Morphology, structure and thermal behaviour of small eutectic Pb-Cd inclusions in aluminium

Small inclusions of eutectic Pb-Cd embedded in a solid Al matrix have been produced by melt spinning and investigated by transmission electron microscopy. Inclusions were found to be phase separated into two parts, one fee and Pb rich, and the other hcp and Cd rich. The two phases maintained an orientation relationship of parallel close-packed planes and directions with each other and with the fee Al matrix. The inclusion shape was anisotropic and each segment displayed facets typical for pure Pb and Cd inclusions found in binary Al-Pb and Al-Cd alloys. Within the observed size range 5-100nm, particles exhibited a single Pb-Cd interface, invariably parallel to the shared close-packed plane. The bimetal inclusions were found to melt at the eutectic bulk melting point regardless of size. Melting was initiated at the triple interface junction of Cd, Pb and Al. By comparison, solidification during cooling required an undercooling of about 35 degrees C and took place very rapidly in less than 1/30s. The volume change during solidification led to an elastic distortion of the surrounding matrix which subsequently decayed over a period of 20 s.