Study On The Microstructures And Mechanical Properties And Electrical Conductivities Of Al-Fe Alloy Processed By ECAP

8xxx series electrical aluminum alloys（Al-Fe alloys）have been widely used in cables due to their high electrical conductivity.However,the trade-off relationship between strength and conductivity leads to low strength,which limited their further application in power transmission.Equal channel angular pressing（ECAP）is a kind of simple,low cost severe plastic deformation method with great commercial potential.It has attracted a lot of attentions the development of electrical aluminium alloys because of the significant strength improvement by the grain refinement through the introduction of large strains.Faced on industrial applications,it is necessary to try to develop Al-Fe alloys with excellent strength and conductivity by using ECAP.Hence,commercial 8176 aluminum alloys（Al-Fe alloys）were studied in this thesis,conventional ECAP process at room temperature（RT）and 150℃were carried on the three different Al-Fe alloys rods:initial state（INT）,homogenization state（HT）and aging state（AT）,respectively.Microstructures,mechanical properties and electrical conductivities of the alloys before and after ECAP were characterized and tested.The influence of pretreatment and ECAP processing parameters on the evolution of microstructure after ECAP were studied,the evolution of microstructure and properties during ECAP were explored,and the strengthening mechanism and conductivity change mechanism were revealed.On this basis,the ECAP process was optimized to obtain Al-Fe alloys with good strength and high conductivity.The microstructures of the INT alloys are a series of rolling grains with width of several to tens microns as well as parallel（sub）micron Al₃Fe or Al₆Fe precipitates.After 8 passes ECAP,there are a series of microstructure evolutions,including dislocations propagation,dissolution and redistribution of micron precipitates,nanoparticles precipitation and grains refinement.The ultra-fined grains（UFG）microstructures contained high density dislocations,nanoparticles,large percentage high angle grian boundary（HAGB）were formed.During the ECAP,the fined rolling grains can effectively induce precipitates fragmentation and Fe dissolution.These supersaturated Fe atoms can accelerate dislocation propagation and promote grain refinement.The minimum average grain size of 2.6μm,as well as the highest percentage of HAGB of 81.3%and the average misorientation of 33.1°were obtained after 8 passes ECAP at 150℃.Due to the combination of dislocation strengthening,particles strengthening and grain strengthening,INT alloys have the significant increases of ultimate tensile strength（UTS）from 125 MPa to189 MPa after ECAP at RT and to 188 MPa after ECAP at 150℃,respectively.The pretreatment microstructures of HT alloys consist of recrystallized grains with an average size of 107μm as well as the parallel（sub）micron precipitates.After 8 passes ECAP,the UFG microstructures were also developed.The elevated ECAP temperature promotes Fe redissolution during ECAP as a result of micron precipitates fragmentation and spheroidization under the effect of temperature and local stress concentration.This resulted in the highest dislocation density of 4.69×10¹⁴ m^-2 after eight passes of 150℃ ECAP processing,and consequently the highest yield strength（YS）of 182 MPa and UTS of 195 MPa.Similarly,there were reductions in the electrical conductivity of the HT alloys from 60.5%IACS to 59.9%IACS after RT ECAP and 59.7%IACS after 150℃ ECAP,because of the increased resistance by the back-soluble Fe atoms,precipitates,grain boundaries and dislocations.The aging treatment microstructures of the AT alloys are recrystallized grains with an average size of 123μm as well as the nanopartilces and parallel（sub）micron precipitates.With the increase of ECAP passes,a large number of dislocations are introduced into the aluminum matrix,and the continuous accumulation of dislocations induces the continuous dynamic recrystallization（CDRX）.Some dislocations gradually rearrange to the dislocation walls or low angle grain boundaries（LAGB）,which merge and absorb dislocations into HAGBs,resulting in the CDRX.The grain sizes were progressively refined to 39.5μm,15.9μm and 3.3μm after1,4 and 8 passes RT ECAP,and to 70.5μm,7.0μm and 3.0μm after 1,4 and 8 passes 150℃ ECAP,respectively.After ECAP,the significant strength increase was achieved in AT alloys.Especially,150℃ ECAP reached the maximum of 165 MPa and 186 MPa respectively at 8passes.Quantitative analysis showed that dislocation strengthening usually contributed the most to the strength increase of the ECAPed Al-Fe alloy,followed by precipitates strengthening and finally grain strengthening.The high conductivity of 61.5%IACS of the AT alloy also decreased after 8 passes.AT alloys after 8 RT ECAP passes retained the highest electrical conductivity of 60.9%IACS in all the 8-passes specimens.The theoretical calculation results show that under different ECAP processes,the concentration of Fe atoms in solid solution changes by 0.02-0.04 at.%and the resistivity changes by 1-2 nΩ·m,which is the main factor affecting the conductivity decrease and resistivity increase in Al-Fe alloys after ECAP.According to the characteristics of microstructure evolution and properties of Al-Fe alloy by the traditional ECAP process,a novel sequential ECAP process was proposed and verified by experiments.After the sequential ECAP,the strength and conductivity of the INT alloys increased simultaneously,with the YS increased by 42%to 168 MPa,UTS increased by 45%to 181 MPa,and the electrical conductivity increased to 60.8%IACS.This electrical conductivity value closes to the 99.5%pure aluminum（60.9%IACS）.In the sequential ECAP AT alloys,the conductivity remained at 61.6%IACS,while the YS increased from 46 MPa to178 MPa and the UTS increased from 88 MPa to 181 MPa,achieving both the excellent strength and conductivity.