Study Of Mg-Gd-Zn Alloys With Long Period Stacking Ordered Structures

In recent years, new developed Mg-RE-Zn alloys with long period stacking ordered (LPSO) structures, exhibiting unique microstructures and excellent mechanical properties especially good high-temperature properties, overcome the disadvantages of conventional magnesium (Mg) alloys such as low strength, ductility and poor high-temperature properties, and have enlarged the application of green environmental material-Mg alloys in automobile and aerospace industries.In this study, Mg-Gd-Zn, Mg-Gd-Zn-Zr, Mg-Gd-Ni and Mg-Gd-Zn-Ni alloys with LPSO structures were prepared by conventional permanent mold casting method combined with heat treatment. The microstructures were analysed by OM, EDS, SEM, TEM and XRD methods. The formation and transformation of LPSO structure phases, the effect of alloying elements addition on the microstructures and mechanical properties were also studied. In addition, the strengthening mechanism of as-extruded LPSO structures Mg alloys was discussed by the direct extrusion and equal channel angular pressing (ECAP) of alloys. The results show that:14H-LPSO structures phases were observeded in as-cast Mg-Gd-Zn alloy, but have little contribution to the mechanical properties because of little amount; Abundant lamellar LPSO structures phases transformed from eutectic structures after solid-solution treatment; After aging treatment, lamellar LPSO structures phases had no evident variation. The properties of alloys elevated a lot due to the appearance of LPSO structures phases and aging strengthening; The grain size of Mg-Gd-Zn alloys was relative to the cooling rate of alloy melt, high cooling rate led to small grain size; as heterogeneous nucleation cores, Zr-rich phases promoted the happening of nucleation, refined the grains; When0.2%Zr (at.%) was added into Mg96Gd3Zn1alloy, the location of LPSO structures phases transformed from grain boubdaries to grains, the conventional lamellar morphology of LPSO structures changed to stripy in aging alloys, which contributed to the homogeneous distribution of LPSO structures phases; As strengthening phase, spherical and needle-like phases contributed to the mechanical properties (UTS=332MPa, YS=235MPa,δ=6.9%); Mg-Gd-Zn(Zr) alloys showed excellent high-temperature properties because of the existence of LPSO phases with high-temperature stability; The mechanical properties of Mg95.8Gd3Zn1Zr0.2alloy were improved by direct extrusion for the area fraction of LPSO phases increased from64%to83%; After1pass ECAP, the LPSO phases were bended with small kinking angles, particle phases fragmented into tiny spherical phases; After2passes ECAP, the kinking angles of LPSO phases increased, introducing abundant plastic deformation, the dispersoid distribution of tiny spherical phases led to dispersion strengthening; The Mg95.8Gd3Zn1Zr0.2alloy subjected to2passes ECAP acquired best mechanical properties (UTS=418MPa, YS=330MPa,δ=7.5%).No eutectic structures existed, but abundant14H-LPSO phases were observed in as-cast Mg-Gd-Ni alloys; The addition of Ni into Mg-Gd-Zn alloys improved the formation of LPSO structures phases in as-cast Mg-Gd-Zn-Ni alloys. The composition of LPSO phases in as-cast Mg-Gd-Zn-Ni alloys was Mg-8±1Gd-2±0.5Zn-2±0.5Ni (at.%). However, the composition of LPSO phases changed to Mg-8.5±1Gd-3±0.5Zn-1.5±0.5Ni (at.%) after heat treatment.