Effects of lithium addition and wheel velocity on the microstructure of aluminium-lithium alloy ribbons

Abstract

The effects of Li addition and wheel velocity on the microstructure of as-cast AI-Li ribbons were studied by optical metallography, scanning and transmission electron microscopy and by X-ray diffraction. Li addition has a marked effect on the ribbon solidification mechanism. Ribbons spun at 12.5 ms−1 and which contained up to 1.79 wt% Li experienced a cooling rate of about 104 Ks−1 and solidified under the melt jet with a growth rate greater than 0.3 m s−1. However, when Li content exceeded about 3 wt%, the ribbons were undercooled (cooling rate ≈ 106 Ks−1) and formed far from the melt puddle effect. Under these conditions, unusual secondary featureless zones occurred through the ribbon thickness and the detected phases corresponded to those expected to be formed under equilibrium conditions. Nevertheless, δ′ (AI3Li) 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 35.7 ms−1 did not affect the ribbon formation mechanism, but it favoured the δ′ precipitation and decreased slightly the grain size.