| The novel Al-Mg-Zn-(Cu)crossover alloy strengthened by the T phase and its precursors,with an excellent combination of high strength,weldability and corrosion resistance of 5xxx and 7xxx series aluminum alloys,has been considered an ideal lightweight material for the transportation field.Recently,by increasing the Zn content,introducing Cu microalloying and developing the final deformation heat treatment process,the strength and age-hardening response of Al-Mg-Zn-(Cu)alloys have been greatly improved.However,the deformed-induced non-uniform distribution of dislocations accelerates the growth and coarsening rate while inducing non-uniform nucleation of T-type phases,significantly reducing the elongation.Therefore,a new alloy design strategy is needed to accelerate the agehardening response while achieving strength-ductility synergy.In the study,an ICME model was developed and used to predict the effect of the content of Mg,Zn and Cu elements and the double-step aging process on the precipitation kinetics of T-type phases.Then,a novel Al-Mg-Zn-Cu alloy with fast age hardening response and strength-ductility synergy was designed.Meanwhile,the double-step aging process was also modified.Finally,the mechanism of strength-ductility synergy and enhanced intergranular corrosion resistance was also studied.The study provides model support and a theoretical basis for designing and developing aluminum alloys with high strength and ductility.By integrating the CALPHAD methodology,KWN model and strength prediction model,an ICME model was developed,which can predict the effect of alloy composition and aging process on the precipitation behavior of T-type phases and the strength of Al-Mg-Zn-(Cu)alloys.With the advantages of strong scalability,high computational efficiency and good accuracy,the alloy development efficiency can be greatly improved by utilizing the ICME model.Subsequently,based on the ICME model,the influence of Zn content and the introduction of pre-aging treatment on the precipitation behavior and mechanical properties of Al-Mg-Zn alloy were successfully predicted with limited input parameters,which verified the extended predictive capability of the ICME model.Meanwhile,the influence of pre-aging temperature and aging temperature on the age-hardening response and strength of Al-5.1Mg-3.0Zn alloy was predicted by the ICME model,which further verified the accuracy of the model.According to the requirement of multi-objective performance(fast age hardening response and strength-ductility synergy),the microstructure characteristics of the designed alloy were determined,that is,after a short-time aging process,high-density nano-sized particles can be precipitated in the matrix.Based on the ICME model,a novel Al-Mg-Zn-Cu alloy with strength-ductility synergy was designed.Meanwhile,the two-stage aging treatment was also modified.Firstly,the effect of the content of Mg,Zn and Cu elements on the nucleation driving force,interfacial energy,growth and coarsening rates of T-type phases was investigated via the CALPHAD simulation.It was found that introducing a high concentration of Cu cannot only promote the nucleation and precipitation of T-type phases but also suppress the growth and coarsening rates.Then,the design strategy of new Al-Mg-Zn-Cu alloys with strength-ductility synergy potential was clarified.A new Al-4.0Mg-3.0Zn-1.5Cu alloy was developed.After this,the influence of the double-stage aging process on the precipitation behavior and mechanical properties of the Al-4.0Mg-3.0Zn-1.5Cu alloy was predicted by the ICME model.By lowering the pre-aging temperature and shortening the pre-aging time,the density of T-type phases can be greatly increased and its size can be refined,which accelerates the aging response rate of the alloy and improves the strength-ductility synergy.On this basis,a double-stage aging process(80℃/3 h+140℃/x h)was modified.According to the experimental study,it is found that the Al-4.0Mg-3.0Zn-1.5Cu alloy could reach the peak aging state within 9 h.The corresponding yield strength and elongation are 386 MPa and 20.6%,respectively.Compared with Al-4.0Mg3.0Zn,the peak aging time was shortened by 14 h,illustrating a fast age-hardening response.Meanwhile,the yield strength was increased by 172 MPa(80.5%)without loss of ductility,thus overcoming the trade-off between strength and ductility.In addition,the intergranular corrosion resistance was also improved.Finally,based on the ICME model and experimental study,the mechanism of strength-ductility synergy and enhanced intergranular corrosion resistance was studied.Firstly,the effect of Cu content on the nucleation,growth and coarsening process of precipitates during the aging process was systematically studied.With the combination of thermodynamic and kinetic simulation,it is found that introducing a high content of Cu into the Al-Mg-Zn alloy can increase the nucleation driving force of the precipitated phase,reduce the critical nucleation radius,and promote the precipitation of stable GP region during the pre-aging process,thus greatly increasing the nucleation rates of T-type phases.Meanwhile,the growth and coarsening rates were inhibited due to the low diffusion rate of the Cu element.Therefore,there is a high density of T-type phases precipitated in the matrix for the Al-4.0Mg-3.0Zn-1.5Cu alloy.The corresponding volume number density and average radius are 1.15×1023/m3(5 times higher than that of 0Cu alloy)and 2.14 nm(only 42.5%of the average radius for 0Cu alloy),respectively.By characterizing the interaction between the T-type phases and dislocations during the plastic deformation,it is found that the high density of nano-sized precipitates can effectively hinder the dislocation movement by forming a large number of dislocation rings,thus inhibiting the dynamic recovery of dislocations and greatly improves the work-hardening ability of the Al-4.0Mg-3.0Zn-1.5Cu alloy.At the same time,the precipitation of the high-density T-type phase reduces the width of the PFZ,which further improves the ductility of the alloy.Based on TEM characterization results near the grain boundary,it is found that the introduction of the high content of Cu elements disrupts the continuity of the grain boundary phase(T phase)and refines the size,thus improving the intergranular corrosion resistance. |
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