Microstructures And Properties Of High-Strength Mg-Zn-Zr-Er Alloys

Be the highest strength in commercial magnesium alloys, ZK60 alloy also has the highest specific strength in all the magnesium alloys. However, it still exist some defect such as low-absolute strength, poor-plasticity. So, to improve the strength while the plastic of the magnesium alloys is improved, became the urgently key problem to be solved in research and applications of magnesium alloy.In the present paper, base on the ZK60 magnesium alloy, in order to meet precipitation strength by rare-earth phase and solution strength by Zn content in matrix, to improve the comprehensive mechanical property, Zn content was improved to 9%, meanwhile different Er(wt%=0,0.5%,1.0%,2.0%,4.0%) content were added. Optical microscope, X-ray diffraction analysis(XRD), Differential Scanning Caborimetry(DSC), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS) and transmission electron microscopy(TEM) were used to study the microstructure of as-cast, heat-treatment before as-extruded, as-extruded and heat-treatment after as-extruded state. The forms of existence of Er in the alloys and its effect to microstructure and mechanical properties were studied under different conditions.The result indicate that the microstructure of as-cast Mg-9Zn-0.6Zr magnesium alloy was mainly consisted ofα-Mg and large amount of Mg-Zn eutectic phase. With Er addition, two types of thermally stable Er-bearing compounds with distinct Zn/Er ratio formed. The volume fraction of the second-phase particles increased with the increase of Er addition. The dendrite arm spacing gradually decreased with the increase of the Er addition, being the smallest at 2.0%Er, the average grain size is about 32μm(102μm without Er addition).Dynamic recrystallization occurred obviously during hot deformation. The addition of Er could bring about homogenous deformation behaviour and recrystallization behaviour to the alloy and it also promote dynamic precipitation of nano-scale spherical MgZn₂ phase to improve the strength of matrix. With the Er addition, the volume of nano-scale spherical MgZn₂ phase decreased. With 2.0%Er addition, the grain size of as-extruded alloy is smallest with its the average long axis and short axis are in the range of 7⁸μm and 2³Î¼m, respectively. The mechanical properties of Mg-9Zn-0.6Zr alloy with 0.5% Er content areσb=366MPa,σ0.2=313MPa,δ=22%.Solution, age and solution plus age treatment have been done to as-extruded alloys with different Er addition. It is suggested that Er addition can increase the microstructure stability drastically and the Er-bearing particles have hindered grain growth during the solution treatment, leading to the smaller grain size in the Er-bearing alloy. Solution treatment at 400℃for 1.5h was carried out after extrusion. Perfect recrystallization occurred in the alloys without Er and with 0.5%Er. With more Er addition, some non-recrystallized grains exist in the alloys. While extend solid solution time to 12h, alloys with Er content 1.0%,2.0%,4.0% were consisted of equiaxial grain with 7⁸μm in average scale. Because of a large amount of MgZn₂ phase were precipitated in age treatment at 200℃for 10h after solution treatment, the yield strength and the elongation of the alloys are increased. Age treatment after extrusion, the yield strength increased. With the Er addition, the increased range of the yield strength is decreased. It is ascribed to the precipitate of the MgZn₂ in the matrix. Compare with age treatment, the dynamic precipitation of fine MgZn₂ particles with a modified spherical morphology occurred during hot extrusion are more effective than static precipitation of MgZn₂ particles through subsequent long-time aging in improving the mechanical property. The mechanical properties of Mg-9Zn-0.6Zr alloy with 0.5% Er contentσb=372MPa,σ0.2=342MPa,δ=16%.