The microstructural development in aluminum-lithium - copper-zirconium alloys

The aging characteristics of an Al-2.1% Li-2.9% Cu-0.12% Zr alloy at 190{dollar}spcirc{dollar}C have been examined using transmission electron microscopy. The microstructures that develop, the nature of the phases present and the interactions between these phases have been documented. Additionally, the intergranular microstructures of an Al-1.1% Li-4.6% Cu-0.12% Zr alloy aged at the same temperature have been determined.; The predominant intragranular precipitate species in the 2.1% Li-2.9% Cu alloy were {dollar}delta{dollar}(Al{dollar}sb3{dollar}Li), {dollar}thetaspprime{dollar}(Al{dollar}sb2{dollar}Cu) and T{dollar}sb1{dollar}(Al{dollar}sb2{dollar}CuLi). The {dollar}deltaspprime{dollar} phase was found to nucleate either during the quench or very rapidly at room temperature. During the early stages of aging {dollar}deltaprime{dollar} was present, not only as small spherical particles, but also as a shell around {dollar}betaspprime{dollar}(Al{dollar}sb3{dollar}Zr) and as a film on the coherent faces of the {dollar}thetaspprime{dollar} precipitates. The dominant nucleation sites for the T{dollar}sb1{dollar} and {dollar}thetaspprime{dollar} phases were matrix and subboundary dislocations. In addition, {dollar}betaspprime{dollar} was found to be an active nucleation substrate for these phases. Aging to the peakaged condition resulted in a decrease in the volume fraction of spherical {dollar}deltaspprime{dollar} and an increase in the T{dollar}sb1{dollar} and {dollar}thetaspprime{dollar} volume fractions. At peakage, {dollar}deltaspprime{dollar} was associated mainly with {dollar}thetaspprime{dollar}. Overaging led to the dissolution of {dollar}thetaspprime{dollar} and a corresponding increase in the volume fraction of T{dollar}sb1{dollar}. The T{dollar}sb2{dollar} phase was found to precipitate within the matrix in the overaged condition. Diffraction analysis shows that this phase displays icosahedral symmetry.; The primary subgrain boundary precipitates in the 2.1% Li-2.9% Cu and 1.1% Li-4.6% Cu alloys were T{dollar}sb1{dollar} and {dollar}thetaprime{dollar}. Low angle grain boundary precipitation in the 2.1% Li-2.9% Cu alloy was also dominated by T{dollar}sb1{dollar} and {dollar}thetaprime{dollar} while T{dollar}sb{lcub}rm B{rcub}{dollar} was found most often at this boundary type in the 1.1% Li-4.6% Cu alloy. T{dollar}sb2{dollar} precipitation at the high angle grain boundaries in the 2.1% Li-2.9% Cu alloy was extensive and led to the development of {dollar}deltaprime{dollar} PFZs. T{dollar}sb{lcub}rm B{rcub}{dollar} and T{dollar}sb1{dollar} formed at the high angle boundaries in the 1.1% Li-4.6% Cu alloy.; Finally, the existence of the T{dollar}sb1prime{dollar} phase has been discounted on the basis of electron diffraction studies and symmetry arguments.