Microstructural development in aluminum-lithium-copper-magnesium alloys and metal matrix composites

Two stage hardening was observed in both materials and somewhat unexpectedly, the aging response of the composite was unchanged as compared to the unreinforced 8090 alloy. Microstructural development during the aging of both materials was studied using electron microscopy and differential scanning calorimetry (DSC). Transmission electron microscopy examination indicated that the {dollar}deltaspprime{dollar} phase (Al{dollar}sb3{dollar}Li) and the S{dollar}spprime{dollar} phase (Al{dollar}sb2{dollar}CuMg) were the two principal strengthening phases in both materials. They formed at different stages during aging and caused the two stage strengthening.; The dislocation distribution in the composite was compared with that in the unreinforced 8090 alloy. The addition of SiC to the 8090 alloy caused a highly non-uniform dislocation distribution in the composite, which is directly responsible for the unchanged aging response in the composite as indicated by microhardness tests.; The T{dollar}sb2{dollar} ({dollar}rm Alsb6CuLisb3{dollar}) phase was found at both grain boundaries and SiC/matrix interfaces and caused {dollar}deltaspprime{dollar} precipitate free zones along these interfaces. The distribution of the S{dollar}spprime{dollar} phase was less affected by the precipitation at grain boundaries and the SiC/matrix interfaces.; The precipitation behavior of the S{dollar}spprime{dollar} phase was studied in detail. According to the morphology and the preferential nucleation sites, the S{dollar}spprime{dollar} precipitates in Al-Li-Cu-Mg alloys have been unambiguously classified as type A, B and C. Type A precipitates nucleate at dislocations and exhibit a "sheet" morphology, type B likely nucleate at vacancy clusters and exhibit the lath morphology, type C nucleate at {dollar}deltaspprime{dollar}/matrix interfaces at the latter aging stage and are distributed uniformly within the matrix as fine laths. The study on the atomic matching across S{dollar}spprime/alpha{dollar} interfaces indicated that the lateral S{dollar}spprime/alpha{dollar} interfaces are coherent or semicoherent.; Dislocation helices and loops were observed in the 8090 alloy which had been naturally aged for {dollar}sim{dollar}6 months. Unlike the dislocation loops observed in other aluminum alloys, which lie on {dollar}{lcub} 110{rcub} {dollar} plane, the loops in the present study lie predominantly on {dollar}{lcub} 210{rcub} {dollar} planes. It has been suggested that prismatic loop glide driven by the precipitation of S{dollar}spprime{dollar} precipitates nucleated at loops is the cause of these {dollar}{lcub} 210 {rcub} {dollar} loops.