B30 and Mo10Fe65B25 (in at.%) annealed at 1403, 1363, 1343, 1023 and 873 K considerable amounts of MoxFe3-xB boride were detected to form at the expense of A1(Fe) or A2(Fe) and Fe2B phases. EBSD results show that the area fraction of this boride is higher when the annealing temperature is lower and the annealing time is longer. The sample Mo6Fe64B30 annealed at 1363 K for 15 h reached three-phase state Mo2FeB2+Fe2B+MoxFe3-xB. The sample Mo10Fe65B25 annealed at 1343 K for 5 h + 1403 K for 45 min + 1023 K for 5 h and quenched in a liquid Ga-In eutectic alloy was almost a three-phase A2(Fe)+Mo2FeB2+MoxFe3-xB material, with only about 2 area % Fe2B. The elaborated thermodynamic description was applied to calculate selected phase equilibria and thermodynamic properties in order to provide a comparison between the calculated and experimental results. The calculations are shown to reproduce available experimental phase equilibria data well. Enthalpies of formation at 0 K of the compounds Mo2FeB2 and MoFe1.93B3.73 were calculated to be equal to −216.0 and −277.4 kJ mol−1, respectively. The present value for Mo2FeB2 matches with first-principles calculations result of Wang at al. (−219.0 kJ mol−1).