Study On Precipitation Behavior And Strengthening Mechanism In Dual-Phase Strengthened Al-Zn-Mg-Cu Alloys

Heat-treatable strengthened Al-Zn-Mg-Cu alloys have been widely used in aerospace and transportation industries as the lightweight structural materials due to their high specific strength,good formability and corrosion resistance.The high strength of this series alloys mainly origins from the formation of numerous nano-sized precipitates with a uniform distribution during heat treatment process.The composition of Al-Zn-MgCu series alloys covers a wide range,and it is closely related to the precipitation behavior and mechanical properties of the alloys.Thus,it is very important to understand the precipitation behavior and the evolution of these precipitates during ageing process and their influence on the properties for the purpose of improving the prefomance and designing composition in Al-Zn-Mg-Cu alloys.Besides,the pre-treatment before ageing process is generaly used in the production of Al-Zn-Mg-Cu alloys in industry,but it still should be clarified that the influence of pre-ageing/stretching on the precipitation behavior and strengthening mechanism during subsequently artificial ageing in Al-ZnMg-Cu alloys with various composition.Thus,five Al-Zn-Mg-Cu alloys with different Zn and Mg contents and Zn/Mg ratios were designed in this work.Techniques and methods,such as scanning electron microscopy(SEM),transmission electron microscopy(TEM)and three dimensional atom probe(3DAP),combined with ageing hardness tests and tensile tests were used to systematically study:(1)the co-preciptation behavior of T’ and η’ phase in Al-Zn-Mg-Cu alloys and the strategy of tailor their phase fraction,(2)the influence of Zn/Mg ratios on the microstructure and mechanical properties of dual-phase strengthened Al-Zn-Mg-Cu alloys,and(3)the effect of pre-treatment on precipitation behavior during ageing process and strengthening mechanism in dual-phase strengthened Al-Zn-Mg-Cu alloys.The main conclusions include:(1)The Zn/Mg ratio can remarkably influence the precipitation sequence of Al-ZnMg-Cu alloys during ageing.The precipitation sequence of T’ phase exists in alloy with the lowest Zn/Mg ratio.The increase of Zn content improves the Zn/Mg ratio of alloy,which promotes the precipitation of η’ phase.As a consequence,the precipitation sequences of T’ phase and η’ phase co-exist in alloy,causing a dual-phase strengthened Al-Zn-Mg-Cu alloys system.(2)T’ phase and η’ phase have different precipitation behaviors at different ageing temperature,which is related to the transformation path of GP zones.The elevated ageing temperature is benefical to the precipitation of T’ phase,while the lower ageing temperature facilitates the precipitation of η’ phase.Duing ageing at 120 °C,the phase fraction of η’phase increases with the aging time,while the fraction of T’ phase decreases.Duing ageing at 150 °C,the fraction of T’ phase and η’ phase remains unchanged during aging.The Zn/Mg ratio of GP zone is the key factor affecting its phase transformation path,tend to transform into T’ phase and GP zones with higher Zn/Mg ratios tend to transform into η’ phase.(3)The strength and ageing response of Al-Zn-Mg-Cu alloys are related to the Zn/Mg ratio and the total content of(Zn + Mg + Cu)solutes.The response rate of ageing process is determined by the Zn/Mg ratio,where higher Zn/Mg ratio accelerates the ageing process.It is influenced by the variation of phase fraction of T’ phase and η’ phase,the increase of Zn/Mg ratio improves the phase fraction of η’ phase accompanies with the decrease of T’ phase.And,η’ phase shows better ageing response and strengthening effect than T’ phase.The strength is proportional to the total content of(Zn + Mg + Cu)solutes,while higher solutes content enhanced the strength of alloys.The increase of solutes content promotes the formation of precipitates,improves the number density of precipitates.As a result,fine and dense precipitates uniformly distribute in matrix with narrow precipitates free zones near grain boundaries.In addition,the strengthening model is modified to be suitable for the Al-Zn-Mg-Cu alloy system strengthened by T’ and η’phases.(4)In the same alloy,the composition of T’ phase and η’ phase are different,while their Zn/Mg ratio is very close.Moreover,the Zn/Mg ratio in precipitates increases monotonically with the increase of Zn/Mg ratio in the alloy composition under peak aged.The composition of precipitates significantly depends on their size.And the concentrations of Zn and Mg(Zn: 30 ～ 45 at.%,Mg: 20 ～ 30% at.%)become higher with the increase of precipitates size,while the concentrations of Al and Cu decrease(Al: 30～ 40 at.%,Cu: <3 at.%).(5)Pre-ageing can effectively improve the precipitation near grain boundaries and narrow the precipitates free zones,casuing the improvement of mechanical properties for Al-Zn-Mg-Cu alloy.Especially for alloys A1,A2 and A5 with medium strength,which peak hardness increased by 10～14 HV,14～18 HV and 26～32 HV,respectively.The alloy with different pre-ageing time at 120 °C can obtain the same strength,but the longer preaging time is beneficial to the formation of smaller precipitates and narrower precipitates free zones in alloys,so as to improve the elongation of the alloy.(6)The effect of pre-stretching on the mechanical properties of Al-Zn-Mg-Cu alloy is related to the alloy composition.In the low Zn/Mg ratio alloy with T’ phase as the main precipitates,the homogeneous nucleation rate of precipitates in the matrix is slow while the dislocation introduced by pre-stretching causes the precipitates to coarsening rapidly,which leads to a significant decrease in the number density of precipitates and the strength of the alloy.In the high Zn/Mg ratio alloy with η’ phase as the main precipitates,the homogeneous nucleation rate is fast in the matrix,and the dislocation introduced by prestretching can act as the nucleation site,which promotes the precipitation and improves the number density of precipitates,leading to an enhanced strength.The pre-aging before pre-stretching is beneficial to improve the dislocation distribution in the alloy during prestretching,casuing the fine size of precipitates with a uniform distribution in the artificial ageing process.