[en] The formation of boride phases is studied in the production of Al-B master alloys, which involves chemical reactions of KBF₄ and aluminum melts. Boron is reduced from KBF₄ and reacts with aluminum to form AlB₁₂ and AlB₂ depending on processing conditions. AlB₁₂ forms at the Al/flux interface and subsequently reacts with aluminum to form AlB₂ in a slow process while moving away from the Al/flux interface. In designed experiments, aluminum reacted with KBF₄ droplets producing dense AlB₁₂ shells trapping KAlF₄ salt inside at high temperatures. In comparison, the majority of individual AlB₂ particles is located near the Al/flux interface inside the aluminum matrix in a circular shape. Meanwhile, a uniform dispersion of AlB₂ particles in the aluminum matrix is also observed away from the Al/flux interface. It is revealed that boron, after being reduced from KBF₄ by aluminum, has a high tendency to build up at the Al/flux interface, forming AlB₁₂ and subsequently transforming to AlB₂. - Graphical abstract: The following figure demonstrates the phase relationships of AlB₂-Al-Flux system. - Highlights: • Boron builds up at the flux/Al interface resulting in the formation of AlB₁₂. • Metastable AlB₁₂ transforms to AlB₂. • Interfacial energies play critical roles in the dispersion of AlB₁₂ and AlB₂. • Interfacial energies: AlB₁₂/Al > Al/flux > AlB₁₂/flux > AlB₂/flux > AlB₂/Al. • AlB₂ has the lowest interfacial energy with aluminum.