First-principles Calculations Of Precipitation Phase In Al-Cu-Mg Alloys

The Al-Cu-Mg alloy is a kind of deformation aluminum alloys with high strength and low specific gravity. Due to its high strength and modulus ratio, good corrosion resistance and excellent high temperature characteristics, the Al-Cu-Mg alloy has been a new type of rigid structural materials. The structure and properities of the microscopic mechanism for the Al-Cu-Mg alloy is still at an exploratory stage. In this thesis, VASP and CASTEP softwares basing on the First Principle were used to calculate the nucleation of the S phase and the thermal stability and elastic modulus of theθandθ'phase in Al-Cu-Mg alloy.In this thesis, I studied the formation of S-phase precipitates in Al-Cu-Mg alloy by caculating the formation enthalpy of solid solution. The results showed that pre-S phase structure could not be formed by directly replacing Al atoms on (021) plane with Cu and Mg atoms in Al matrix and then structural relaxing, but the distribution of metastable Cu and Mg atoms may be directly relaxed into structures similar to S phase by inducement of dislocations. In the Al matrix, the special structure formed by two Cu-Mg atom layers sandwiching two Al layers on (021) plane showed high stability. Theoretical calculations showed that the nucleation of S-phase could be formed by direct replacing Al atoms on (021) plane of fcc structure with Cu and Mg atoms, it is consistent with the results observed by TEM.In the next part of this thesis, I calculated the phonon spectrum and density of phonon states ofθ'-Silcock,θ'-Preston andθphase in Al-Cu-Mg alloy by the linear response method. In this way I got their free energy at different temperatures. Our calculation results showed that theθ-phase formation enthalpy plus zero-point energy is higher thanθ'-Silcock andθ'-Preston phase at the temperature of 0K. When the temperature is higher than about 450K, the free energy ofθ-phase is even lower than that ofθ'-Preston phase, and when the temperature is higher than about 500K, the free energy ofθ-phase is even lower than that ofθ'-Silcock phase which can well explain the generally accepted sequence ofθ-phase precipitation. And all of results agree with the a large number of experiments which shown that that the final stable phase isθ' when the Al-Cu-Mg alloy is aged at the temperature lower than 200℃, Equilibriumθphase only exists over a long time aging above this temperature. In this part, we also studied the mechanical properties ofθandθ'phase by calculating their elastic modulus.