Effect Of Li On Microstructure And Properties Of Mg-gd-zn Alloys With Long Period Stacking Ordered Structures

Owing to excellent comprehensive mechanical properties at room temperature and high temperature, the rare earth magnesium alloy have a increasingly broad application prospects in the field of aerospace, automobile and so on. The presence of long period stacking ordered(LPSO) strengthening phase, gives promising mechanical properties and unique microstructures to rare earth magnesium alloys, which have already become the focus of attention at home and abroad.In this paper, Mg-Gd-Zn-Li alloys only with lamellar LPSO structures within the matrix were prepared successfully by conventional permanent mould casting method and heat treatment process. Then alloy grains were then further refined by the addition of Zr. Finally, the refined Mg-Gd-Zn-Li-Zr alloy was deformed by forward extrusion. The microstructure evolution were investigated by OM,XRD,SEM and TEM. The effect of Li on microstructure and properties of Mg-Gd-Zn alloy were studied in different processes. The results show that:(1)Li addition can effectively refine?-Mg dendrites in as-cast MgGd3Zn1 alloy and uniform the distribution of eutectic phases. However, the amount of eutectic phase was increased significantly once the addition was up to 6%(at. %), which would deteriorate mechanical properties.(2)The optimum solution treatment for Mg96-xGd3 Zn1Lix(at.%)(x=2, 4, 6) alloys was 500 ? ?45h.(Mg,Zn)3Gd phases granulated during solution treatment with no solid transformation. Li addition could suppress the formation of LPSO phases during the solid solution treatment of MgGd3Zn1 alloy. Coarse LPSO phases at grain boundaries were suppressed strongly. There was no obvious suppressing effect on the lamellar LPSO phases in ?-Mg matrix until Li addition was more than 4%.(3)Zr addition can effectively refine MgGd3Zn1Li4 alloy grain. Dendrites in as-cast alloys were eliminated and small equiaxed grains were formed. A much better homogeneity of structure than MgGd3Zn1Li4 alloy in as-cast and subsequent solution-treated state was obtained. The optimum addition 0.3%( at. %).(4)Under the conventional casting process, Li addition could bring much casting defects MgGd3Zn1 sample, which would deteriorate the casting performance of MgGd3Zn1 alloy. Therefore, Li addition can't effectively improve the comprehensive mechanical properties of MgGd3Zn1 alloy under as-cast and heat treatment state.(5)Dynamic recrystallization(DRX) took place in MgGd3Zn1Li4Zr0.3 alloy after extrusion, which refined the grains extremely. The DRXed grain size of solution-treated MgGd3Zn1Li4Zr0.3 was much smaller than that of as-cast alloy after extrusions, which is due to the inhibition effect of grain growth from lamellar LPSO phases formed in solution treatment. The second phases in as-cast MgGd3Zn1Li4Zr0.3 alloy were all broken in submicron particles after extrusion, while second phases in solution-treated MgGd3Zn1Li4Zr0.3 alloy were all broken into particles with(submicron + micron) bimodal size.(6)The comprehensive mechanical properties of MgGd3Zn1Li4Zr0.3 alloy was significantly improved after extrusion. Solution-treated and extruded MgGd3Zn1Li4Zr0.3 alloy exhibited the best comprehensive mechanical properties after aging treatment and its UTS was 425 MPa, with an elongation of 10%. Microstructure refinement and uniformity of MgGd3Zn1 alloy can be improved obviously via Li addition, heat treatment, and extrusion simultaneously. which can significantly improve the comprehensive mechanical properties of MgGd3Zn1 alloy.