Aluminothermic reduction of boron oxide for the manufacture of Al–B alloys

Powder metallurgy processing route was explored to produce Al–AlB2 master alloys from B-rich boron compounds in an environmentally friendly fashion. Al/B2O3 powder mixtures were ball milled and subsequently heated isothermally at elevated temperatures, T > 700 °C. There is hardly any evidence for a measurable combustion reaction across the section of the aluminium grains after ball milling for 1 h at room temperature. The interpenetrating Al and B2O3 layers start to react once the as-milled powder blend is heated to 875 °C. The XRD spectrum of the powder blend held at 875 °C reveals reflections of Al2O3, evidencing a thermite reaction of the type, B2O3 + 2Al → 2B + Al2O3. B thus made available reacts with the excess Al to produce AlB2 particles across Al grains. Al2O3 generated with the above thermite reaction reacts with the remaining B2O3, when the isothermal heat treatment is performed at 1000 °C. B atoms form the high temperature AlB10 compound, rather than AlB2, at this temperature. An additional borate, Al18B4O33, starts to form at still higher temperatures either by the direct reaction between Al2O3 produced by the thermite reaction and the unconverted B2O3 or by the decomposition of Al4B2O9 formed earlier.

► Ball milling at 25 °C fails to establish a reaction between Al and B2O3.
► Al and B2O3 react once the ball-milled powder blend is heated to 875 °C.
► Thermite reaction, B2O3 + 2Al → 2B + Al2O3, takes place at 875 °C.
► The freed B reacts with the excess Al to produce AlB2 particles at 875 °C.
► Borates, Al4B2O9 and Al18B4O33, and AlB10 are produced at T > 1000 °C.