Effect of the magnesium content on the mechanochemical behavior in ternary system Mg–B2O3–C

• Mg content plays a key role in the mechanochemical behavior of ternary system Mg–B2O3–C.
• By increasing milling time, an exothermic reaction occurs before magnesium melting.
• Magnesiothermic reaction provided enough heat for activating carbothermic reaction.
• Mg amount, within a range of 9/2–6 mol occurred in MSR mode.

The influence of Mg content in Mg–B2O3–C mixture on the MgO–B4C composite formation and mechanism of reactions during ball milling process was investigated. In keeping with this aim, a mixture of boron oxide powder along with different amounts of Mg (4–10 mol) and C (1–3 mol) was activated in a ball mill. Thermodynamic calculations and differential thermal analysis (DTA) results revealed that Mg value played a main role, thereby; overall reaction enthalpy and adiabatic temperature (Tad) changed by variation of magnesium content. Tad enhanced with the increase in the Mg content and found maximum value (2852 °C) at stoichiometric ratio (Mg = 6 mol). DTA results showed that increasing the activation time to 5.5 h could decrease the temperature of combustion reaction to 530°C before magnesium melting. According to experimental findings, in the mixture of powder with 4 mol Mg, magnesiothermic reaction occurred in MSR mode and no carbothermal reaction took place. However, when the Mg content reached within a range of 9/2–6 mol, the magnesiothermic reaction occurred in MSR mode and activated the carbothermal reaction. Further enhancement in Mg content (10 mol Mg), as a diluent agent, led to MSR magnesiothermic reaction and no carbothermal reduction occurred.