Study On The Microstructure And Mechanical Properties Of Quasicrystal-strengthening Mg-Zn-Gd Alloys Processed By Cyclic Extrusion And Compression

Quasicrystal is a new kind of secondary phase which has rotational symmetry without translation periodicity. It can not only effectively improve the mechanical properties of Mg alloys but also facilitate dynamic recrystallization and randomization of the texture in Mg alloys during plastic deformation. In this study, three kinds of Mg-Zn-Gd alloys with different alloying element contents were designed and fabricated, including Mg-1.5Zn-0.25 Gd,Mg-3.5Zn-0.6Gd,Mg-4.5Zn-0.75Gd(at.%). Their microstructures, tensile properties and wear properties were studied. These three alloys were subjected to cyclic extrusion and deformation(CEC) at 250°C,300°C,350°C and 400°C respectively. Consequently their microstructures, texture evolution and mechanical properties were studied.Firstly, the microstructures and mechanical properties of the as-cast alloys were investigated. It is found that the alloys consist mainly of ?-Mg and I-phase which exists as large dendritic secondary phase. With the increase of alloying element contents, the volume fraction of I-phase increases and the microstructures of the as-cast alloys were refined. I-phase can drastically improve the strength of the as-cast alloys while undermining the plasticity. As the increase of alloying element contents, the yield strength(YS) and ultimate tensile strength(UTS) were improved and the elongation was decreased. I-phase can effective improve the wear resistance and antifriction property of the as-cast alloys. With the increase of volume fraction of I-phase, the weight loss greatly decreased and the friction coefficient were also reduced. The wear mechanism changed with load from oxidation wear to abrasive wear, and finally fretting wear.It is found in the annealed Mg-1.5Zn-0.25 Gd alloy that, large quantities of secondary phase, including Mg4Zn7?MgZn2 and I-phase, precipitated in the Mg matrix and distributed by partition in the grains, which can be attributed to the composition segregation of the alloying elements in the ascast alloys. The content of alloying element is found to be crucial to the precipitation of nanoscale I-phase. The content of alloying element is found to be crucial to the precipitation of nanoscale I-phase. The nanoscale I-phase ranges in size from a few to dozens of nanometers and grows during annealing, revealing that the precipitation process satisfies the classical nucleation and growth theory.Three alloys were subjected to cyclic extrusion and compression(CEC) at 250°C, 300°C, 350°C and 400°C respectively, the influence of temperature and I-phase fraction on the microstructures, textures and mechanical properties were investigated. With the increase of CEC passes, the microstructures of Mg-Zn-Gd alloys were gradually refined and transformed from partial recrystallization to fully recrystallization, and large dendritic Iphase was broken and dispersed homogeneously in the matrix, meanwhile large quantities of nanoscale I-phase particles precipitated and increased in size, the textures of the wrought alloys were obviously weakened. The YS decreased and the elongation increased with passes. The YS, UTS and elongation of the Mg-1.5Zn-0.25 Gd alloys after 8 passes CEC at 350°C are 159MPa?251MPa and 31.4% respectively.With the increase of processing temperature, the grain size of the CECed samples increased, nanoscale I-phase precipitated in larger scale and increased in size. The YS of the CECed alloys decreased with temperature, while the elongation increased firstly, peaked at 350°C and then decreased. With the increase of the volume fraction of I-phase, the grain size of the CECed alloys decreased, YS increased and elongation decreased.