Formation of core-type microstructure in Al–Bi monotectic alloys

The macroscopic morphologies and core-shell dimensional accuracy for Al–Bi immiscible alloys were investigated. Irrespective of compositions, the shell always consists of a Bi-rich phase due to surface segregation. With decreasing the superheat degree above the highest critical point of Al–65.5 wt.% Bi alloy, the shell of particles changed from irregular, annular to crescent shapes under the actions of Marangoni motion and gravity. The simulation suggests that the superheating and particle size affect markedly the temperature gradient and cooling rate of droplets, and thus influence the Marangoni and Stokes motions. At a superheat degree of 100 K, a perfect core-shell microstructure of Al–65.5 wt.% Bi particles with a diameter of 0.9 mm was successfully obtained. The dimensional relationship between the core and whole particle could be described by a function of Dcore = 0.9137 Dparticle − 0.0312.

Research highlights
► The shell consists of a Bi-rich phase in core-type Al-Bi binary alloy particles.
► Conditions to produce the perfect core-type microstructure are discussed.
► For core-type Al-65.5 wt.% Bi alloys, Dcore=0.9137 Dparticle−0.0312.