First-principles Calculation And Characterization Of The Influence Of Zn And Cu On The Aging Precipitation Of Al-Mg-Si Alloy

Al-Mg-Si(6000 series of aluminum)alloys are widely used in automobile due to their excellent comprehensive mechanical properties,good formability,high specific strength high quality of welded surface and good corrosion resistance.They can be strengthened by heat treatment and have higher strength in the peak-aged state.In order to make the alloy have good formability and sufficient strength after short-time baking at low temperature,it is necessary to improve the aging response speed of the alloy.The current studies generally promote the formation of aging precipitates by adding Zn and Cu elements to Al-Mg-Si alloy.Therefore,first-principles calculations,combined with experimental characterization methods such as TEM,HRTEM,HAADF-STEM are applied to systematically investigate the effects of Zn and Cu on the aging precipitation sequence of Al-0.6Mg-0.9Si alloy and the structure of the aging precipitation phase in this work.A theoretical basis is provided for the study of the aging precipitation behavior of Al-Mg-Si alloys for automotive plates and the development of alloys.First-principles calculations are applied to investigate the influence of Zn and Cu addition on the evolution of precipitates in Al-Mg-Si alloys during aging.The formation enthalpy of Mg-Si GP zone and ?" phase is lower than that of Mg-Zn GP zone and ?'phase.It is difficult for Zn to induce the formation of Mg-Zn phases in Al-Mg-Si alloy.The doping of Zn can reduce the formation enthalpies of Mg-Si GP-zones and ?" phase,and increase the covalent bond strength.The formation of GP-zones and ?" phase during the aging stage of Al-Mg-Si alloys is promoted due to the improved stability of the structures.Cu atoms can also be doped into the phase and replace Si3/Al atoms.However,when Zn and Cu are both added in Al-Mg-Si alloys,Zn atoms are more preferentially doped into the Mg-Si GP zone and P" phase than Cu.The formation enthalpy of the Q' phase is lower than that of the ?" phase and the ?" phase doped by Cu.Therefore,Cu atoms are preferentially to induce the formation of Q'phase in the Zn added Al-Mg-Si alloy.The effects of Zn addition on the interface between ?" phase and Al matrix are systematically investigated.Three kinds ofinterfaces:(010)?"//(010)Al,(100)?"//(130)Al,(001)?"//(320)Al can be formed between ?" phase and Al matrix.The interfacial energy of(010)?"/(010)Al interface is the highest.Thus?" phase grows along the[100]Al direction with needle morphology.In addition,(100)?"//(130)Al and(001)?"//(320)Al are generally formed at the position of the inversion center of ?" phase,which locates at the Si3/Al atom.The energy of Zn atoms replacing Si3/Al atoms in the internal of ?" phase is lower than interface.Therefore,Zn atoms preferentially enter the internal of ?" phase.But when the Zn content is sufficiently high,Zn atoms may also occupy Si3/Al positions on the interface.With the increase of the Zn atoms occupying the interface,the separation energy of the(100)?"//(130)Al and(001)?"//(320)Al interfaces increases.The strength of the interface is also higher,and the strengthening effect of the ?" phase is improved.The types and crystal structures of the precipitates in the initial and peak-aging state of the Al-Mg-Si alloys with Zn and Cu added are studied by TEM HRTEM and HAADF-STEM,and the calculation results are verified.At the beginning of aging,the precipitation phases of Al-Mg-Si alloys with Zn and Cu added are mainly Mg-Si GP zones and a small amount of Q' phase substructures "Cu cluster-like zone".In addition,the Mg-Si GP zones are doped with Zn atoms.The precipitation phases of the peak-aged state are mainly ?" phases,which are also doped with Zn atoms.The interface structures formed between ?" phase and the matrix are consistent with the first-principles calculation results.Zn atoms also occupy Si3/Al positions on the(001)?"//(3 20)Al interface,but not on the(100)?"//(130)Al interface.A small amount of transition state precipitates from ?" phase to Q' phase as well as Q' phases are observed in the peak aging state,indicating that the mechanism of Cu atoms promoting alloy aging precipitation is mainly due to changing the aging precipitation sequence of Al-Mg-Si alloy and inducing the formation of Q' phases and Q" phases,while only ?" phases doped with Zn atoms are found in the peak aging state of Al-Mg-Si alloy with only Zn added.