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[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.