The Mechanism Of Atomic Scale Displacive Phase Transition In Aluminum Alloys

Aluminum alloy is widely used in aerospace,transportation and other fields because of its excellent properties such as low density and high specific strength.The fine and dispersed particles precipitated in the aging process can hinder dislocation slip and strengthen the aluminum alloy.Therefore,it is very important to understand the phase transition mechanism of precipitated phase for the design of high strength aluminum alloy.Phase transition of macroscopic system can be described by the landau mean field theory,if the system is only nanometer level or even reaches the atomic scale,then nano thermodynamics or small system can be used to study.However,when the system is atomic scale particles and is surrounded by a matrix,such as precipitates common in aluminum alloy,the existing classical theories has lost its statistical significance in such a system,so need to have a new approach to study.The development of density functional theory and first-principles calculation methods has laid a foundation for solving this kind of problem.In this paper,the first principles method is used to study the mechanism of displacive phase transition at atomic scale from the following two aspects.Firstly,by summing up the common phase transition rule of GPB zones and β’’ phase in Al-Cu-Mg and Al-Mg-Si series alloys,it is found an obvious characteristic that aluminum atoms jump from the lattice point of face centered cube to the central position of octahedron during the phase transition process.We have proposed a new displacement phase transformation model,which successfully explains the evolution of early precipitates in Al-Mg-Si and Al-Cu-Mg series alloys,and reveals the microscopic mechanism of displacive phase transition at the atomic scale.The simulation results show that the transition energy barrier of each atom in pure aluminum is as high as 2.4 eV,but the thermal activation energy of the atom is only 0.043 eV even at 500 K aging temperature,and the displacive phase transition cannot be generated over the barrier.However,some solute atoms,such as Cu,Mg and Si,can effectively reduce the energy barrier of displacive phase transition.The interaction of two solute atoms can even make the energy barrier disappear,and lead to the change of the potential energy surface of the atom.The anharmonic effect leads to the occurrence of displacive phase transition.The model is then applied to predict the series of aluminum alloys that may undergo displacement phase transition.Based on the radius,work function and the interaction potential between atoms,the solute atom combinations which are favorable to the displacive phase transition are selected preliminaries.Furthermore,the ternary aluminum alloy which may have the displacement phase transformation is predicted by the CI-NEB method for the alloy composed of these solute atoms.The mechanism of displacive phase transition and the database presented in this paper enrich the atomic scale phase transformation theory and are helpful for the design of new aluminum alloys with early aging strengthening.