Microstructure And Mechanical Properties Of AZ31Mg Alloy With Surface Mechanical Attrition Treatment

In recent years, gradient nano structure materials have attracted much attention due to these special microstructure and excellent properties. Surface mechanical attrition treatment(SMAT), as the most effective surface nanocrystallization way to obtain the gradient nano structure, has been successfully applied to copper, nickel, stainless steels and other metal materials, and has made significantly improvement in their mechanical properties. However, there are few reports on the mechanical properties of magnesium(Mg) alloy with gradient nano structures. Therfore, in the present work, SMAT has been carried out on the AZ31 Mg alloys prepared by equal channel angular pressing(ECAP) and commercial rolling, respectively. By means of optical microscopy(OM), X- ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), microhardness test and mniaxial tensile test, the microstructure and mechanical properties of the AZ31 Mg alloys with SMAT are investigated. The effect of SMAT on the microstructure and mechanical properties of the two types of AZ31 Mg alloys has been discussed and analyzed. Finally, the dynamic tensile deformation behavior and fracture behavior of the AZ31 magnesium alloy with gradient structure for different grain sizes have been studied by using SEM in situ tensile test.The main conclusions are as follows:1. After SMAT, a gradient nano structure is formed in the surface of the ECAPed AZ31 Mg alloy along the thickness direction. The surface layer of the alloy subjected to SMAT for 10 min consists of nanosized grains with an average grain size of 27.4 nm and random crystallographic orientations. The thickness of the nanocrytalline surface layer is about 50μm. Then an ultra-fined grain layer appears to the depth about 80 μm to 90 μm. Below about 90 μm depth are typical as-received structures, but with a certain volume fraction of twins induced by SMAT. After SMAT for a long time, the volume fraction of twins can reach to 30%. SMAT can improve the microhardness of the ECAPed alloy. A depth-dependent gradient microhardness is formed in ECAPed AZ31 Mg alloy after SMAT. At the same time, the yield strength and the ultimate tensile strength of the alloy are significantly increased, but with a decrease in fracture elongation. As compared to the as-received alloy, the SMATed one exhibits 121.8 % higher yielding strength after SMAT for 3 min. However, with the increase of SMAT duration, the improvement of yield strength becomes smaller. Moreover, the SMATed samples exhibits a quite different strain hardening behavior with the as-received alloy. In the hardening stage III, the SMATed alloy shows a higher strain hardening rate and the strain hardening rate increases with the increasing duration of SMAT process. While, the SMATed alloy shows a lower strain hardening rate and the strain hardening rate decreases with the increasing duration of SMAT process in the stage IV.2. A gradient nano structure can also be observrd in the surface of the commercial rolled AZ31 Mg alloy along the thickness direction after SMAT. The thickness of the gradient nano structure layer increase with the increase of the SMAT duration. The surface layer of the alloy subjected to SMAT for 10 min consists of nanosized grains with an average grain size of 42 nm and random crystallographic orientations. An ultra-fined grain layer appears in the depth of 165 μm. Below 165 μm depth are typical as-received structures. A depth-dependent gradient microhardness is also formed in commercial rolled AZ31 Mg alloy after SMAT. Similarly, the yield strength and the ultimate tensile strength of the commercial rolled AZ31 Mg alloy are significantly increased after SMAT, but with a decrease in fracture elongation. With the increase of SMAT duration, the improvement of yield strength becomes smaller. Moreover, the plastic anisotropy of the commercial rolled Mg alloy increases significantly after SMAT for a long time, which is related to the special deformation behavior of gradient nano structure.3. A comparson in microstructure and mechanical properties between ECAPed and commercial rolled AZ31 Mg alloys with SMAT has been done, whose average grain size is 5.5 μm and 2.8 μm, respectively. The results show that a gradient nanocrystalline surface layer with similar thickness is formed on both ECAPed and commercial rolled AZ31 Mg alloys after SMAT for 3 min. However, the ECAPed alloy retains respectable ductility, due to its good strain harden behavior. Therefore, the ECAPed AZ31 Mg alloy subjected to SMAT exhibits good strength- plasticity.4. The difference in the tensile deformation behavior between fined-grain and coarsed-grain microstructure, has been investigated by using the SEM in-situ tensile test. The four different tensile stages of the fined-grain and coarsed-grain microstructure within the onside gradient nano structure, including initial state, elastic deformation, plastic deformation and fracture, has been dynamicly observed. During the tensile deformation, the intergranular crack initiation, grain deformation, intergranular crack growth, parallel slip line, transgranular crack initiation and growth appears continuously. These features are all affected by the grain size. Through the dynamic observation on the tensile deformation and the tensile fracture analysis, the effect of the gradient structure on the tensile deformation of surface nanocrystallized Mg alloy has been put forward.