In situ studies of precipitate formation in Al-Pb monotectic solidification by X-ray transmission microscopy

Al-1.5 wt pct Pb monotectic alloys were unidirectionally solidified. X-ray transmission microscope (XTM) observations, both during and after solidification, revealed various new morphological/compositional features in the melt and solid. In the melt, nonuniform lead-rich interfacial segregation layers and droplets were observed to form well ahead of the interface. In the solid, periodic striae formed at translation/solidification velocities as low as 6 X 10(-6) m/s. The striae shape does not replicate that of the interface. The striae spacing decreases from 4 to 2 X 10(-4) m with an increasing solidification rate between 6 and 16 X 10(-6) m/s. High resolution post-solidification XTM examination reveals that these striae consist of Pb-rich particles of 2 to 3 X 10(-6) m diameter. At translation/solidification velocities below 6 X 10(-6) m/s, Pb incorporation into the solid occurs in the form of continuous fibers and strings of particles of about 5 X 10(-6) m diameter. Bands, parallel to the interface, in which these fibers were aligned in the solidification direction, alternated with bands of poor fiber alignment. The width of these bands is comparable to the striae spacings obtained at the high solidification rates.