The Corrosion Mechanism Of Intermediate Phase In Aluminum Alloy:First-principles Calculations

Aluminum and its alloys are widely used in various fields due to its excellent properties.Its production is next to that of steel,ranking first in non-ferrous metals.Similar to steel,Al alloys also suffer from the corrosion.Much attention has been paid on the corrosion resistance of Al alloys.As we all know,Al alloys inevitably precipitates some secondary phases during the casting and heat treatment process,such as?phase,T1 phase and S phase,et al.Based on electrochemical analysis,it can be found that the electronic work function difference between the second phases and the Al matrix in alloys is the main driving force for the corrosion of Al alloys.S and T1 phase are common precipitation phases in Al-Cu alloys.The corrosion behaviour of S and T1 phase has a crucial effect on the corrosion resistance of Al-Cu alloys.This work investigates the corrosion mechanism of S and T1 phase on the atomic scale.First-principle calculation method based on density functional theory is applied to calculate the surface energy and work function of S and T1 phase,and discuss the effect of the stress and electrochemical environment?H₂O and Cl^-?on T1 phase.The following conclusions were dawn.1.The surface energy,surface electronic work function,and interface substitution energy of S phase were calculated.The different surface terminations have surface properties,which depend on the surface atomic density and the elemental composition of terminations.High Mg content in surface means low work function and weak corrosion resistance.The substitution energy results show that alloying atoms prefer to segregation at the interface.Due to small atomic radius and high electronegativity,Fe and Si significantly promote the precipitation of S phase.2.The surface energy,surface electronic work function,and interface substitution energy of T1 phase were calculated.The work function of T1 phase can be attributed to the Li content in surface,because Li has minimal electronegativity and is easy to lose electrons.As a result,the surface terminations containing Li atoms are susceptible to corrosion.The effect of alloying elements on the precipitation of T1phase is studied.Al?111?/T1?010?interface is built and the substitution energy of Mg,Zn and Ag are calculated.Comparing with Mg and Zn atom,the energy of Ag atom to substitute the interfacial one is low,meaning that Ag can relax the strain in the interface.Ag atom has the closest atomic radius to Al atom,and the same chemical valence as Li atom.Therefore,Ag atom is more likely to promote the precipitation of T1 phase.3.Different from pure metal,work function of some T1 surfaces shows unique behavior under stress state.The?010?T1 surface with Al and Cu termination has an increasing work function under the action of tensile strain.In fact,tensile strain induces the first and second surface layer to merge,which can improve the surface electronic density and raise work function.As a result,the corrosion resistance can be enhanced.In addition,T1/H₂O and T1/Cl^-interfaces were constructed to investigate the corrosion mechanism in different environment.The result suggests that Li and Mg prefer to bond with Cl or O atoms and precipitate in solution.Consequently,the work function of T1 phase surface is improved and corrosion resistance is enhanced after precipitation.