Physical metallurgy of aluminum-lithium alloys

Abstract

The addition of lithium to aluminium reduces the density and increases the elastic modulus; precipitation of the metastable δ'(Al₃Li) phase from supersaturated Al-Li solid solution leads to appreciable increase in strength. The enhanced values for specific modulus and specific strength favour the use of the Al-Li alloys as structural materials for aerospace applications. However the binary alloys suffer from problems of poor ductility and toughness associated with strain localisation (resulting from the ease with which δ' particles are sheared during deformation), the presence of δ'-free zones near grain boundaries and the heterogeneous nucleation of the equilibrium δ phase on the grain boundaries. These problems have been overcome by the development of ternary and quaternary alloys containing copper and magnesium. A small amount (~0.1%) of zirconium is added to these alloys to improve the recrystallisation characteristics. The properties of alloys developed for commercial exploitation are briefly discussed. An overview of the physical metallurgy of the Al-Li alloys is presented with emphasis on the following features: (i) phase equilibria and precipitation reactions in Al-Li, Al-Cu-Mg, Al-Cu-Li and Al-Mg-Li systems and extension of these results to Al-Li-Cu-Mg alloys, (ii) insoluble particles and their effect on precipitation in the alloys, (iii) microstructural studies on Al-2.3%Li-1.2%Cu-0.7%Mg-0.12%Zr alloy, (iv) lithium depletion during solution treatment, (v) coarsening of δ' particles and development of precipitate-free zones near grain boundaries and (vi) microanalysis of the lithium containing phases.