Influence of AlB₁₂ Particles on the Strengthening Effect in Aluminum Matrix Composites

The structure and mechanical properties of Al–AlB₁₂ powder composites were studied. Composites containing 2.5, 5, 10, and 15 vol.% α-AlB₁₂ particles in an aluminum matrix were produced by heating and melting a mixture of Al and α-AlB₁₂ powders using a high-frequency heating unit in an inert atmosphere at 1250–1500°C. The melt temperature was increased with higher α-AlB₁₂ content to maintain the required melt fluidity. Aluminum powder with an average particle size of 40 μm and α-AlB₁₂ powder with an average particle size of 2.5 μm were used to produce the composites. The structure was analyzed with X-ray diffraction and scanning electron microscopy. The mechanical properties were determined through compression testing. X-ray diffraction and metallographic analyses revealed that all composites represented an aluminum matrix reinforced with finely dispersed α-AlB₁₂ particles. Metallographic analysis demonstrated uniform distribution of strengthening α-AlB₁₂ particles in the aluminum matrix at 2.5–5 vol.% content. Inhomogeneous particle distribution, with the formation of α-AlB₁₂ agglomerates, was observed at 10 and 15 vol.% α-AlB₁₂. In the composites with 2.5, 5, and 10 vol.% α-AlB₁₂, the integral hardness ranged from 510 to 570 MPa. The hardness of the Al–15 vol.% AlB12 composite was 540 MPa. Mechanical tests of the composites with 2.5 and 5 vol.% α-AlB₁₂ phase revealed their precipitation strengthening, which was well described by a modified Orowan equation. In the composite with 10 vol.% α-AlB₁₂ particles and above, the strengthening effect was diminished, and even softening occurred in the composite with 15 vol.% α-AlB₁₂ particles. This was attributed to the formation of a large number of α-AlB₁₂ particle agglomerates. The minimum content of α-AlB₁₂ particles promoting the precipitation strengthening in Al–AlB₁₂ composites was estimated. Considering the calculation error, it was 1 vol.%.