Microstructure And Mechanical Properties Of High Strength Mg-Zn-Al(-X) Alloys

Due to the remarkable casting characteristics and exceptional aging strengthening potential,Mg-Zn-Al based magnesium alloys have received a lot of interest from researchers in recent years.However,the low room temperature strength of Mg-Zn-Al based alloys restricts the industrial application of them to a certain extent.Therefore,there is significant scientific and practical benefit in developing novel high-strength Mg-Zn-Al alloy.This study employs Sm,Mn,and Gd elements for microalloying treatment on a Mg-5Zn-4Al(wt.%,ZA54)alloy basis.High-pressure die-casting and gravity casting techniques were used to produce the Mg-5Zn-4Al(-X,wt.%,X=Sm,Mn,Gd)alloys,and experiments with extrusion were performed on the gravity cast alloys.In this work,the aging behavior of Mg-5Zn-4Al(-X)alloys in high-pressure die-casting and differential temperature extrusion states were investigated along with the microstructure and room temperature mechanical properties of these alloys.The results show that the average grain size of high-pressure die-cast alloys are less than 10 μm.The semi-continuous network distributed I-phase is the primary intermetallic phase of die-cast alloys.Aging treatment at 120 °C during 150 hours resulted in a considerable rise of the yield strength of die-cast alloys.The yield strength and aging performance of the die-cast alloys were not considerably impacted by the inclusion of trace amounts of Sm and Mn elements.However,the fluidity during the die-casting process of alloy melt and the plasticity of alloys were enhanced by Sm and Mn elements.After aging treatment,a significant amount of roughly spherical I-phase precipitates with equivalent radii and volume fractions of ～8.3 nm and ～1.7%,respectively,formed in the Mg-5Zn-4Al alloy.The yield strength of alloys significantly rises as a result of the distributed distribution of the precipitated I phase in the Mg matrix.The appearance of the Al-Sm phase,Al-Mn phase,and Al-Gd phase,respectively,in the gravity cast Mg-5Zn-4Al(-X)alloys was formed by the addition of trace amounts of the elements Sm,Mn,and Gd,and its resulted in a reduction in the network Mg-Zn-Al phases in the alloys.The network Mg-Zn-Al phases in the Mg-5Zn-4Al(-X)alloys exhibit solid solution behavior that is influenced by the trace Sm,Mn,and Gd elements,and those elements enhances the high-temperature resistance of alloys.After being extruded(designated as DTE1 for the temperature of extruder and mold is 140 °C and the temperature of the ingot is 330 °C,and DTE2 for the temperature of extruder and mold is70 °C and the temperature of the ingot is450 °C)and aged,the experimental values of the tensile yield strength of the DTE1-ZA54,DTE1-ZASm540,DTE2-ZAM540,DTE2-ZAGd540 was ～305 MPa,～328 MPa,～360 MPa and ～363 MPa.When compared to DTE1-ZA54 alloy,increase in yield strength of DTE1-ZASm540 alloy is primarily the result of solid solution strengthening,whereas the increases in yield strength of DTE2-ZAM540 and DTE2-ZAGd540 alloy are primarily the result of dispersion strengthening.