Effects of reductant type on the combustion synthesis of NiB

Ni–B intermetallic alloys are important filler materials which are used in brazing applications. Carbothermic and aluminothermic synthesis routes are main methods to produce NiB. Carbothermic synthesis is carried out in electric arc furnaces and consumes high amount of energy whereas aluminothermic synthesis mainly uses its own reaction energy. Thus, via aluminothermic synthesis, the consumption of energy drops. Aluminothermic synthesis is a sub-group of combustion synthesis methods and, its name is based on the Al which is used as reductant in reactions. In the present study, aluminothermic synthesis conditions of NiB (mainly containing 15% B by wt.) were investigated for increasing B₂O₃ stoichiometry in reactants. Moreover, some experiments were conducted to understand the magnesiothermic synthesis conditions, as an alternative reductant. Results were compared to each other and to thermochemical simulations which were carried out by using HSC Chemistry 6.12 and FactSage 7.1 softwares. Chemical analysis, SEM-EDS and XRD were the main techniques to characterize raw materials and the products. Increasing B₂O₃ stoichiometry and the use of the Mg as reductant positively affected the chemical content of obtained NiB phases although metal recovery yields were lower for magnesiothermic experiments than that of aluminothermic experiments. For instance, in the magnesiothermic experiment carried out with 100% B₂O₃ stoichiometry, the obtained metallic fraction consisted of 84.2% Ni, 15.1% B and 0.1% Mg.