The effect of solidification conditions on the structure and properties of the Al-6wt% Li-15wt%Cu eutectic alloy containing T₂ phase

Abstract

The morphology of T₂ and of αAI-T₂ eutectic dendrites has been studied in two ingot samples of nominally eutectic composition (Al-6wt% Li–15wt% Cu) and the effect of growth velocity in Bridgman solidification on eutectic interlamellar spacing and hardness has been determined for one of the ingot compositions. The results show that the αAI-T₂ eutectic dendrites forming in these samples are characterized by the main stem and secondary arms growing along the five-fold rotational axes of the T₂ phase of the eutectic and that this growth mode is an example of the complex regular morphology exhibited by eutectics with entropy of solution exceeding 23 kJ mor⁻¹ and approximately equal volume fractions of the constituent phases. The eutectic spacing, λ, in Bridgman samples depends on the growth velocity, V, in the range 0.1 to 1 mm sec⁻¹ according to the relationship λV^1/2 = 19.6 ± 0.5 µm^3/2 sec^−1/2 while hardness, H, depends on λ according to a modified Hall-Petch relationship H= H₀ + Kλ⁻¹ with H₀ = 137 ± 6 kg mm⁻² and K = 0.048 ± 0.003 kg mm⁻¹.