| As-cast magnesium alloys can be refined by thermal deformation.The comprehensive mechanical properties of magnesium alloys can be improved by means of fine-grain strengthening,dispersion strengthening and texture strengthening,so as to meet their different needs.Demand in areas such as the automotive,aerospace industry,and other fields.Adding cheap Si element to magnesium alloy can form Mg2Si phase with thermal stability,which can significantly improve the high temperature performance of magnesium alloy and overcome the disadvantage of high cost of rare earth magnesium alloy,but the solid solubility of Si in Mg is Small,almost 0,when the content is greater than1.34wt%,massive Mg2Si phase will be formed,which will reduce the mechanical properties of the alloy,so the fragmentation of Mg2Si phase,thereby improving the performance of the alloy has become an important research hotspot.In this paper,Mg-5Zn-2Al-x Si(x=0.5,1,1.5wt.%)alloys is prepared by conventional casting method,the structural characteristics of the as-cast alloy are studied,and the as-cast alloy is subjected to different homogenization treatments to determine the best homogenization treatment process.On this basis,the homogenized Mg-5Zn-2Al-x Si alloys was subjected to extrusion and equal channel angular extrusion(ECAP)to study the structure and performance characteristics of the extruded alloy.However,after a single deformation,the bulk Mg2Si phase in the Mg-5Zn-2Al-1.5Si alloy was not completely broken,so the two-step ECAP extrusion deformation of the extruded Mg-5Zn-2Al-1.5Si alloy to achieve low-temperature ECAP extrusion Deformation,the characteristics of the structure and properties of the alloy after cooling deformation were studied,and the mechanism of Mg2Si phase and alloy grain refinement was discussed.For the Mg-5Zn-2Al-1.5Si alloy after being extruded,the creep resistance mechanism of the alloy was explored.The main conclusions are as follows:(1).As-cast Mg-5Zn-2Al-x Si Alloy is mainly composed of?-Mg,Mg32(Al,Zn)49 and Mg2Si phases.With the increase of Si content,the Mg2Si phase gradually changed from a filament to a short rod shape.Finally,when the Si content was 1.5wt.%,massive Mg2Si phase appeared,and both the yield strength and tensile strength of the alloy increased,and the elongation rate has been decreasing.(2).After homogenization treatment of Mg-5Zn-2Al-1.5Si alloy,the coarse Mg32(Al,Zn)49 phase disappears,the thin rod-like Mg2Si phase is passivated into short rods,and the bulk Mg2Si phase gradually transforms into a polygon;The best homogenization heat treatment process is 345?×40h.(3).Mg-5Zn-2Al-x Si(x=0.5,1,1.5wt.%)after extrusion deformation,all the alloys undergo complete dynamic recrystallization,and the grains are significantly refined.With the increase of Si content,the volume fraction of Mg2Si phase is increasing,resulting in the gradual decrease of grains,which are5.11?m,5.64?m and 4.33?m in sequence.Mg-5Zn-2Al-0.5Si has the best performance,with a yield strength(YS)of 157 MPa,a tensile strength(UTS)of282 MPa,and an elongation of 26.3%.The main strengthening mechanism of the strength increase is grain boundary strengthening,and the second phase dispersion strengthening,The alloy has a strong fiber texture after extrusion.(4).After the extrusion of Mg-5Zn-2Al-1.5Si alloy and the composite deformation of ECAP,the distribution of Mg2Si phase is more uniform,and the micro Mg32(Al,Zn)49 phase is precipitated.These small phases can pin the grain boundaries,Refine the grain size and improve the performance of the alloy,and(0002)texture migration 32°.(5).Mg-5Zn-2Al-1.5Si alloy,under the condition of stress of 50-70 MPa and temperature of 150?,the stress index is 10.28.The reason for the higher stress index is that the part is not broken after positive extrusion The Mg2Si phase cracks during the extrusion process,which causes Mg2Si to crack during the creep process;under the conditions of stress of 50MPa and temperature of150?-200?,the creep activation energy is 139KJ.mol-1,dislocation climbing is the main control mechanism in the creep process. |
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