| 7xxx series aluminum alloys have been widely used in many fileds such as aviation,aerospace,continental drilling due to their excellent mechanical properties.However,they also have poor formability and low stress corrosion resistance due to its high alloying degree.In order to improve itheir performances,microalloying or changing the proportion of alloy elements is necessary to adopted.However,the content adjustment of one element often have an impact on other elements in the alloys,thus it is of great significance to investigate the interactions between alloy components upon designing an alloy.In addition,it's also a promising method to make some changes to clusters,then the whole ageing precipitatation process and finally the mechanical properties by deformation.And this method demands a fully understand of interactions between dislocations and clusters.In this paper,a medium-strength 7xxx series aluminum alloy with low Zn content was selected as the research material,and the content of constituent elements was changed one by one.By using transmission electron microscopy(TEM),three dimensional atom probe(3DAP),microhardness and tensile test,several aspects were systematically investigated as bellow:(1)Effect of trace Ag addition on precipitation strengthening and microstructure of Al-Zn-Mg-Cu alloy during artificial ageing;(2)Composition evolution in the matrix and precipitates of Al-Zn-Mg-Cu(-Ag)alloys during artificial ageing;(3)Effect of Cu and Zn concent on microstructure and properties of Al-Zn-Mg-Cu-Ag during artificial ageing;(4)Effect of different compression deformation on microstructure of natural aged Al-Zn-Mg-Cu-Ag alloy.The main conclusions are as follows:(1)Al-4.2Zn-2.8Mg-1.0Cu-x Ag(x = 0,0.2,0.4)alloys aged at 120?,150? and180? all have a characteristics of co-precipitation of strengthening phase T? and ??,and the proportion of ?? phase increases with the increase of Ag content.The addition of Ag facilitates and refines the precipitation and hence improves the ageing strengthening effect,comprehensive tensile properties.(2)At the early stages of ageing at 120?,150? and 180?,the order of solute precipitation percentage(Ag>Zn>Mg>Cu)in the Al-4.2Zn-2.8Mg-1.0Cu-x Ag(x = 0,0.2,0.4)alloys proves that the nucleation of precipitates conforms to the "vacancy cluster mechanism" not the "solute cluster mechanism".The addition of Ag can inhibit the precipitation of Zn and Mg atoms in the very early ageing stages(1-5 min)at the three ageing temperatures,but the effect of Ag on precipitation of Cu atoms is related to temperature(at 120?,Ag has no effect on precipitation of Cu atoms,while at 150?and 180?,Ag can promotes the precipitation of Cu atoms).With increasing the ageing temperature,the precipitation percentage and average concentration in precipitates of each solute element increase.(3)The effect of Ag on the precipitation behavior of Zn,Mg and Cu is related to ageing time.During ageing at 150?,Ag inhibits the precipitation of Zn and Mg at the very early stages(1-5 min)and later stages of ageing,and promotes the precipitation in the middle stages of ageing;Ag promotes the precipitation of Cu in the early and middle stage of ageing but inhibits the precipitation in later stages of ageing.The promotion effect of Ag on ?? phase is related to the increase of(Zn+Cu)/Mg ratio in the nucleation stage.The inhibition effect of Ag on coarsening of precipitates is likely related to the composition characteristics of two-stage differentiation of precipitates.Most of the precipitates have the characteristics of low Zn/Mg ratio,high Cu content and high Ag content,which are conducive to improve the stability of precipitates.(4)The addition of Cu produces a rapid early aging strengthening effect in Al-4.2Zn-2.8Mg-0.4Ag alloy,which is mainly due to the promotion of Cu on the segregation behavior of solute elements in the solution-treated alloy.The addition of 1.0wt.% Cu has the most predominated enhancement to Ag,and 2.3 wt.% Cu has a great enhancement to Zn,Mg and Ag.The addition of Cu can delay the coarsening behavior of precipitates,which is related to the enrichment of Cu atoms near the interface between precipitates and matrix.(5)With the increase of Zn content,the ageing precipitation kinetics and ageing strengthening ability of Al-x Zn-2.8Mg-1.0Cu-0.4Ag alloy were significantly improved.For both low Zn and high Zn alloys,Zn and Ag are the elements with higher precipitation degree at the peak-aged state,while Mg and Cu are the elements with higher precipitation degree during the over ageing stages.However,the increase of Zn content can also accelerate the coarsening of ?? precipitates and promote the precipitation of Mg and Cu.Therefore,increasing Zn content results in a faster over-ageing softening behavior.(6)In the Al-4.2Zn-2.8Mg-1.0Cu-0.4Ag alloy,dislocations has appeared in the elastic deformation stage but with small amount and discrete distribution;while the dislocations greatly proliferated and tangled or even evolved into dislocation cells at plastic deformation stage.As the deformation degree increase,the number density and volume fraction of clusters decreased first and increased later.The precipitation degree of solute atoms first decreased and then increased,and the change order from high to low is Ag > Zn > Mg > Cu.In terms of the composition of clusters,the Zn concentration decreased first and then increased,while the Mg concentration increased first and then decreased,as the Cu concentration increased and the Ag concentration decreased.(7)In terms of the method of alloy composition design,Ag and Cu can be added to the Al-Zn-Mg alloys at the same time.In terms of the method of deformation,the combination of pre-ageing and low strain deformation is beneficial to obtain microstructures of fine and dense clusters.This method will provides a good foundation for the subsequent transformation of strengthening phases and is expected to increase the strength of alloys. |
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