Microstructure evolution and mechanical properties of Mg-x%Zn-1%Mn (x=4, 5, 6, 7, 8, 9) wrought magnesium alloys

The roles of Zn content and thermo-mechanical treatment in affecting microstructures and mechanical properties of Mg-x%Zn-1%Mn (mass fraction, x=4, 5, 6, 7, 8, 9) wrought Mg alloys were investigated. The microstructure was extremely refined by dynamic recrystallization (DRC) during extrusion. With increasing Zn content, the DRC grains tended to grow up, at the same time, more second phase streamlines would be present, which restricted the further growing. During solution treatment, the DRC grains would rapidly grow up; however, higher Zn content could hinder the grain boundary expanding, which results in finer ultimate grains. MgZn2 dispersoid particles which are coherent with the matrix would precipitate from the supersaturated solid solution during the one-step aging process, and nano-sized GP zones formed during the pre-aging stage of the two-step aging provide a huge amount of effective nuclei for the MgZn2 phases formed in the second stage, which makes the MgZn2 particles much finer and more dispersed. The mechanical properties of as-extruded samples were not so sensitive to the variation of Zn content, the tensile strength fluctuates between 300 and 320 MPa, and the elongation maintains a high value between 11% and 14%. The strength of aged samples rises as a parabolic curve with increasing Zn content, specifically, the tensile strength of one-step aged samples rises from 278 to 374 MPa, and that of two-step aged ones rises from 284 to 378 MPa, yet the elongation of all aged samples is below 8%. When Zn content exceeds its solid solution limit in Mg-Zn system (6.2%, mass fraction), the strength rises slowly but the elongation deteriorates sharply, so a Mg-Zn-Mn alloy with 6% Zn possesses the best mechanical properties, that is, the tensile strengths after one-and two-step aging are 352 and 366 MPa, respectively, and the corresponding elongations are 7.98% and 5.2%, respectively.