Research On Microstructural Transformation Of Al Alloys In The Early Stage Of Aging Based On Phase Field Crystal Model

With the continuous advancement of global environmental protection and sustainable development,energy conservation and emission reduction has become one of the development directions of various industries around the world.In this context,lightweight aluminum alloy has become an important material widely used in various fields such as production construction,furniture and building materials,automobile manufacturing,and portable electronic equipment due to its light weight,high strength,and good corrosion resistance.An in-depth understanding and quantitative analysis of the microstructural transformation mechanism of aluminum alloys in the early aging stage is conducive to the design of specific aluminum alloy materials that meet various application scenarios.A large amount of early experimental data and the current supercomputing platform for efficient computing provide a sufficient foundation for the numerical calculation of aluminum alloys.This work builds a numerical calculation model of aluminum alloy microstructure based on the phase field crystal method,and simulates the structural transformation process of the alloy material under stress and the segregation,nucleation,and coarsening of solute.Besides,the influence mechanism of the lattice misfit on the solute segregation process is discussed.To solve the problem of observing the positions of specific atoms,the atomic-scale segregation mechanism of solute atoms was studied by molecular dynamics and Monte Carlo simulation.The main innovative achievements are as follows:(1)Analyzed the microstructural evolution of defect structures in metallic materials such as grain boundaries and dislocations during stress aging,as well as the selection of nucleation points for structural transformation,and found that stress can promote grain boundary slip and dislocation movement,thereby affecting Plastic behavior and mechanical properties of materials.(2)It is the first to reveal the microstructure evolution mechanism of three stages in the process of grain boundary segregation of Al alloy,solute enrichment stage: this stage can be divided into two stages according to the direction of solute migration.Early stage,the solute migrates from the grain to the grain boundary to reduce the grain boundary energy.When the value of the grain boundary concentration is higher than a certain critical value,there are obvious periodic concentration fluctuations on the grain boundary,and the region with a higher concentration enters the second stage,the nucleation stage,when the solute concentration is higher than the critical nucleation concentration.A more obvious structural transformation can be observed in the nucleation stage,which is mainly reflected in the transformation of the ordered structure into a disordered precursor structure,and then into a stable second phase structure.And enter the third stage,the grain boundary segregation coarsening stage,in this stage the crystal nuclei will absorb the surrounding solute and grow rapidly until the surrounding solute is exhausted.(3)The microcosmic control mechanism of the lattice mismatch degree on the aging precipitation process is explained systematically.It is found that a higher lattice mismatch can increase the migration rate of solute in the solute enrichment stage and reduce the critical nucleation concentration.For alloys with a high degree of misfit,the nucleation and coarsening of the second phase appear at the grain boundaries of the entire layer almost at the same time,showing a layered growth mode.However,the grain boundary with a low lattice mismatch degree needs to further increase the local concentration through the amplitude modulation decomposition inside the grain boundary after the grain boundary segregation,showing an island growth mode.In addition,higher grain boundary angles promote layered growth,while lower grain boundary angles promote island growth.(4)For the first time,combined molecular dynamics and Monte Carlo hybrid simulation to study the segregation mechanism at the atomic scale,which solved the deficiency of the analysis of the local structure by the crystal phase field.It was found that the segregation of solute Cu atoms in the grain boundary segregation of Al-Cu alloy can be divided into two modes,displacement segregation and vacancy segregation.For grain boundaries with higher grain boundary density such as(111)grain boundaries,Cu atoms segregate mainly through replacement,while for grain boundaries with lower grain boundary density such as(331)grain boundaries,Cu atoms mainly segregate through vacancies.The proportion of these two segregation mechanisms in the grain boundary segregation is closely related to the grain boundary structure and segregation degree.(5)Groundbreakingly combined the crystal phase field method and molecular dynamics method to derive a new composite crystal phase field model for simulating microstructure evolution.Based on this,the processes of grain boundary segregation,amplitude modulation decomposition,and nucleation structure transformation in the early aging stage were simulated.Compared with the structural crystal phase field method,it can directly combine the mechanical parameters and lattice structure of specific materials,and can calculate the position of solute atoms,and better characterize complex alloy structural units.Compared with molecular dynamics,the calculation efficiency is higher,and it does not completely depend on the role of the potential function.Microstructural transitions can be simulated over wider temporal and spatial scales.