Study On Fabrication Of Quasicrystal-reinforced Mg-Zn-Y Alloys And Strengthening And Toughening Mechanisms

With the increasing demand for lightweight and high-strength structural materials in equipment manufacturing fields such as transportation and aerospace,Mg alloys are favored for their advantages such as light weight and high specific strength.However,Mg alloys have poor room temperature deformation ability and low absolute strength,which limit their wide application.The icosahedral quasicrystal phase（I-phase,Mg₃Zn₆Y₁）has excellent mechanical and physical properties due to its unique atomic arrangement structure,which can not only effectively strengthen Mg alloys,but also have a great impact on dynamic recrystallization during plastic deformation of Mg alloys.In this paper,Mg-6x Zn-x Y（x=0.17,0.25,0.34 at.%）alloys containing I-phase were prepared by conventional casting.Nano-sized I-phase of Mg-Zn-Y alloy was achieved by semisolid+hot extrusion composite processing technology,that is,the nano lamellar eutectic（I-phase+α-Mg）structure was formed through semisolid isothermal treatment process,and then broken into nano I-phase particles in the subsequent hot extrusion process,which significantly improved the comprehensive performance of Mg-Zn-Y alloy.In order to investigate the influence of high temperature deformation on quasicrystalline reinforced Mg-6x Zn-x Y（x=0.17,0.25 and 0.34at.%）alloy,the mechanical behavior of high temperature deformation was studied by high temperature compression experiment,and the changes of I-phase during high temperature deformation were analyzed.The constitutive equation and machining diagram of high temperature deformation of Mg6x Zn-x Y（x=0.17,0.25 and 0.34at.%）alloy were established,which provided theoretical basis and data support for the development and manufacturing of the subsequent quasicrystalline reinforced Mg-Zn-Y alloy components.Due to many process parameter variables involved in semisolid+hot extrusion composite processing technology,it is difficult to determine the optimal combination of process parameters.In order to further improve the preparation process of Mg-Zn-Y alloy and optimize its mechanical properties.In this paper,Mg1.5Zn-0.25Y（at.%）alloy（remelting）was selected.Based on the efficient global optimization algorithm of Kriging model,the process parameters of quasicrystalline reinforced Mg-1.5Zn-0.25Y alloy by semisolid+hot extrusion were optimized,and the strengthening and toughening mechanism of Mg-1.5Zn-0.25Y alloy was analyzed.The main research conclusions are as follows:1.The Mg-6x Zn-x Y（x=0.17,0.25 and 0.34 at.%）alloys containing I-phase were obtained by conventional casting.Then the as-cast alloys were homogenized and subjected to primary and secondary extrusions,and the mechanical properties of as cast,primary extrusion and secondary extrusion alloys and the evolution law of quasicrystal phase in microstructure were analyzed.In the as cast alloy,the I-phase mainly exists in the micron lamellar eutectic structure at the grain boundary.After one extrusion,the micron level lamellar eutectic structure is broken into micron level granular quasicrystal phase,which is distributed in strips along the extrusion direction.After secondary extrusion,the micron sized granular quasicrystal phase was further crushed and still distributed along the extrusion strip.Direct secondary extrusion can improve the strength of Mg-6x Zn-x Y（x=0.17,0.25 and 0.34 at.%）alloys,among which Mg-1.0Zn-0.17Y alloy has the largest increase in secondary extrusion at 250℃,and its tensile strength is about 30%higher than that of primary extrusion alloy.2.The quasicrystal phase and matrix grain structure in Mg-6x Zn-x Y（x=0.17,0.25 and0.34 at.%）alloys were controlled by semisolid+hot extrusion composite processing technology to improve its comprehensive mechanical properties.After semisolid isothermal treatment,the alloy can form nano lamellar eutectic structure at the grain boundary.The thickness of nano lamella in Mg-1.0Zn-0.17Y,Mg-1.5Zn-0.25Y and Mg-2.0Zn-0.34Y are 86nm,66 nm,and 72 nm respectively;Then the semisolid alloy was hot extruded,and the nano lamellar eutectic structure was broken into nano particles and micro particles,which were distributed in strips along the extrusion direction.In addition,the semi-solid+hot extrusion composite processing technology is used to change the grain structure into a typical bimodal grain structure.After semi-solid+hot extrusion composite processing technology,the plasticity of Mg-1.0Zn-0.17Y alloy is the best,and the elongation reaches 44.0±2.6%,which is 124.5%higher than that of the primary extrusion state,and 120%higher than that of the direct secondary extrusion state.The strength of Mg-2.0Zn-0.34Y is the highest,and the tensile strength reaches 409±0.6 MPa,which is 54.4%higher than that of the primary extrusion state and 51.5%higher than that of the direct secondary extrusion state.The Mg-1.5Zn-0.25Y alloy has the best comprehensive mechanical properties.Its yield strength（YS）,tensile strength（UTS）and elongation（EL）are 307±6.6 MPa,396±0.8 MPa and 15.4±0.1%respectively.3.The Mg-6x Zn-x Y（x=0.17,0.25 and 0.34 at.