Origin of the separated α-Al nanocrystallization with Si added to Al86Ni9La5 amorphous alloy

An understanding of nanocrystallization is an important precondition for controlling the microstructure and properties of nanocrystal-reinforced amorphous alloys. In the present work, the microstructures of Al86Ni9La5 and (Al86Ni9La5)98Si2 amorphous alloys and their annealed products were investigated systematically to reveal why the nanocrystallization behavior of primary α-Al was considerably changed with Si addition. Upon heating, α-Al nucleates in the Al-rich regions of two amorphous alloys. The addition of Si significantly shrinks the size of single Al-rich region in the as-quenched amorphous sample, but raises the local maximum Al concentration and slows down the rate of diffusion of solute atoms. As a result, the primary crystallization of α-Al first takes place at a higher nucleation rate in the Al-rich regions at lower temperatures. However, the residual amorphous phase enriched with solute elements does not crystallize until the alloy is heated to a high enough temperature for massive atomic diffusion to occur, with further crystallization of α-Al occurring through epitaxial growth of the existing α-Al crystals. The high bonding strength between Si with Ni, and especially La, is responsible for the splitting of α-Al nanocrystallization into two stages. [Display omitted] •An abnormal two-stage precipitation of α-Al was distinctly addressed.•Morphology of α-Al was observed by transmission electron microscopy.•Elemental distribution at different crystallization stages was determined by 3DAP.•The second crystallization of α-Al is due to epitaxial growth of existing α-Al crystals.•Strong bonding of Si-Ni/La is responsible for the splitting of α-Al nanocrystallization.