The Microstructures And Mechanical Properties Of High-Aluminum Magnesium Alloys Processed By ECAP

Magnesium alloys are one of the lightest structural materials for application in aerospace and other transportations. However, magnesium exhibits poor ductility due to its HCP structure with limited slip systems. Moreover, the increasement in their specific strength is limited because of the lack of effective reinforcement measurements. Grain refinement has been one of the preferringly considered measures in improving the strength and plasiticity of magnesium alloy. Equal channel angular pressing (ECAP) provides a technique for producing ultra-fine grained bulk materials in the submicrometer or manometer range by imposing severe plastic deformation.In this paper, the binary magnesium alloys with high aluminum content Mg10Al, Mg15Al, Mg20Al and Mg25Al were cast and then ECAPed for 4 passes by route Bc using a die with 0=90°at 280℃. The alloy Mg15Al after ECAP shows the highest strength, which was futher ECAPed for another 4-pass. Their microstructures before and after ECAP were investiagted by Optical microscopy(OM) scanning electron microscope(SEM) and transmission-type electron microscope(TEM), and the mechanical properties were measured by hardness test and tension at room temperature and compression at 250℃.The main results can be summarized as follows: After 4 passes pressing, the network ofβ-Mg₁₇Al₁₂ existing at the grain boundary ofα-Mg changed into submicron equiaxed particles which not very uniformly distributed into the matrex. The grain size of theα-Mg was refined from 50μm to 1μm in the areas with shortage ofβ-Mg₁₇Al₁₂; while in the areas where a- and (3-phase coexist the grain size was refined below 200 run which was far smaller than the conventional magnesium alloys (AZ61,AZ91) after ECAP pressing. The mutual restriction between the two phases a andβgreatly reduced the speed of their dynamic recrystallization during ECAP.The mechanical properties, such as hardness, tensile strength and elongation, of the high-aluminum magnisium alloys were all improved significantly after ECAP. The ECAPed Mg15Al exhibited the highest strength 268MPa and the elongation was improved from 0.05% to 7.4%; For Mg20Al, the tensile strength was improved from 42 MPa to 209 MPa, and the elongation from near 0% to 3.3%; The strength of Mg25Al improved from 54MPa to 182MPa, and the from near 0% to 1.3%. According to the microstructures of Mg alloys, it can be thought that the content of Mg₁₇Al₁₂ in Mg10Al is low, so the grain refinement is not obvious, and so is the strengthening. When the content of Al was too high, Mg25Al for instance, the fraction of Mg₁₇Al₁₂ phase is too large to improve the mechanical properties of the alloy. The fracture morphology showed that fracture type changed from cleavage fracture to quasi-cleavage afer pressing, and there was large quantity of tough torn grain in the fractures. Comparing the mechanical properties of the specimens sufferd 8 passes pressing with that of the 4 passes pressing we found that the strength decreased while the elongation increased, the possible reason was the formation of texture. The present work indicates that the excellent mechanical properties can be obtained in the as-cast binary high-aluminum magnisium alloy with Al content ranging 15-20 % after ECAP processing. It is expected that these properties can be further improved by alloying with other elements such as Zn, Mn etc. as that employed in the conventional Mg-Al alloy system.