Microstructure morphologies during the transient solidification of hypomonotectic and monotectic Al–Pb alloys

Abstract

The solidification cooling rate ( T), growth rate ( v), temperature gradient ( G), interphase spacing ( λ) and diameter ( d) of the Pb-rich phase have been experimentally determined for a hypomonotectic Al–0.9 wt%Pb and a monotectic Al–1.2 wt%Pb alloys directionally solidified under unsteady-state heat flow conditions. It is shown for both cases that, from the cooled bottom of the casting up to a certain position along the casting length, the microstructure was characterized by well-dispersed Pb-rich droplets in the aluminum-rich matrix, followed by a region of morphological transition (with the Pb-rich phase formed by droplets and fibers) and finally by a mixture of fibers and strings of pearls for positions closer to the top of the casting. It has been also observed that such microstructural transition was anticipated for the alloy with higher solute content. It is shown that the correlation between the morphology of the Pb-rich phase and the growth rate can be synthesized as follows: Al–0.9 wt%Pb alloy, droplets for v > 1.0 mm/s and fibers for v < 0.65 mm/s; Al–1.2 wt%Pb alloy, droplets for v > 1.1 mm/s and fibers for v < 0.87 mm/s. Experimental growth laws relating the interphase spacing to both G and v are proposed.