Effect Of Heat Treatment On The Microstructure And Mechanical Properties Of Mg- Er-Zn-(Zr) Alloys

Magnesium and magnesium alloys have good electromagnetic shielding performance, high specific strength and stiffness, outstanding machinability, easy recycled and low density, are increasingly used in aerospace, automotive, electronics and other fields. However, the low tensile strength and ductility greatly limits its further development. The paper investigated the effects of the zinc and zirconium additions on microstructure and mechanical properties of Mg-Er-Zn-(Zr) alloys by optical microscope(OM), scanning electron microscopy(SEM) with energy dispersive spectrum(EDS), X-ray diffraction(XRD), differential thermal analysis(DTA), hardness testing and compressive testing machine. The effects of solid solution temperature, time and cooling ways, aging temperature and time on microstructure and mechanical properties of Mg-Y-Zn alloys were also investigated. Through this subject study, some conclusions can be drew:1). Through the research of different Zn in Mg97.82-xEr2ZnxZr0.18 alloys, the addition of Zn element changes and refines the microstructure of the as-cast Mg97.82-xEr2ZnxZr0.18 alloys significantly. With the increase of the content of Zn, the grain sizes of the alloys present decrease trend. The microstructure of Mg97.82Er2Zr0.18 alloy without Zn is composed of the α-Mg phase. The Mg97.57Er2Zn0.25Zr0.18, Mg97.32Er2Zn0.5Zr0.18 and Mg97.07Er2Zn0.75Zr0.18 alloys are all composed of α-Mg phase and lath-shaped X phase. While the Mg96.82Er2Zn1Zr0.18 alloy is composed of α-Mg phase, X phase and a few net-shape W phase. The volume fraction of X phase increases with the increase of zinc content.The mechanical properties testing show that the alloys which add zinc element have higher vickers hardness and strength than the alloys without Zn. With the increase of the content of Zn, the vickers hardness and strength of Mg97.82-xEr2ZnxZr0.18 alloys increase, but the elongation decrease. When the Zn content is 1.0 at.%, the vickers hardness, compressive strength and yield strength of the alloy come to the maximum, while the elongation reach to minimum, the values are 63.9Hv、320.6 MPa、146.5MPa and 12.4%, respectively.2). As the addition of Zr, the microstructure of the as-cast Mg97.5Er2Zn0.5 alloy is obvious refined, the volume fraction of X phase in grain boundary decreases, the solid solution time of X phase is shortened, and the mechanical properties of the as-cast and solution-treated al oys are improved significantly.3). According to the optimizing of solid solution treatment parameters, the optimum solid solution treatment conditions for the Mg97.82-xEr2ZnxZr0.18 alloys are as97.82-xr2x0.18follows: Mg97.57Er2Zn0.25Zr0.18— 540℃×12h, Mg97.32Er2Zn0.5Zr0.18— 540℃×16h, Mg97.07Er2Zn0.75Zr0.18—540℃×32h, Mg96.82Er2Zn1Zr0.18—540℃×32h. So at the same solid solution temperature, the solid solution time of Mg97.82-xEr2ZnxZr0.18 alloys increase with the increase of Zn content. The X phases of the first three alloys completely dissolve into α-Mg matrix and have no new phase precipitate. The X phases of Mg96.82Er2Zn1Zr0.18 alloy completely dissolve and however, W phases precipitate.The mechanical properties testing show that the mechanical properties of solution-treated Mg97.82-xEr2ZnxZr0.18 alloys improve obviously comparing to the as-cast alloys. With the increase of Zn content, the vickers hardness and strength of the solution-treated alloys increase, but the elongation decreases. Comparing to water cooled alloys, the alloys which use furnace cooled have more obvious LPSO phase in Mg matrix, and the vickers hardness and strength are lower, while the elongation is higher.4). After aging treatment at 180℃, 300℃, 400℃ and 450℃, respectively, the Mg97.82-xEr2ZnxZr0.18 alloys have no obvious peak aging behavior, the phase constituents show little change and the mechanical properties change a little compare to the solution-treated alloys. With the increase of Zn content, the mechanical properties of the age-treated alloys increase. Thus the aging treatment can not improve the microstructure and mechanical properties of Mg97.82-xEr2ZnxZr0.18 alloys effectively.