Evaluation of properties and microstructure of non-heat treatable Al–Mg–Li–C–O alloys with variable Li concentration

Abstract

Fine grained Al–Mg–Li–C alloys, with lithium concentrations from 0.7 to 1.5 wt-%, have been produced by a mechanical alloying–powder metallurgy route. An initial range of compositions was chosen for manufacture into 10 kg billets which were uniaxially forged into plate; subsequently two compositions, alloy A (Al–3.7Mg–0.7Li–1.0C (wt-%) and alloy B (Al–4.4Mg–1.4Li–1.0C), were down-selected for a 20 kg scale-up exercise. Billets were forged at 300°C, using an 8:1 reduction ratio, which provided a sufficient level of work to develop properties, while avoiding excessive grain growth. Alloy B exhibited tensile properties (0.2% proof stress 450 MPa; ultimate tensile strength 510 MPa; strain to failure 6%) that exceeded the AECMA specification for AA 5091. Both alloys were confirmed as non-heat treatable and therefore exploitable in the as forged T1 condition. Microstructural analysis has confirmed that a fine grain size (<1 µm) and nanoscale Al₂O₃/Al₄C₃ and MgO dispersoids provided significant Hall–Petch and Orowan strengthening, respectively, capable of increasing the 0.2% proof stress to 450 MPa. Although optimisation of thermomechanical practice is still required, these Al–Mg–Li–C alloys show considerable potential for aerospace, land, and space applications.