| Due to the demand for light-weight alloys for structural applications, magnesium alloys are receiving attention as the structural material with high specific strength. In the past few years, the Mg-Zn-RE(RE,rare earth) systems have been regarded to be of the highest strength. Among them, Mg–Zn–Y alloys have received considerable attention due to distinctive microstructures and excellent mechanical properties.In this work, Mg-Zn-Y-Mn(-Ca) alloys containg LPSO structures and W phases were prepared by conventional permanent mold casting method. The microstructures were analysed by XRD, OM, SEM and TEM methods. The influence of Zn/Y atom ratio on the microstructural evolution of Mg-Zn-Y-Mn alloy, the effects of Ca alloying on the precipitation of LPSO phase, the formation and transformation of LPSO structure during solid solution heat treatment as well as the relationship between microstructures and mechanical properties were investigated systematically. In addition, the strengthening mechanism of as-extruded Mg-Zn-Y-Mn-Ca alloys was discussed comprehensivly. The results are showed as follows:(1) The secondary phases in four investigated as-cast Mg94-x Zn2.5+x Y2.5Mn1 alloys evolve with Zn/Y ratio increase. The phases identified in microstructures are α-Mg+X-phase+W-phase, when Zn/Y ratio is 1.0; α-Mg+W-phase, when Zn /Y ratio is 1.5; α-Mg+W-phase+I-phase, when Zn /Y ratio are 1.9 and 2.3. And the grains were not refined with the increasing of Zn content.(2) The investigated Mg94-x Zn2.5Y2.5Mn1Cax(x = 0,0.17,0.34,0.51,0.67) alloys are consisted of α-Mg phase, X-phase(Mg12YZn) and W-phase(Mg3Zn3Y2). With 0.34 at.% Ca addition, the grains have been greatly refined.(3) Ca was experimentally proved useful as an alloying element to facilitate LPSO precipitates. Adding moderate(0.34 at.%) calcium to the Mg–Zn–Y–Mn alloy can greatly increase the volume fraction of LPSO phase in the case of a low Y/Zn atomic ratio.(4) The mechanical properties of as-cast Mg-Zn-Y-Mn alloy was improved significantly by 0.34 at.% Ca addition, due to the strengthening from microstructural refinement and the formation of large LPSO phases, which achieved a good balance of the strength and ductility.(5) The morphology and distribution of the LPSO phases changed obviously after solid solution heat treatment. The 18 R LPSO phases in the as-cast alloy are mainly block shaped and distributed in the vicinity of the grain boundaries. The fine lamellar 14 H LPSO phases in the solid solution treated alloy is experimentally considered to be thermodynamically stable, as it remains in the grain interior.(6) The netted W-phase broken into small granules which evenly distributed in α-Mg matrix due to the effect of solid solution heat treatment. The globular W-phase strongly contributed to the improvement of mechanical properties, especially benefit the ductility of the alloy.(7) Mg93.66Zn2.5Y2.5Mn1Ca0.34 alloy processed by hot extrusion resulted in basal plane texture formed, occurrence of dynamic recrystallization and grain refinement significantly, thus improved mechanical properties apprantly, the tensile strength(UTS), yield strength(YS) and elongation(?) reached 415 Mpa, 348 MPa and 16.4%, respectively.(8) Fragmented W-phase particles reveal a more dispersive distribution during hot extrusion. In addition, the LPSO phases were bended with small kinking angles due to the severe plasticity deformation. These make great contributions to the improvements of mechanical properties. |
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