Stability Design Of Grain Boundaries And Precipitates In Al-Zn-Mg-Cu Alloys

Al-Zn-Mg-Cu alloys can hardly maintain excellent high-strength properties in long-term thermal environment due to the poor thermal stability of microstructure.In order to broaden the heat treatment temperature and application range of 7XXX alloy,it is necessary to further improve the microstructure stability.In this paper,the stabilization design of grain boundary（GB）and main precipitated phases（’phase,phase and L1₂phase）is carried out by first-principles calculation.The interfacial segregation and bulk substitution behaviors of elements are used to regulate and predict the microstructure stability from the perspective of thermodynamics,and the appropriate microstructure stabilizing elements were screened out,which provided a theoretical basis for the optimization of alloy composition.Then combined with the alloy preparation and heat treatment experiments,the stabilizing effect was verified by microstructure characterizations of GBs and precipitates and mechanical properties testing.The main conclusions are as follows:（1）Thermodynamic calculations examining the behavior and effects of elements segregation at GBs found that the low-energyΣ3（111）andΣ11（113）are immune to vacancy segregation,while high-energy Al GBs,such asΣ13（320）,Σ9（221）,Σ5（210）,andΣ5（310）,can attract both vacancies and solutes.Under-sized elements（Ni,Fe,Co,Cu）and similar-sized elements（Si,Zn,Ag,and Ti）prefer interstitial or vacancy sites at the GB interface,while over-sized elements（Mg,Zr,Sc,Er）tend to substitute Al or vacancy-neighboring sites at the GB interface.Segregated vacancies weaken GBs.Under-sized Ni,Co,Cu,similar-sized Ti,and over-sized Zr,Er,can directly enhance Al GBs,while similar-sized Ag and over-sized Mg reduce the GB binding strength.Solute strengthening or weakening effects tend to be always mitigated,more or less,by GB-segregated vacancies.（2）Among the GB strengthening elements,Sc,Zr,Er and Ti can form L1₂precipitates.The calculation results show that no matter which Al₃X（X=Sc,Zr,Er or Ti）phase is preferentially precipitated in the matrix,Ti tends to appear at the interface between L1₂precipitates and the matrix i.e.,the structure of Al₃Ti coated outside Al₃Z（Z=Sc,Zr or Er）is more stable in thermodynamics.The energy of Al₃Ti/Al interface is only 60.7m J/m²,and the interfacial bonding force is much higher than that’and,therefore,Ti can not only improve the stability of Al₃Z phase,but also can further improve the precipitation strengthening effect.（3）The calculation results of Ti substitution and interfacial segregation in’andphases show that Ti can stabilize’andphases in different ways.Ti can enter into’phase at the early aging stage,which reduces bulk energy and delays phase transformation.When the’changes to₂phase,it gradually exudes Ti from the bulk phase,and Ti tends to segregate to the₂/Al interface,which reduces the growth momentum and improves the stability ofphase.（4）Through the stability experiment,it was found that the recrystallized grains of 0.15Ti alloy did not grow after solution treatment,and no PFZ was formed during long-term over-aging,indicating that Ti addition was indeed helpful to stabilize GB structures and achieved the expected effect of calculation design.In addition,during aging,Ti can segregate to Al₃（Sc,Zr）interface to form Al₃（Sc,Zr,Ti）phase,which improves the coarsening resistance of Al₃（Sc,Zr）phase during over-aging as predicted by calculation.Moreover,the segregation of Ti at the/Al interface delayed the growth of phase.The actual stabilization effect of Ti on precipitate is in agreement with the calculated results.Due to the improvement of the microstructure stability,the 0.15Ti alloy still retains the high strength of 757.4 MPa after 96 h aging,and has a good elongation of 9.2%.Compared with 0Ti alloy,0.15Ti alloy has good comprehensive properties,and the strength and plasticity are increased by11.6%and 17.9%,respectively.