| Compared to traditional aluminum matrix composites,nanoparticle reinforced ultrafine grained aluminum matrix composites exhibit higher specific strength,specific stiffness,and other properties due to their excellent particle and grain refinement strengthening effect.They possess a wide range of application prospects in aerospace,automobile and other fields,and are favored by many researchers.High energy milling plus spark plasma sintering is an advanced method for preparing this kind of material.During high energy milling,nanoparticles can be well dispersed and simultaneously ultra-fine grianed aluminum matrix can be obtained.During spark plasma sintering,grain growth can be effectively inhibited due to the rapid and low-temperature sintering characteristics.At present,some achievements have been made in the preparation of ultra-fine grained aluminum nanocomposites by this method.However,most studies have been limited to focusing on the microstructure and properties of sintered samples.The effect of milled powders’ state on the sintering behavior,as well as the microstructure and properties of sintered samples,is not very clear.In addition,the effect of nanoparticle and ultra-fine grain structure on the materials’ subsequent hot deformation and heat treatment is rarely reported.Therefore,in this paper,Ti N nanoparticle reinforced ultra-fine grained Al2024 composites were first prepared by high energy milling and spark plasma sintering,and then hot deformation and heat treatment were carried on the materials.The evolution of microstructure and mechanical properties during the processing of materials’ preparation,hot deformation and haet treatment were studied and analyzed deeply.The results obtained are as follows.The effect of milling time on the morphology and microstructure of the powders,as well as the microstructure and properties of the sintered samples,was investigated.Results show that with the prolongation of milling time,the Al2024-2Ti N powder tends to be equiaxed and decreases gradually.With 30 h milling or more,the morphology and particle size of powder no longer change significantly,and the grain size is about 36 nm.The stacking density of the powder and the relative density of the sintered sample also increase gradually.The hardness and compressive strength of the sintered sample with 40 h milling are the highest.The increase of milling time promotes the grain growth during sintering,after sintering the grain sizes of the powders milled for 40 h and 50 h are 145.4 nm and 289.2 nm,respectively,which are the same before sintering.The grain growth of powder with 50 h milling is more obvious.The effect of Ti N contents on the morphology and microstructure of the powders,as well as the microstructure and properties of the sintered samples,was investigated.Results show that Ti N is beneficial to accelerate the milling process,promote the solid solution of solute atoms and the crush of Al2 Cu,and refine the grain.With 40 h milling,the grain sizes of the powders with 1-4 wt.% Ti N addition are reduced from 87 nm to 30-50 nm.Ti N can restrain the grain growth during sintering,and the average grain size of Al2024 and Al2024-2Ti N powders after sintering are 576.5 nm and 145.4 nm,respectively,which are 6.6 and 4 times that before sintering.The yield and compressive strength of the Al2024-2Ti N nanocomposite are 730 MPa and 871 MPa,respectively,increased by 185 % and 35 % compared to the Al2024 alloy.It still shows 10 % engineering strain.The hot deformation behavior of ultra-fine grained Al2024 alloy and Al2024-2Ti N nanocomposite was investigated using Gleeble thermal simulation tester.True stress-strain curve was analyzed.Constitutive equation was established.Processing map was constructed.Deformation mechanism was discussed.Results show that the flow stress of Al2024 and Al2024-2Ti N samples decreases with the increase of temperature and the decrease of strain rate.Under the same deformation conditions,the steady flow stress of Al2024-2Ti N is lower than that of Al2024.The deformation activation energy of Al2024 and Al2024-2Ti N samples are 239.260 k J·mol-1 to 749.386 k J·mol-1,respectively.Compared with Al2024,the optimal hot working area of Al2024-2Ti N has a lower deformation temperature and higher strain rate.Ti N nanoparticles are beneficial to restraining grain growth and promoting dynamic recrystallization during hot deformation.The deformation mechanism of Al2024-2Ti N is grain boundary sliding,accompanied with dynamic recrystallization,dynamic recovery and grain growth,while that of Al2024 is intra-granular sliding,accompanied with dynamic recovery and grain growth.The effect of solution and aging parameters on the microstructure and properties of Al2024 and Al2024-2Ti N samples was investigated,and the strengthening mechanism and kinetics of aging were studied.Results show that the optimum heat treatment parameter of Al2024-2Ti N is 490 °C/2 h+180 °C/6 h.Under this condition,the yield strength,tensile strength and elongation after heat treatment are 359 MPa,783 MPa and 9.4 %,respectively,while those of Al2024 are 376.5 MPa,567.9 MPa,and 12.7 %,respectively.The aging strengthening effect of composites is weakened.The increase in yield strength of Al2024 and Al2024-2Ti N after heat treatment is 32.9 % and 11.8 %,respectively,which is due to that the solid solution of the second phase is suppressed by Ti N,causing the lack of solute elements and reducing the number of precipitated phases,and thus weakening the aging strengthening effect.Ti N restrains the grain growth during heat treatment.The average grain sizes of Al2024 and Al2024-2Ti N after aging are 3.2 μm and 452.4 nm,respectively.The accerating effect of Ti N on the aging kinetics is greater than the suppressing effect of ultrafine grain,and thus the aging kinetics of Al2024-2Ti N is higher than that of Al2024.The thermal diffusion activation energy of S’ precipitates in Al2024 and Al2024-2Ti N samples is 78.98 k J/mol and 82.06 k J/mol,respectively. |
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