Reaction pathways of in situ ZrB2 nanoparticles during flux-assisted synthesis in Al melt

Abstract

Based on the Al-K 2 ZrF 6 -KBF 4 system, in situ ZrB 2 /Al nanocomposite was fabricated using flux-assisted synthesis in Al melt at 850 °C. A time-quenched technique was utilized to reveal the reaction mechanisms and particle evolution processes. Microstructural observations reveal that ZrB 2 generates through three parallel pathways, i.e., molten Al reacts with salt droplets (type I), K 2 ZrF 6 -dominated (type II), and KBF 4 -dominated (type III) powders, respectively. As for type I and type II, the initial appearance of lamella and flocculent agglomerates enrich supersaturated [Zr] and [B] solute atoms, upon which high-density ZrB 2 nuclei spontaneously nucleation and grow over time. While in type III, Zr-diffusivity and surface tension dissolution drive the transformation of the rod-like AlB 2 phase into ZrB 2 nanoparticles. It is noteworthy that all of these reactions are basically complete within 15 min, and hereafter the multiply melt stirring has a significant shearing effect, resulting in relatively dispersed particle agglomerates with sizes of 5–20 μm. • Time-quenched technique helps to trace synthesis processes of salt-metal reactions. • Three parallel reaction pathways and associated reasons are clarified. • ZrB 2 nanoparticles form mainly through the nucleation-growth mechanism. • Reaction pathways and melt stirring affect particle cluster configurations.