Microstructural Morphologies and Experimental Growth Laws during Solidification of Monotectic and Hypermonotectic Al-Pb Alloys

Al-Pb alloys with monotectic and hypermonotectic compositions were directionally solidified under unsteady-state heat flow conditions. The cooling curves recorded during solidification allowed solidification thermal parameters such as the cooling rate (T˙), growth rate (v) and thermal gradient (G) to be experimentally determined. Different microstructural patterns have been associated with the alloy solute content, i.e., Al-1.2 and 2.1 wt% Pb. A sequence of morphologies from the bottom to the top of the Al-1.2 wt% Pb alloy casting (monotectic) can be observed: Pb-rich droplets in the aluminum-rich matrix, followed by a region of microstructural transition formed by droplets and fibers and finally by a mixture of fibers and strings of pearls. A completely fibrous structure (without transition) has been observed along the entire Al-2.1 wt% Pb alloy casting (hypermonotectic). The interphase spacing (λ) was measured along the casting length, and experimental correlations between λ and experimental solidification thermal parameters have been established. Power laws with a −2.2 exponent expressing λ as a function of the growth rate, v, were found to better represent the fibrous growth of both Al-Pb alloys. Moreover, a single experimental law expressing λ as a function of both G and v was found to describe the fibrous growth of both the monotectic and the hypermonotectic alloys experimentally examined.