Research On The High Strength Mg-Y-Zn Alloys With Long-period Stacking Ordered Structures

Magnesium alloys, as one of the lightest structural materials with high specific strength, have become increasingly attractive due to their applications in automobile and aerospace industries. However, these applications are seriously limited by their disadvantages, such as low strength and poor high-temperature properties. In the last decade, magnesium alloys with long-period stacking ordered (LPSO) structures, exhibiting excellent mechanical properties and unique microstructures, have been developed in the ternary Mg-RE-Zn systems (where RE represents rare earth elements), accordingly attracting progressive interest.In this paper, Mg-Y-Zn alloys with long-period stacking ordered structures were processed by the method of conventional permanent mould casting and equal channel angular pressing (ECAP). The forming and strengthening mechanism of LPSO structures were investigated. In addition, the microstructure evolution and mechanical properties of Mg-Y-Zn alloys in different technological conditions were also studied. The results show that:1. The Mg-Y-Zn alloys with LPSO structure were developed successfully by conventional permanent mould casting method. The Mg94Zn2Y4alloy has the optimized strength in the Mg100-3xZnxY2x alloys at room and elevated temperature. The matrix of the Mg94Zn2Y4alloy is consisted of major2H-Mg with slices stacking faults (SF) in it. The X-phase in the alloy is composed of mainly18R LPSO structure and some Mg segments sandwiched within it.2. The mechanical property was improved by adding trace of Sr (0.13wt.%) to the Mg94Zn2Y4alloy. Compared with the Mg94Zn2Y4alloy, the elongation is increased from2.7%to4.6%. The improvement in ductility of the alloy is attributed to the grain refinement and appearance of stacking faults (SF) in the α-Mg matrix.3. The fine plate-like precipitates with14H-LPSO structure are generated in the supersaturated a-Mg matrix during solution treatment at500℃for35h. The forming of this14H-LPSO structure in the α-Mg matrix results in precipitation hardening.4. Formation of LPSO structure has been examined and studied in the Mg94Cu2Y4and Mg94ZniCuiY4alloys. In contrast, no LPSO structure was found to be formed in the Mg94Zn2Cu2Y2alloy, when the content of Y decreased.5. The high strength Mg93.96Zn2Y4Sr0.04alloy was developed successfully by conventional permanent mould casting combine with ECAP processing. The ultimate tensile strength and elongation of the ECAP processed alloy at room temperature and at elevated temperature (200℃) are408MPa,5.1%and375MPa,5.7%, respectively.6. X-phase reveals a more dispersive distribution during the ECAP process. In addition, dislocation piles up at kink bands and subboundary appeares caused by high density of dislocation and dislocation tangle in the LPSO structure, which leading fine grain strengthening.