Papers by Keyword: System Al-Bi

Abstract: Al-Bi and Al-In of exact monotectic composition were directionally solidified with solidification velocities ranging from 0.3 µm/sec to 5 µm/sec and temperature gradients from 2 K/mm to 6.5 K/mm. Three distinct microstuctures could be observed: fibers, strings of pearls or irregular arrays of second phase droplets. The transitions from fibers to strings of pearls is discussed theoretically and compared with the experimental results. The origin of this transition seems to be a Nichols and Mullins instability of the fibers during cooling from the monotectic to the eutectic temperature caused by surface perturbations driving volume diffusion of solute in the liquid fibers of the second phase.

Abstract: We prepared Al-Bi alloys with our new aerogel counter gravity casting facility and Al-Pb alloys by simple mould casting with variable cooling rates. By counter gravity casting, it is possible to have directional solidification with flat isotherms and without forced melt flow during the casting process. Both methods allow studying the nucleation rate in liquid-liquid decomposition. The most important result is that a separation between monotectic and hypermonotectic particles is possible by using suitable cooling rates. The difference of the frequency maxima is a function, depending on cooling rate and starting composition. By casting experiments with variable cooling rates, we found that the average particle diameter is a function of the cooling rate, according to the theory of Ratke and Zhao, but the nucleation rate is higher than assumed in their theory.