High Strength Mg-Y-Zn Alloys

Magnesium alloys are the lightest structural materials with high specific strength, good electric conduction, thermal conduction, damping capacity, electromagnetic shielding, formability, as well as easy recycled. It has great application prospect on automobile, electro-communication, aviation, and military industry. However, an important disadvantage of Mg alloys is their low strength, which limited their development and application. The paper investigated the effects of the yttrium, zinc and zirconium additions on microstructure and mechanical properties of Mg-Y-Zn alloys. The effects of cooling rates during solidification, solid solution temperatures and cooling ways, extrusion and two-step extrusion temperatures on microstructure and mechanical properties of Mg-Y-Zn alloys were also investigated. The high strength Mg-Y-Zn alloys can be prepared by equal channel angular pressing (ECAP). Moreover, the hot simulation was conducted on Mg₉₇Y₂Zn₁ alloy to investigate its hot deformation behavior. The strengthening effects of long period stacking ordered structure on Mg-Y-Zn alloys were also analyzed.It is found that the addition of yttrium, zinc and zirconium play very important role in improving microstructure and mechanical properties of Mg-Y-Zn alloys. The as-cast Mg₉₈Y₁Zn₁, Mg₉₇Y₂Zn₁, Mg₉₆Y₃Zn₁, Mg_95.5Y₃Zn_1.5 and Mg₉₄Y₄Zn₂ present a typical dendrite structure. The XRD pattern of as-cast alloys indicates that the Mg₉₈Y₁Zn₁ alloy is consists ofα-Mg, W-Mg₃Zn₃Y₂ and X-Mg12ZnY while Mg₉₇Y₂Zn₁, Mg₉₆Y₃Zn₁, Mg_95.5Y₃Zn_1.5 and Mg₉₄Y₄Zn₂ alloy are consist ofα-Mg and X-Mg12ZnY. After extrusion at 400℃, the dynamic recrystallization (DRX) occurs around secondary phase particles and distorted grains boundaries in Mg₉₇Y₂Zn₁, Mg₉₆Y₃Zn₁ and Mg_95.5Y₃Zn_1.5 alloys, but the DRX process is incomplete. The extruded Mg-Y-Zn alloys exhibit excellent mechanical properties both at ambient temperature and elevated temperature. The addition of zirconium has no effect on phase constituents, but the microstructure of Mg₉₇Y₂Zn₁ alloy is refined and the mechanical properties of Mg₉₇Y₂Zn₁ alloy are improved by the addition of zirconium.The paper investigated the effects of cooling rates of solidification, solid solution temperatures and cooling ways, extrusion and two-step extrusion temperatures on microstructure and mechanical properties of Mg-Y-Zn alloys. It was found that the higher cooling rate of solidification, the finer dendrite and higher mechanical properties for Mg-Y-Zn alloys. It was found that at the temperatures above the melting temperature of X-phase eutectic pockets, the solid solution can has significant effects on microstructure. After extrusion at 400℃, the DRX process is completed in Mg₉₇Y₂Zn₁ alloy solution treated at 415℃, but the DRX process is incomplete in the alloy solution treated at 560℃. After extrusion and equal channel angular pressing (ECAP), the alloy solution treated at 560℃exhibits higher strengths and lower elongation than that of alloy solution treated at 415℃. After extrusion and two-step extrusion, the lower extrusion temperature results in the finer grain size. With the extrusion temperature increasing, the grain size is increasing. Both extrusion and two-step extrusion have similar grain size. The DRX process is completed in extruded Mg₉₇Y₂Zn₁ alloy and the DRX process is incomplete in two-step extruded Mg₉₇Y₂Zn₁ alloy.After ECAP, the secondary phases were broken into uniformly distributed sections. The ultrafine grains were observed after ECAP. The grain size of Mg₉₇Y₂Zn₁ alloy was refined to 200-500nm,and the grain size of Mg_95.5Y₃Zn_1.5 alloy was refined to about 200nm。It is found that the LPS structure contributes to the formation of ultrafine grain size of ECAP processed Mg₉₇Y₂Zn₁ alloy. The ECAP processed Mg-Y-Zn alloy exhibits excellent mechanical properties. The high strengths with the yield strength (YS) of 406.2MPa and the ultimate tensile strength (UTS) of 455.2MPa were obtained for ECAP processed Mg₉₇Y₂Zn₁ alloy. The YS and UTS of ECAP processed Mg_95.5Y₃Zn_1.5 alloy are 444.6MPa and 472.7MPa correspondingly. The elongation is decreased with pass number increasing. It is due to that the process of ECAP introduced micro-cracks in the X-phase, which accelerated the growth and coalescence of the cracks during tensile test.The hot simulation was conducted on Mg₉₇Y₂Zn₁ alloy to investigate its hot deformation behavior. With the increasing of strain rate and decreasing of temperature, the flow stress is increasing gradually. And Mg₉₇Y₂Zn₁ alloy has high stress exponent value.It is also found that, the dislocations accumulated at the interfaces of X phase, the LPS structure and the matrix. The inhibition of the basal slip in Mg-Y-Zn alloys is due to the formation of X phase and the LPS structure. The deformation twin is deflected in the LPS structure region. The stacking fault energy of Mg-Y-Zn alloy is quite low due to high Y, Zn additions. During the deformation, the stacking faults can be easily introduced in and the dislocation would be accumulated at the front of stacking fault region. The dislocation glide can cross stacking faults and leads them to be bent and form steps. Therefore, it is concluded that interaction between LPS structure and dislocation contributes to the strengthening of Mg-Y-Zn alloy.