| In This paper, (1)The effect of Zn content and heat treatment on microstructure and properties of Mg-xZn-1Mn(ZMx1) alloy were investegated by using microstructure observation, scanning electron microscopy (SEM), EDS, tensile test and X-ray diffraction(XRD)analysis; (2) The morphology and distribution of precipitates in ZM61 alloy, orientation relationship and interface between precipitates and matrix, and the form of Mn elements exist in matrix were studied by transmission electron microscopy (TEM), constant-potential electrolysis deep-etching, high-resolution transmission microscopy analysis (HRTEM) and Matalab simulation; (3) The hardening effect of ZM61 alloy for single, two-step aging at different temperatures were researched by micro-hardness testing, while the microstructure evolution and the influence of pre-deformation on hardening effects of ZM61 were discussed by TEM during aging at 180℃,The results showed that, (1)with the Zn content increasing, the segregation is more serious in as-cast alloy; homogenization effect is more difficult; recrystallization grains grow up is more easy during extrusion, while the increased second phase streamlines can hinder the grain growth; the recrystallization grains growth would be restricted during solution treatment, and finer final grain size would be obtained; the grain size of as-aged alloy is consistent with the as-solution treated. The mechanical properties of as-extruded alloy are not sensitive to the changing Zn content, while the strength of as-aged alloy is increased like a parabolic with Zn content increase. When Zn content is higher than solubility limit(6.2wt.%) inα-Mg, the alloy is become more easy to over age, strength increases is slow, and the excess Zn will precipitate as-network intermetallic at the grain boundary and form large particles in grain, they results in less increase in strength but obviously decrease of elongation. Therefore, ZM61 has the best mechanical properties, tensile strength can up to 366Mpa.(2)There are two types precipitates resulted in age-hardening effect in Mg-Zn-Mn alloy, which are the rod shapedβ1'phase and plate likeβ2' phase, both them have laves structure like MgZn2. Two-step aging makes precipitates finer and dispersive which enhanced the strengthening effect. Using the"orienrelat"program written by Matalab can accurately draw the two-phase super-posited diffraction patterns. The orientation relationship ofβ1' andβ2' with matrix are [0 001]β'1 //[11(-|2)0]α, (11(-|2)0)β'1 //(0001)α,å'Œ(0001)β'2//(0001)α, [11(-|2)0]β'1//[1010]α. The interface ofβ1' with matrix andβ2' with matrix is all semi-coherent.(3) Most Mn element soluted in matrix in as-cast microstructure; a small amount of fine Mn dispersoids were formed during homogenization and extrusion process; the majority of Mn element precipitated as regular shape Mn dispersiods after solution treatment, the main shape of these Mn particles are regular polygon, spherical and rod-like; the form of Mn element existence in as-aged samples is similar with those solution treated samples. The result also show thatα-Mn particles can act as heterogeneous nucleus for MgZn2 phase during aging, but those MgZn2 are always coarser than which didn't nucleate onα-Mn particles.(4) At the same temperature, two-step aged ZM61 alloy has faster hardening effect, and the peak hardness is higher than single-step aged alloy. At different temperatures, regardless of single or two-step aging, the higher the aging temperature, the shorter the time to reach peak hardness, the lower the peak hardness, and the easier to over age. The microstructure evolution law of two-step aging is similar to single-step aging for ZM61 alloy. Rod-shapedβ1' phase nucleation and elongation is the main feature of the microstructure before and being reach peak hardness, and the density number and aspect ratio of rod-shapedβ1' phase reached the maximum value at the peak time; After peak time, the number of plate-likeβ2' phase is gradually increase, and rod-shapedβ1' phase is become coarse. But there are also two differences: First, there is almost no plate-likeβ2' phase before and being reach peak hardness in two-step aged ZM61 alloy, while small amounts of plate-likeβ2' phase in single-step aged; Second, two-step aged alloy is more easier to over-aged, and the rapid of plate-likeβ2' phase increasing is faster than single-step aged alloy after peak time.(5) Pre-deformation can significantly accelerate the precipitation kinetics of ZM61 alloy aging, the greater the deformation, the effect of accelerates more obvious. Pre-deformation can also improve the peak hardness of single-step aged ZM61 alloy, but there is little effect on those two-step aged samples. TEM analysis found out that, compared with the undeformed specimens, the precipitates density were increased obviously in which pre-deformed 5% samples, and the precipitates density in pre-deformed 10% alloy consists with pre-deformed 5%. |
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