Study On The Microstructures And Mechanical Properties Of Mg-zn-si Alloys Processed By Ecap

Due to their merits such as high specific strength and specific stiffness, good size stability, favorable damping of vibration and easy reclamation, magnesium alloys are known as"green engineering metal structure materials of the 21th century". Intermetallic compound Mg2Si is a perfect reinforcing phase of light mental based composite, because it has high melting point, high rigidity, high modulus, low density and low coefficient of thermal expansion. In the as-cast alloys, course Chinese character-shaped and long-strip Mg2Si phase can cut crack matrix easily, reduce the mechanical properties, so its application is limited. By producing severe plastic deformation, equal channel angular pressing (ECAP) can refine the matrix and the second phase effectively and achieve the purpose of improving mechanical properties of materials.In this paper, the three kinds of alloys ZK31,ZK31+2%Si, ZK31+4%Si were cast and then ECAPed for 4 passes and 8 passes by route Bc using a die withΦ=90°at 300℃. By means of optical microscopy (OM) and scanning electron microscopy(SEM), the microstructure of the as-cast and ECAPed alloys were examined.The results showed that, in the as-cast alloys, the Chinese character-shaped or long-strip Mg2Si phase can cut the matrix seriously, resulting in mechanical properties of the as-cast ZK31 +2%Si alloy and ZK31 +4%Si alloy lower than that of the as-cast ZK31 alloy.After four passes of ECAP, the matrix and Mg2Si phase of the three kinds of alloys were significantly refined, but the refinement level of the ZK31 +2% Si alloy and ZK31 +4% Si alloy was greater than ZK31 alloy. After ECAP, the Chinese character-shaped or long-strip Mg2Si phase was broken into small particles. The small particles could hinder the dynamic recovery in the ECAP process, which was conducive to the refined alloy substrate. After 4 passes of ECAP, the mechanical properties of the three kinds of alloys were greatly improved, and high temperature creep properties of ZK31 and ZK31 +4% Si alloy was also improved. Tensile strength of ZK31 alloy was improved from 175.4MPa to 224.3MPa; elongation was improved from 13.81% to 18.15%. Tensile strength of ZK31+2%Si alloy was improved from 129.5MPa to 246.7MPa; elongation was improved from 5.58% to 28.06%. Tensile strength of ZK31+4%Si alloy was improved from 138.7MPa to 248.5MPa; elongation was improved from 9.55% to 22.60%. The improvement of mechanical properties of ZK31 +2% Si alloy and ZK31 +4% Si alloy was greater than that of ZK31 alloy. Due to the elimination of microstructure defect, the high temperature creep performance ZK31 alloy and ZK31 +4% Si alloy was greatly improved. Compared with the 4 passes, after 8 passes, the alloy matrix growed up a little, and there was no significant improvement in mechanical properties.Fracture morphology analysis showed that fracture mode of as-cast alloys was brittle fracture and the fracture was mainly formed by dissociation steps and tearing edges. After ECAP, fracture mode of ZK31 was still brittle fracture, and fracture was formed dissociation steps and a small amount of dimples. Fracture mode of ZK31 +2% Si and ZK31 +4%Si alloy was ductile fracture and the fracture was composed of dimples.This study indicates that ECAP can refine microstructure of alloys and improve mechanical properties of at room temperature and high temperature creep properties significantly. Addition of Mg2Si phase is beneficial to the improvement of refinement and mechanical properties.