Microstructural development during transient directional solidification of hypermonotectic Al–Bi alloys

Directional unsteady-state solidification experiments were performed with hypermonotectic Al–5.0 wt%Bi and 7.0 wt%Bi alloys. Thermal parameters such as the growth rate (v) and the thermal gradient (G) were experimentally determined by cooling curves recorded along the casting length. The predominant Bi-rich phase was characterized by droplets embedded in the aluminum matrix. Both the interphase spacing (λ) and the Bi-rich particles diameter (d) were measured along the casting length. These microstructural features were correlated to the solidification thermal parameters: growth rate, cooling rate and thermal gradient. An experimental law expressing λ as a function of both G and v was found to better represent the growth of hypermonotectic Al–Bi alloys. Moreover, it was found that the interphase spacing decreases with increasing alloy bismuth content.