| 7075 aluminum alloys were widely used in military transport and aerospace manufacturing due to their high specific strength,high fracture toughness and good stress corrosion resistance.With the rapid development of the manufacturing industry,higher requirements for the performance of aluminum alloys have been put forward.The development of a lightweight,high-strength,high-plastic 7075 aluminum alloy has become a top priority.In-situ nanophase(co-addition of Sc(Er)and Zr)and nanophase(graphene,abbreviated GNP)addition emerged as an important technical means to improve the properties of alloys.In this work,the nano-phase reinforced 7075 aluminum alloys were prepared by casting method that can be used for large-size ingots and mass production,while achieving matching of its cost and performance.X-ray diffraction(XRD)was used to explore the evolution of the phase structure of 7075 aluminum alloy.The influence of nanophase on the multi-scale microstructure and mechanical properties of 7075 aluminum alloy was systematically clarified by scanning electron microscope(SEM)and transmission electron microscope(TEM),and the strengthening mechanism of the alloy was analyzed.Combined with electron backscatter diffraction(EBSD),the static recrystallization behavior of 7075 aluminum alloy containing Al3(Sc,Zr)phase was studied.The nano-phase provides an effective heterogeneous nucleation core for the?-Al phase during the solidification process,resulting in a significant refinement of the grains and the non-equilibrium eutectic phase at the grain boundary of the as-cast 7075 aluminum alloy.The average grain sizes of the 7075,Er-Zr-7075,Sc-Zr-7075 aluminum alloy,and GNP/7075aluminum matrix composites are 96.2?m,47.6?m,29.6?m,and 45.0?m,respectively.The refinement of the non-equilibrium eutectic phase at the grain boundary of the as-cast alloy enables the Er-Zr-7075,Sc-Zr-7075 aluminum alloy,and GNP/7075 aluminum matrix composites to obtain a better homogenization microstructure.After the two-stage homogenization treatment process at 400?/12h+465?/12h,the network non-equilibrium eutectic phase at the grain boundaries of the Sc-Zr-7075 alloy was more fully dissolved,and the morphology changed from long strip to spherical shape.The main strengthening phase GP zone and?'phase of the alloy were refined by the nanophase and the precipitation density was increased.The Al3(Sc,Zr)phases of Sc-Zr-7075 aluminum alloy in two-stage homogenization process(400°C/12h+465°C/12h)were finely dispersed compared to that the single-stage homogenization process(465°C/16h).The average size of Al3(Sc,Zr)phase was decreased from 24.6nm to 18.3nm,and the corresponding volume fractions were 1.2×10-3and 1.8×10-3,respectively.The fine and coherent Al3(Sc,Zr)phase((L12))strongly pined the dislocations and grain boundaries,preventing the dislocations from rearranging into sub-grain boundaries and from developing into high angle grain boundaries,which further hindered the formation and growth of recrystallized core of the alloy.The addition of Sc and Zr made the Sc-Zr-7075Al alloy remained the most of fibrous structure and high-density dislocations formed in the extrusion process,the recrystallization fraction of the alloy decreased from 35%to 22%,and the corresponding percentage of substructure increased from 59%to 67%.The addition of nanophase graphene led to a higher dislocation density of GNP/7075aluminum matrix composites compared to that of 7075 aluminum alloy.This was mainly attributed to the large difference in thermal expansion coefficient between graphene and aluminum matrix,which produces high residual stress at the interface.The residual stress produces a large number of dislocations during the release process.The graphene and aluminum matrix interface were firmly bonded and no brittle Al4C3phase was found at the interface junction,which was conducive to the full load to transfer effect of GNP/7075aluminum matrix composites.Compared to that of the 7075 aluminum alloy,the Er-Zr-7075 aluminum alloy had improved strength while maintaining high elongation,synchronous enhancements in strength and ductility was obtained in Sc-Zr-7075 aluminum alloy.The ultimate tensile strength(UTS)of the four alloys(T6)were 598MPa,650MPa,696MPa,679MPa,the yield strength(YS)were 548MPa,601MPa,652MPa,628MPa,and the elongation(EL)were 7.5%,7.5%,8.5%,8.0%,respectively.The addition of Er(Sc)and Zr led to the increase in strength of 7075aluminum alloy mainly through grain refinement strengthening,Orowan strengthening and precipitation strengthening(GP zone and?'phase).The increase in strength of GNP/7075aluminum matrix composites was mainly attributed to grain refinement strengthening,dislocation strengthening and second phase strengthening. |
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