Computer Simulation Of The Al-li Alloy Aging And Regression Mechanism

In this paper, the atomic-scale computer simulations on retrogression and ageing-reageing of Al-Li alloys were firstly worked out. The computer simulation programs are based on the microscopic diffusion equation with the discrete format and nonequilibrium free energy, which can be applied to all precipitation process, including atomic clustering, ordering, growth and coarsening, and the counter process such as disordering and dissolution during retrogression, and the compositional regions from metastable to instable. It comes from nonlinear equation, and can be applied to the system with higher compositional gradation. Furthermore, Any prior assumption on the new phase structure or the mechanisms of precipitation and retrogression was unnecessary, the emergence and dissipation of possible nonequilibrium phases could be described automatically without any database to support, and atomic pictures, order parameters, volume fractions ,structure function and correlation function of precipitates could be further obtained.For different volume fractions, structure function and correlation function were both scaled at late stage of phase precipitation, but the peak and shape of scaling structure function profiles were different, the maximum value of which increased with the volume fractions. When skipping to instable regions, the variations of structure function profiles with time were the same: the peak of structure function was decreased and the location of the peak of the structure function shifted towards smaller reduced wave vectors.In the course of retrogression, the evolutions of ' phase were in the order: stoicheometric ' phase-nonstoicheometric ordered phase-disordered matrix, including the diffusion of solute atoms and the disordering of ordered phases. The solute-rich regions were formed around the ordered phases as the phases were dissolved during the retrogression, and in the end, the ordered phases would be replaced by solute-rich regions completely. But the process of solute-rich regions turning into disordered matrix was very slow, and it was hard to come back to the original states during retrogression.The mechanisms of phase precipitation were very different at different compositional regions from metastable to instable, while the mechanisms of retrogression were consistent: the composition drop among a small range. The rangeof composition drop was smaller with the increase of volume fractions. In metastable region, the precipitation was "composition fluctuation among a small range", and the retrogression tended to be the counter process of the precipitation. In instable region, the precipitation mechanism was "composition fluctuation among a large range", and the retrogression could not be simply regarded as the counter process of the precipitation. In transitional region, with the increase of volume fractions the retrogression characteristic transformed from that of metastable to instable region gradually.In the course of retrogression, the disordering of ordered phase and atomic diffusion was very fast at early stage, and then turned to be slow. For a single ordered phase, the maximum value of long range order parameter had a sharp drop, while that of the composition parameter had a relaxation , even a sharp increase in higher composition region at early stage of retrogression.