| First FactSage software has been used to optimize the composition of the Mg-Zn-Cu alloy which has the best strengthening effect. The best alloy compositions are Mg-6Zn-1Cu and Mg-6Zn-2Cu alloys. By doping the forth element Bi Mg-6Zn-1Cu-0.5Bi and Mg-6Zn-2Cu-0.5Bi alloys were obtained respectively. Mg-6Zn-(1-2)Cu(-0.5Bi) magnesium alloy has been prepared via metal mold casting method. Using microhardness tester, optical microscope (OM), X-ray diffractometer (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM), has been analyzed magnesium alloys. The experimental results showed that:The microstructure of as-cast Mg-6Zn-(1-2)Cu magnesium alloy was mainly composed of a-Mg, eutectic structrue (a-Mg+CuMgZn+MgZn2) formed at grain boundary. The morphology of as-cast Mg-6Zn-(1-2)Cu changed from discontinuous distribution into a continuous distribution of grain boundary along with the increase of the content of Cu. At the same time, the hardness was gradually increased. By doping element Bi, the Mg3Bi2phase in the as-cast distributed along the grain boundary of eutectic structure, the hardness was improved at the same time. By FactSage software and DSC analysis of the as-cast alloys, selected solid solution temperature for each alloy respectively. With the increase of Cu content, solid solution temperature was increased. After solid solution treatment, most of the non-equilibrium eutectic organization in the grain boundary was dissolved. In the process of aging, the continuous rod eutectic structure on the grain boundary was changed into round particles to some extent. Age-hardening curves of Mg-6Zn-(1-2)Cu(-0.5Bi) magnesium alloys at160℃showed typical aging effect. Mg-6Zn-1Cu alloy was higher than that of the Mg-6Zn-2Cu alloy. The peak aging hardness for Mg-6Zn-1Cu alloy alloy aged34h was HV75.94. With the adding of Cu formated CuMgZn phase and the MgZn2phase was decreased. The increasing of Cu make increasing the formation temperature of MgZn2phase, which reduce the precipation amount of MgZn2. Adding element Bi the hardness increased with the aging process, it is because Mg3Bi2was formed with the adding of Bi and element Bi promoted the formation of MgZn2phase. Microstructure analysis showed that the Mg-6Zn-lCu alloy aged at160℃after8h, a large number of crystal defect such as edge dislocation, dislocation loops, jog which hinders the dislocation motion existed in matrix and improved the strength of the alloys. A small amount of moire stripe occurred due to the existence of some precipitates which lattice stripe is inconsistent with the matrix in early aging precipitation. Besides, plate-like G.P. zones were detected. With the increase of aging time, a large number of granular-like, short rod-like and bulk-like precipitated distributed in the matrix of Mg-6Zn-1Cu alloy. The short rod-like precipitated grew up certain direction and gradually interconnected. This phenomenon inhibited β1’-MgZn2to grow up to a certain extent and delayed the occurred of overaging phase of β. It is worth mentioning that the interconnected rod-like phase never through the boundary, the precipitate free zone (PFZ) was kepe at the grain boundary. By adding Bi, the Mg-6Zn-1Cu-1Bi alloy compared with Mg-6Zn-1lCu alloy at the long time ageing, the precipitation was more and smaller and the phenomenom of short rod-like phase grew up at the certain diretion interconnected each was decreased. This is because the Mg3Bi2and β1’-MgZn2phase was bond together, which can promote nucleation of β1’-MgZn2phase and inhibite grow up of β1’-MgZn2. To help improve the quality of alloy aging hardness, and slow the aging phase alloy aging hardness speed. |
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