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+?-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, ?H2980, 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