Effects of order-disorder transition on phase relationship, elastic strength, and mechanical anisotropy of Al-Li alloys

Abstract

In this work, the effects of order-disorder transition on phase relationship, elastic strength, and mechanical anisotropy of Al-Li system are investigated by first-principles calculations with the help of cluster expansion, Monte Carlo simulation and quasi-harmonic Debye model. The results show that at higher temperature disordered FCC is energetical favorable at xLi < 0.59, while BCC becomes stable at xLi ≥ 0.59. The order-disorder transition at low temperature, deduced from metastable phase diagram at Al-rich side (0.00 ≤ xLi ≤ 0.54), promotes the transformation of disordered α FCC to ordered δ’ L12 Al3Li and δ B32 AlLi structures. The elastic strength indicates the precipitate hardening of Li addition originates from the α – δ’ ordering, while softening is found after the α – δ ordering or δ’ – δ phase transition. The change of FCC, either ordered or disordered, to BCC structures incurs strong mechanical anisotropy, as a result of centrosymmetry breaking and angular electronic bonding. The newly identified long-range ordering of AlLi4 tetrahedron pairs and corresponding bonding electron morphology play the key role in the order-disorder transition and mechanical anisotropy. Our findings not only provide clues for the rational design of Al-Li alloys with higher Li concentration, but also shed lights on the complex experimental phenomena of Al-Li alloys.