Effect Of External Field And Ca And Sr On Microstructure And Mechanical Properties Of Mg-Zn-Y Alloy Reinforced By Quasicrystals

Due to its quasi-periodic lattice structure, icosahedral quasicrystals phase(I-phase) possesses unique properties such as high hardness, high strength, low interface energy and high thermal stability, etc., which make it become a natural choice as a strengthening phase. When magnesium alloys consist of I-phase as a second phase, they exhibit excellent room- and high-temperature mechanical properties. However, magnesium alloys reinforced by I-phase have a dendritic cast structure with coarse intergranular I-phase, which is harmful to the mechanical properties of these alloys. Therefore, in this thesis, the effects of pulsed magnetic field(PMF), ultrasonic treatment and Ca and Sr elements on the microstructure and mechanical properties of Mg93Zn6 Y alloy were investigated. The main conclusions can be summarized as follows:1)With increasing discharging voltage, microstructure of Mg93Zn6 Y alloy were further refined in the range of 0~300V. The morphology of primary ?-Mg was modified from dendritic to rosette-like and particle-like. The morphology of I-phase was modified from continuous reticular to discontinuous reticular and average area ratio of I-phase was decreased. With increasing discharging frequency, microstructure of alloy were further refined in the range of 0~10Hz. The morphology of primary ?-Mg was modified from dendritic to rosette-like. The morphology of I-phase was modified from continuous reticular to discontinuous reticular and island-like, and average area ratio of I-phase was reduced. With decreasing the casting temperature, primary ?-Mg was further refined, but the size of I-phase increased first and then decreased.2)With increasing discharging voltage or discharging frequency, the tensile strength and elongation of Mg93Zn6 Y alloy were further increased. With decreasing pouring temperature, the tensile strength and elongation of alloy increased first and then decreased. When discharging voltage, discharging frequency and pouring temperature were 300 V, 10 Hz and 720?, the tensile strength and elongation of alloy were increased by 77.6% and 90.9% respectively, compared with those of the alloy without PMF treatment.3) With increasing ultrasonic power or processing time, microstructure of Mg93Zn6 Y alloy were further refined. The morphology of primary ?-Mg was modified from dendritic to rosette-like and particle-like. The morphology of I-phase was modified from continuous reticular to discontinuous reticular and island-like, and average area ratio of I-phase was also decreased. With decreasing processing temperature, the primary ?-Mg was refined gradually, but the size of I-phase increased first and then decreased.4)With increasing ultrasonic power or processing time, the tensile strength and elongation of Mg93Zn6 Y alloy were further increased. With increasing processing temperature, the tensile strength and elongation of alloy increased first and then decreased. When ultrasonic power, processing time and processing temperature were 600W?90s and 720?, the tensile strength and elongation of alloy were increased by 80.4% and 83.3% respectively, compared with those of the alloy without ultrasonic treatment.5)Except ?-Mg and I-phase, the addition of Ca element to Mg93Zn6 Y alloy generated Ca2Mg6Zn3 phase. With increasing the addition amount of Ca element, microstructure of alloy were refined gradually. The morphology of primary ?-Mg was modified from dendritic to rosette-like. The morphology of the second phase was modified from reunion and discontinuous reticular to discontinuous reticular and island-like, and average area ratio of the second phase was decreased. With increasing the addition amount of Ca element, the tensile strength and elongation of alloy were increased gradually. Upon adding 0.8wt.% Ca element, the tensile strength and elongation of alloy were increased by 38.6% and 172.7% respectively, compared with those of the unrefined alloy.6)The addition of Sr element to Mg93Zn6 Y alloy generated S-phase(Mg6Zn2Sr) except ?-Mg and I-phase. With increasing the addition amount of Sr element, the grain size of primary ?-Mg decreased gradually, and the morphology of primary ?-Mg was modified from dendritic to rosette-like. The morphology of the second phase was modified from reunion and discontinuous reticular to discontinuous reticular and island-like. With increasing the addition amount of Sr element, the tensile strength and elongation of alloy were increased gradually. Upon adding 0.8wt.% Sr element, the tensile strength and elongation of alloy were increased by 31.5% and 127.3% respectively, compared with those of the unrefined alloy.