Thermodynamics of the intermediate phases in the Mg-B system

The high temperature vaporization thermochemistry of the intermediate phases in the B-rich portion of the Mg-B phase diagram (above 80% at. B) has been extensively studied by Knudsen Effusion-Mass Spectrometry (KEMS) and Knudsen Effusion-Weight Loss (KEWL) techniques. The temperature dependence of the Mg vapor pressures in equilibrium over the binary alloys in the composition range 87.5-95.2 and 95.2-100%at. B (corresponding to the two biphasic regions MgB7+MgB20 and MgB20+β-B, respectively) has been measured in the temperature range 1162-1467 K. The enthalpy changes of the decomposition reactions for MgB7 and MgB20 are presented together with previously determined data for the Mg-richest phases MgB2 and MgB4 and a self-consistent set of formation enthalpies for all the four Mg-B phases has been derived. The proposed experimental formation enthalpies, ΔfH2980, are MgB2: −41.1±2.2, MgB4: −29.9±2.0, MgB7: −20.3±1.5, MgB20: −8.5±0.6 kJ/mol atoms. Using the present and literature data the Mg-B thermodynamic description has been newly assessed using Thermo-Calc software and the whole phase diagram redrawn. Good agreement was obtained between the calculated results and the experimental data.