Agglomerates of in-situ particles are the key detrimental defects in particulate reinforced aluminum matrix composites (PRAMCs) by acting as Achilles' heel leading to the premature failure of the materials. Effective methods to disperse/eliminate these agglomerates rely on in-depth understanding of the agglomeration mechanism of the in-situ particles. In this work, the agglomerating behavior of TiB ₂ particles in Al-Ti-B system was investigated through the thermit reactions between mixed fluorides and molten aluminum. The results indicate that the morphological patterns of TiB ₂ agglomerates are inherited from the preformed Al ₃Ti intermedium. The formation of flocculent, shell and flaky TiB ₂ agglomerates are the results of the diffusing boron atoms continuously reacting with Al ₃Ti. The in-situ Al-TiB ₂ was used as a precursor to prepare TiB ₂p/7075 Al composites. In particular, ultrasound vibration treatment was applied to explore if a locally forced oscillation can deagglomerate the TiB ₂ agglomerates. Microstructural observations strongly support such speculation. Thanks to the deagglomeration and dispersion of TiB ₂ particles, both the strength and ductility of the PRAMC have been improved drastically. The fracture surface of the composites was transformed from particle debonding to the dominance of ductile fracture.