Understanding the role of ultrasonic cavitation assisted casting of boron nitride nanotube-reinforced aluminum matrix composite

Abstract

Integrating and dispersing one-dimensional (1D) nano reinforcements in metal matrix composites (MMC) is challenging due to their higher specific surface area, strong van der Waals forces within the nano reinforcement, and poor wettability with the matrix. In the present study, ultrasonic cavitation-assisted casting was employed to fabricate boron nitride nanotubes (BNNT)-reinforced aluminum matrix composites. The ultrasonic treatment (UST) demonstrated excellent deagglomeration, and dispersion of BNNTs in molten aluminum along with enhanced grain refinement potency. A remarkable ∼62% grain refinement efficiency in Al-BNNT composite compared to pure Al was achieved owing to the combined effect of UST, BNNT reinforcement and cold rolling. The increase in low-angle grain boundaries (LAGBs) by ∼58% in Al-BNNT composite, as revealed by electron backscatter diffraction (EBSD) maps, demonstrates the pileup of dislocations resulting in strength improvement. The yield strength exhibited a 97% improvement in the BNNT-reinforced composite compared to pure Al. The strengthening is attributed to UST, enhanced dislocation density and efficient bridging effect between BNNTs and the matrix. Thus, the present study constitutes the first successful report on the role of UST in the mechanism of BNNT dispersion, grain morphology and enhanced mechanical properties, including cold rolling of BNNT-reinforced MMC. Successful dispersion of BNNTs by ultrasonic cavitation confirms that UST technology is a promising method for manufacturing high-strength nanoparticle-reinforced lightweight metal matrix composites.