A study of the early stage decomposition behaviour of Al-Li alloys at room temperature

Abstract

Our understanding of the early stage decomposition behaviour of Al - Li alloys with a lithium content around 3% or less is a subject of controversy. The primary concern is the origin of the metastable δ precipitate having an Li₂ ordered structure and the state of solid solution prior to its formation. Two theoretical predictions based on model calculations of free energies and phase diagram exist. One model, due to Sigli and Sanchez(1), predicts the existence of a low temperature miscibility gap for the disordered solid solution. The second model, due to Khachaturyan et al.(2), predicts a homogeneous ordering of the alloy of starting composition. In this model the ordered solid solution, having an order parameter less than the perfect value, separates into lithium rich and lithium lean regions with the onset of disorder in the latter region. The results of a limited number of investigations are available(3 - 11) which aim at resolving this issue. The present communication presents some additional experimental results and discusses the issue in the light of these results.