%）alloys after semisolid+hot extrusion composite technology has four typical deformation characteristics of hardening,transition,softening and steady-state rheology during high temperature compression deformation.The essence is the result of the competition between work hardening and dynamic recovery and recrystallization softening.When the hot compression deformation temperature decreases,the strain rate increases,and the content of I-phase decreases;the peak stress and steady-state flow stress of the alloy after semisolid+hot extrusion composite technology increase accordingly.Based on the Arrhenius model,the high-temperature deformation constitutive equations of the Mg-6x Zn-x Y（x=0.17,0.25 and 0.34 at.%）alloys after semisolid+hot extrusion composite technology are established,which are as follows:1.0Zn-SE 250℃:（?）=e^48.67[sinh（0.0153σ）]^6.35exp?（-270000/RT）1.5Zn-SE 250℃:（?）=e^28.55[sinh（0.0148σ）]^4.75exp?（-163000/RT）2.0Zn-SE 250℃:（?）=e^15.19[sinh（0.0128σ）]^3.94exp?（-92000/RT）The high temperature compression experiment shows that the dominant mechanism of high temperature deformation of Mg-Zn-Y alloy after semi-solid+hot extrusion processing technology is dislocation climbing,which can be judged by combining the average stress index（n）and the average apparent activation energy（q）.At the same time,the high temperature compression experiment also proved that the quasicrystal phase has high stability at the experimental temperature.4.Combined with machine learning and semisolid+hot extrusion composite processing technology,the comprehensive mechanical properties of Mg-1.5Zn-0.2Y alloy were significantly optimized.By establishing the sample data set and Kriging proxy model between the semisolid process parameters and mechanical properties of Mg-1.5Zn-0.2Y alloy.Based on the efficient global optimization algorithm of Kriging model,the best combination of process parameters for semisolid+hot extrusion composite processing is obtained through only three iterations:semisolid（562℃+29 min）+hot extrusion（300℃+1.0mm/s）.Mg-1.5Zn-0.2Y alloy with tensile strength of 439±0.48 MPa was obtained.Co MPared with 1.5Zn as extruded alloy,its tensile strength increases by 65.7%.It is proved that the machine learning method is suitable for optimizing semi-solid process parameters and improving the strength of Mg-Zn-Y alloy.5.Based on machine learning method,this paper obtains the optimal semisolid+hot extrusion composite processing parameters of Mg-1.5Zn-0.2Y alloy.The micro quasicrystal phase containing nano quasicrystal particles and agglomerated by nano particles was prepared,and the bimodal grain structure composed of Nano fine crystals and micro coarse crystals was obtained.The strengthening sources of Mg-1.5Zn-0.2Y alloy prepared by combining machine learning and semi-solid+hot extrusion composite processing technology are mainly:（1）Strengthening effect caused by quasicrystal phase.The first is the second phase strengthening provided by the agglomerated large micron quasicrystal phase.The other is the dispersion strengthening provided by the nano quasicrystal phase dispersed by fragmentation.（2）Strengthening effect caused by bimodal grain structure.（3）Precipitation strengthening provided by nano quasicrystal phase precipitated in the grain of Mg-1.5Zn-0.2Y alloy during hot extrusion.The yield strength of semisolid+hot extruded Mg-1.5Zn-0.2Y alloy is higher than that of once extruded Mg-1.5Zn-0.2Y alloy.Quantitative analysis shows that about 75.6MPa comes from fine grain strengthening,and about 172.5 MPa from dislocation strengthening.In summary,this paper pioneered the combination of semisolid isothermal treatment and extrusion process,that is,the semisolid+hot extrusion composite processing technology was used to regulate the content and morphology of the I-phase,and realized the nanocrystallization of I-phase.The preparation process,room temperature toughening mechanism and high temperature deformation behavior of quasicrystalline reinforced Mg-Zn-Y alloy were systematically investigated.In addition,the machine learning method was applied to the semisolid+hot extrusion composite processing technology to further optimize the process parameters and achieve the optimization of the comprehensive properties of the quasicrystallized Mg-Zn-Y alloy.The research results will play an important role in promoting the development of the basic theory of quasicrystalline application,and have important practical significance for the development and application of high strength and toughness deformed Mg alloys.