The formation of AlB2 in an Al-B master alloy

The formation of borides in an Al-3 wt.%B master alloy, produced via chemical reactions of KBF4 and aluminium has been investigated. The chemical reactions produce boron, which dissolves into molten aluminium and subsequently forms aluminium borides. Backscattered electron imaging (BEI) of the Al-3 wt.%B master alloy under a scanning electron microscope (SEM) revealed the presence of two types of phases that contain different levels of boron. Combined with X-ray diffraction (XRD) results, the two types of phases are identified as AlB2 on AlB12. This gives a direct evidence for a peritectic reaction of AlB12 and aluminium, which produces AlB2. The thermodynamic properties of the reactions that may be involved are examined, and the presence of AlB12 phase in the master alloy explained. The observed microstructure is explained according to the peritectic reaction in an Al-B phase diagram. The stability of AlB2 and AlB12 at lower temperature than 975 °C is clarified.