Molecular Dynamics Simulation Study Of Special Grain Boundaries Of Aluminum And Aluminum-Copper Alloys With Low Mille Index Rotation Axis

Since the formation of coherent twin boundary is inhibited by the high stacking fault energy of the material,the corrosion resistance of aluminum alloys cannot be improved by traditional grain boundary engineering.Recently,a new concept of grain boundary engineering has been proposed by our research group,which considers that the corrosion resistance of the material has priority on the grain boundary inter-connection（GBIC）over theΣvalue.And found that there is a class of near-singular boundaries（{1 1 1}/{1 1 1}）in aluminum alloy materials with high stacking fault energy.However,it is still not clearly understood about the thermal stability and atomic structure distribution characteristics of near-singular grain boundaries,which has greatly hindered the research process of using the new grain boundary engineering to improve the corrosion resistance of aluminum alloy.In this paper,grain boundary models of pure aluminum with different grain boundary surfaces were constructed,and the thermodynamic state of the models was calculated by molecular dynamics method.The influencing factors of grain boundary energy are analyzed and discussed,and the preliminary criterion of near-singular grain boundary is given.The main results are as follows:（1）A number of grain boundary models,including twisted grain boundary with low Miller index（as{1 1 1},{1 1 0}and{1 0 0}）and symmetric tilting grain boundary with lowΣvalue,were constructed.The thermodynamic state of these models was simulated after full relaxation at300K and 600K.The influences of layer faults and defects on the grain boundary energy were analyzed and discussed.The results show that the grain boundary energy is increased by the stacking fault energy and does not change with the torsion angle.The grain boundary energy is increased by point defects,and the influence of interstitial atoms is greater than that of vacancy atoms.The value of the structure factorτ,which is positively correlated with the grain boundary energy,is then proposed and can be used to estimate the thermodynamic properties of grain boundaries.The grain boundary energy range of 178-640 m J·m^-2and the structure factorτ<0.05 are required for the near-singular boundary.The main types of near-singular boundary matching are:<1 1 1>{1 1 1}/{1 1 1},<0 0 1>{0 0 1}/{0 0 1}（misorientation<25°）,Σ11-{11 3}/{1 1 3},Σ15-{1 2 5}/{1-2 5},Σ17-{0 1 4}/{0 1 4},Σ19-{3 3 1}/{3 3 1}。（2）It is found that a low energy grain boundary has the ability to inhibit impurity precipitation by comparing the response of different temperatures to grain boundary energy.Furthermore,the effects of different grain boundary types on the kinetics and thermodynamics of Cu coherent precipitation（as monolayer GP zone）have been studied using Al-Cu alloy as an example.The results show that the thermal stability of the system is improved due to the reduction of the free energy with the addition of the Cu layer.The Cu layer is not affected by the position,and it is not easy to spontaneously form the GP zone during the coherent twin boundary model.It is easy to spontaneously form GP zones but not aggregation on the near singular boundary model.It is liable to precipitate and agglomerate at general grain boundaries for Cu and Al₂Cu phase is formed based on the diffusion phase transformation mechanism.（3）In order to further understand the influence of different grain boundary types on the mechanical properties of the Al-Cu alloy,the fracture behavior of different grain boundary models was observed during the simulated tensile process.The results show that the fracture mode of the coherent twin boundary model and the near singular grain boundary model is ductile fracture,while the general grain boundary model is brittle fracture.The ductile fracture was found in both the coherent twin boundary model and the near singular boundaries,while the brittle fracture behaviour was observed in the general grain boundary model.In summary,there are many types of near-singular boundary that can inhibit impurity precipitation both thermodynamically and kinetically.And the mechanical properties of the near-singular boundary are similar to those of the coherent twin boundary.Therefore,it is feasible to control the impurity phase precipitation by adjusting the proportion of near-singular grain boundary,which provides a new idea and theoretical basis for improving the comprehensive properties of aluminum alloy.