ELECTRONIC STRUCTURE OF DIFFERENT REGIONS AND ANALYSIS OF STRESS CORROSION MECHANISM OF Al-Zn-Mg-Cu ALLOYS
Al-Zn-Mg-Cu系铝合金中不同区域电子结构及应力腐蚀机理分析

The atomic cluster models of α--Al, η--phase and large angle grain boundary of α--Al in Al--Zn--Mg--Cu alloys have been constructed by computer program. The environment--sensitive embedding energies of Zn, Mg, Cu and H atoms, interaction energies, Fermi energies and densities of state have been calculated by recursion method. The stress corrosion cracking behavior of Al--Zn--Mg--Cu alloys has been analyzed according to the calculated electronic parameters. The results show that Mg, Zn and H atoms are easy to segregate on grain boundaries. Mg promotes the segregation of H on grain boundary, which leads to the embrittlement of grain boundary because of the attraction of Mg to H. Zn increases the difference of electrode potential between boundary and grain, which deteriorates the stress corrosion resistance of Al--Zn--Mg--Cu alloys. Cu reduces the difference of Fermi energies between grain and grain boundary, and lowers the electrode potential difference between grain and grain boundary, which helps to slow up the corrosion process. The calculated results also indicate that the Fermi energy of η--phase is the highest, so η--phase will decompose firstly in the corrosion process as anode. Discontinuous distribution of η--phase along grain boundary can weaken the segregation of H on the grain boundary because of the capture of η--phase to H, and improve the stress corrosion resistance of Al--Zn--Mg--Cu alloys, while the corrosion channel can form and speed up the corrosion process when η--phase distributes continuously on the grain boundary.