Metal-matrix nanocomposites (MMNCs) have great potential for a wide range of applications. To provide high performance, effective nanoparticle (NP) dispersion in the liquid and NP capture within the metal grains during solidification is essential. In this work, we present the novel synthesis and structural characterization of surface-clean titanium diboride (TiB2) NPs with an average particle size of 20 nm, by ultrasonic-assisted reduction of fluorotitanate and fluoroboride salts in molten aluminum. The high-intensity ultrasonic field restricts NP growth. Using a master nanocomposite approach, the as-prepared TiB2 NPs are effectively incorporated into A206 alloys during solidification processing because of their clean surface, showing partial capture and significant grain refinement.