New Aluminum-lithium Alloy Precipitation Phase Precipitation And Related Mechanisms

The objective of this investigation were to study the effects of small additions of Mg,Zn alloying elements on the micro-structures and mechanical properties of Al-Cu-Li alloys aged in low temperature, middle temperature and high temperature. Various testing methods were used, including Micro Vickers hardness measuration, Transmission electron microscopy (TEM) observation, Computer Simulation. The main conclusions were as follows:(1) In Al-Cu-Li alloys, Mg and Zn, we find routine strengthen phase: T₁ phase andθ' phase including a mass of cubic phase at the T6 peak-aged condition (170℃), however it does not find a mass of cubic phase in other aging technology. For this unique phenomenon, we did not find relative report; From the chemical constitution, precipitatin feature and lattice parameter, it was different from the cubic phase which had been reported, therefore the cubic phase in this work should be a kind of new phase.(2) The microstructure was observed by TEM after T6 (solution treatment+artificial aging) and T8(solution treatment+cold-forming+artificial aging). The experiment results show that much precipitation phase as the alloy is aged at the T6 peak-aged condition (170℃) was cubic phase, but the existence of much cubic phase was not found at other conditions.(3) From the cubic precipitation rule, they have the competition precipitation character at aging kinetic between cubic phase and T₁ phase: pre-deformation before aging improve T₁ phase density in evidence, and it can precipitate a mass of distributing T₁ phase in equality, for pre-deformation bring high density dislocation, provide great heterogeneity nucleate site, accelerate T₁ phase dispersion precipitation, hence restrain the cubic precipitation; if in low temperature, main precipitated phase areδ' phase and G.P zone, G.P zone precipitated speed are very fast, so it restrain the cubic precipitation; if in high temperature, main precipitated phase are G.P zone and T₁ phase, T₁ phase have priority precipitated condition, so the cubic phase precipitation are restrained.(4) In Al-Cu-Li alloys, Li clusters are generated rapidly and Cu clusters are retarded due to strong interaction between Li atom and vacancy. The addition of Mg to Al-Cu-Li alloys is responsible for the accelerated formation and coarsening of Cu clusters and restrained formation and coarsening of Li clusters and maked strong tendency to form vacancy clusters; The addition of Zn to Al-Cu-Li alloys has no obvious effect on the evolution of Cu clusters and Li clusters from our simulationresults.(5) The addition of Zn and Mg to Al-Cu-Li alloys is responsible for the precipitationof small and disperse Cu cluster and has no effect on the formation of Li clusters.Moreover, Zn clusters, Mg clusters and Mg-Zn co-clusters are formed in Al-Cu-Lialloys due to the addition of Zn and Mg, which can be used to explained the formationof small and disperse Cu clusters.(6)From the computer simulation result during the initial aging time: Cu-Mg atomenrichment zone around Mg-Zn cluster become the perfect nucleate point for thecubic phase formation. Meanwhile much vacancy around the Mg cluster providecondition for solute atom celerity diffusion. Mg-Zn clusters can reduce the interfacialtension and shearing strain energy. A mass of Mg-Zn atom cluster may be the keyfactor for the cubic occurrence.