Research On The Mg-Y-Zn-Mn Alloys With Long-Period Stacking Ordered Structures

In recent years, Magnesium alloys have become increasingly attractive due to their applications in automobile and aerospace industries. The new developed Mg-RE-Zn alloys with long period stacking ordered (LPSO) structures overcome the disadvantages which limited the application of Mg alloys such as low ductility, strength and poor high-temperature properties, exhibited excellent microstructures and unique mechanical properties especially good high-temperature properties.In this study, Mg-Zn-Y-Mn alloys with LPSO structures were prepared by conventional permanent mold casting method combined with heat treatment. The effect of Mn addition on formation and transformation of LPSO structure phases, microstructures and mechanical properties were studied.The microstructures were analysed by XRD, EDS, SEM, TEM and OM methods. The microstructure evolution of Mg-Y-Zn-Mn alloys in different technological conditions were also studied. Besides, the strengthening mechanism of as-extruded LPSO structures Mg94Zn2.5Y2.5Mn1alloys was discussed by the direct extrusion. The results show that:The as-cast Mg95.xZn2.5Y2.5Mnx alloy is consisted of a-Mg phase, X-phase (Mg12Y,Znl518R-LPSO) and the W-phase (Mg3Zn3Y2) located at grain boundaries.The area fraction of X-phase increased to about17%after adding1.0%Mn, which strengthened the alloy. And W-phase reduced by5%. The grain size of the alloys has been obviously decreased, which produced a fine-grain strengthening effect.The fine plate-like precipitates with14H-LPSO structure are generated in the supersaturated a-Mg matrix after solution treatment at500℃for35h and air-cooling. The forming of this14H-LPSO structure in the a-Mg matrix results in precipitation hardening. After solution treatment, the W-phase was partly dissolved and broken into small granules,which reduced the probability of crack during the deformation process.Granules Mg3Zn3Y2phase reveals a more dispersive distribution during direct extrusion. After direct extrusion, the X-phases and14H-LPSO phases were bended with small kinking angles. After200℃aging treatment for70hours, the microstructures were homogenized, the defects of extrusion and the residual stresses were removed.The high strength Mg95.xZn2.5Y2.5Mnx alloy was developed successfully by direct extrusion. The ultimate tensile strength and elongation of the as-extruded alloy at room temperature are345MPa,22.4%.