Microstructure Evolution of 7N01 Alloy with Synergism of Zn,Mg and Mn Elements
7N01 alloy is one of the commonly used alloys in the fields of rail transit,air transport and new energy vehicles.It is the construction materials of the main parts of high-speed train body,aircraft beam and car crossbeam,which has excellent work-ability,form-ability and weld-ability.From the composition of 7N01 alloy,it can be found that the composition of 7N01 alloy varies over a large-scale range,with the main elements Zn,Mg and Mn raging of 4.0％～5.0％,1.0％～2.0％,and 0.2％～0.7％,respectively,in ad-dition other components of elements are limited in a low range.The using stand of 7N01 alloy for the construction materials of high-speed train body is that the tensile strength(σb)and yield strength(σ0.2)are not less than 350 and 225 MPa respectively,with a loss of strength and elongation at most 15％in a neutral solution containing Cl-.It has been proved that the content of Zn element signifi-cantly affects the toughness and the resistant of corrosion behavior of 7xxx alloy with high Zn and Mg contents,the density of η'phase in Al matrix and grain boundary of the alloy is directly related to the content of Mg element,and Mn element has a great influence on the elongation of 7xxx aluminum alloy with high Zn and Mg contents.However,there are few literature reports on the effects of Zn,Mg and Mn elements,as well as their synergistic effects,on 7xxx alloy.In this work,six effective components of 7N01 alloys were de-signed,and the hardness and mechanical properties of the alloys with different compositions during aging were tested by means of hard-ness tester and universal testing machine.The microstructure of the alloys with different components were characterized and analyzed by means of three different transmission electron microscope(TEM)imaging modes,which named transmission electron microscope bright field image(BFTEM),selected area electron diffraction(SAED)and high angle annular dark field image(HAADF-STEM).The results of hardness and tensile test showed that during peak aging at 120 ℃,the hardness curves of the six alloy samples all in-creased firstly,then briefly decreased before increasing again in the end,as a result there were two peaks in the hardness curves.The two peaks were related to the precipitation of GP zones and η'phase.Guinier Preston(GP)zone was coherent to Al matrix and the dis-locations pass through by cutting,which increased the less value to the strength of the alloy;while η'phase was semi-coherent to Al matrix and the dislocations passed through by passing,which significantly increased the strength of the alloy,as a result all the hard-ness values and tensile strength values of the samples at the first peak were all higher than those of the samples at the second peak.By comparing with the hardness curves of different samples,it could be seen that alloys with high Mg or Mn content had higher hardness and tensile strength value,and the hardness peak of the alloy gradually decreased with the increase of Zn/Mg ratio.This result could be attributed to the formation of more η'phase and AI6Mn phase in the matrix.TEM images showed that in the early stage of aging,two different types of precipitates were generated,named GPI and GPII respectively,which both could transform to η'phase with different sizes.By comparing with SAED pattern and BFTEM images of different samples,it could be seen that the type and quantity of GP zone,η'phase in matrix was significantly influenced by Mg content of the alloy,the size of precipitation free zone(PFZ)in the grain boundaries was influenced by Zn content,and the size and density of precipitates in matrix and grain boundary were related to the con-tent of Zn and Mg contents.HRTEM images indicated that the quantity and distribution of Al6Mn precipitates was directly related to the content of Mn content,and the increase of Mn content in 7N01 alloy slightly increased the density of GP zone and η'phase.At the same time,Al6Mn was widely distributed in the matrix or grain boundaries,which could easily lead to dislocation accumulation.A large number of dislocation loops and lines appeared around Al6Mn phase,which made it difficult for dislocations to move and block-around Al6Mn phase boundary,resulting in a decrease in the elongation of the alloy.
7N01 alloycompositionhardnessmechanical propertiesevolution of precipitates
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