EFFECTS OF LITHIUM ADDITION AND WHEEL VELOCITY ON THE MICROSTRUCTURE OF ALUMINUM-LITHIUM ALLOY RIBBONS

The effects of Li addition and wheel velocity on the microstructure of as-cast Al-Li ribbons were studied by optical metallography, scanning and transmission electron microscopy and by X-ray diffraction. Li add ition has a marked effect on the ribbon solidification mechanism. Ribb ons spun at 12.5 m s(-1) and which contained up to 1.79 wt% Li experie nced a cooling rate of about 10(4) K s(-1) and solidified under the me lt jet with a growth rate greater than 0.3 m s(-1). However, when Li c ontent exceeded about 3 wt%, the ribbons were undercooled (cooling rat e approximate to 10(6) K s(-1)) and formed far from the melt puddle ef fect. Under these conditions, unusual secondary featureless zones occu rred through the ribbon thickness and the detected phases corresponded to those expected to be formed under equilibrium conditions. Neverthe less, delta' (Al3Li) phase did not occur in ribbons containing up to 1 .79 wt% Li or in the featureless zones of the most Li-rich ribbons (Li > about 3 wt%). Changing of the wheel velocity from 12.5 to 22.2 or 3 5.7 m s(-1) did not affect the ribbon formation mechanism, but it favo ured the delta' precipitation and decreased slightly the grain size.