Study On The Structural And Energy Properties Of Vacancy Defects In Aluminum

Aluminum(Al)alloy,as the most widely used non-ferrous metal in industrial production,has outstanding advantages such as light weight,high strength,good formability and corrosion resistance.It has a broad application prospect in aerospace,petrochemical,transportation,national defense and other fields.However,when aluminum alloy is affected by environmental factors such as high temperature quenching,plastic deformation and irradiation,various vacancy defects will inevitably appear in the material,such as voids,stacking fault tetrahedra(SFT)and vacancy-type dislocation loop(VDL).The existence of these vacancy clusters will cause a lot of harm to the mechanical properties of materials,such as hardening,strengthening,embrittlement and plastic instability.One of the keys to understanding the adverse effects of these vacancy defects on the mechanical properties of materials is to accurately characterize their formation and evolution,in which their structure and energy are the most basic property parameters.However,there are relatively few researches on the structure and energy properties of vacancy defects in aluminum,and it is difficult to provide theoretical support for defects in actual production.Therefore,the structure and energy properties of vacancy defects(voids,SFT,VDL)in Al are studied in detail by using first-principles and molecular statics/dynamics.The main research contents and conclusions of this paper are listed as follows:(1)The reasons for the difference in energy properties between single vacancy and double vacancy in Al reported in the literature are studied by the first-principles,and the stability of small vacancy clusters with fewer than 8 vacancies is studied.The results show that the exchange correlation functional is the reason for the large difference of mono vacancy formation energy in previous literatures.Compared with the experimental results,LDA predicts the best mono vacancy formation energy,followed by PBE,PBEsol,PW91 and AM05.For the divacancy,the first-principles predict that it is energy unstable,which is contrary to the experimental results,because of the unreasonable interpretation of the non-Arrheniustemperature-dependent relationship of vacancy concentration in the experiment.For small vacancy clusters in Al,SFT is the most stable configuration,followed by voids and vacancy plates on the {111} plane,while the vacancy plates on the {111} surface are always energy unstable.(2)The first-principles and molecular statics systems are used to study the structure and energy properties of void-type vacancy clusters,and the correlation law between structure and energy is revealed.Based on this,a program software to accurately determine the stable structure and energy of void-type vacancy clusters is developed.The results show that the vacancy clusters with the largest total first nearest coordination number are usually the most stable structures for void-type vacancy clusters with a specific size,and their formation energies has a perfect positive linear relationship with their Wigner-Seitz area.As for the energetic behavior of an additional vacancy on the vacancy clusters surface,we find that it is mainly influenced by the first nearest neighbor coordination environment.Therefore,the binding energy of any void-type vacancy cluster can be predicted by knowing the Wigner-Seitz area difference of Vn cluster.In addition,the void structure predicted by the model is in agreement with the experimental results.(3)The effect of vacancy plate shape on the construction of SFT and VDL structures was investigated by first-principles,molecular statics and dynamics,as well as further studies around the structure and energy of SFT and VDL.The results show that the initial shape of vacancy plate,namely the initial shape of Frank loop,plays an important role in the transformation mechanism of various defect nanostructures.The triangular vacancy plates can form SFT by Silcox-Hirsch mechanism,while the hexagonal vacancy plates can form VDL by further nucleation and growth.For small vacancy clusters,SFT has the highest stability,and as the size of vacancy clusters increases,the stability of voids and Faulted loop will be higher than that of SFT.In addition,the relative stability results of the three vacancy clusters predicted by the model were found to be consistent with the experimental observations.