| Rare earth elements(RE) is the most effective strengthening elements in magnesium alloy, develop the Mg-RE series alloys, improve the mechanical properties at room temperature and high temperature can make the magnesium alloy widely used. The WE series alloys(Mg-Y-Nd-Zr) become more and more attractive for aerospace and automotive industries because of their high room temperature mechanical properties, excellent high temperature creep resistance and good corrosion resistance. Sm and Gd maximum solubility and precipitation hardening effect in magnesium are stronger compared with the same group of Nd and Y, thus replace Nd and Y by Sm and Gd in WE series alloy may have better mechanical properties. In this paper, adding elements Sm, Gd and Zn to the magnesium matrix, developed a new heat-resistant magnesium alloy, study the effect of heat treatment on Mg-Sm alloy microstructure and properties.Study the effect of Zn and Gd on microstructure and mechanical properties of Mg-3Sm-xGd(x=0.5,1.5)-yZn(y=0,0.3,0.6)-0.5Zr alloy-0.5Zr) alloy by Using optical microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction equipment. The optimized alloy chemical composition ratio is Mg-3Sm-1.5Gd-0.3Zn-0.5Zr. The alloy was mainly composed ?-Mg and (Mg,Zn)3(Sm,Gd)1eutectic phase in as-cast state.(Mg,Zn)3(Sm,Gd)1has a FCC structure with lattice parameters a=0.727nm. After the solution treatment at510?for4h and subsequent peak aged at200?, alloy exhibited remarkable mechanical properties. The ultimate tensile strength, yield strength and elongation of the alloy were282MPa,185MPa and6.1%at room temperature. These good mechanical properties were mainly attributed to the fine microstructure and fine metastable phase precipitates in the matrix during aging process.The decomposition of a-Mg supersaturated solid solution in Mg-3Sm-1.5Gd-0.3Zn-0.5Zr alloy during isothermal aging at200?is as follows: Mgssss??"(D019)??'(bct)??1(fcc)??(fcc).At the beginning of aging process, ?" precipitates, with the aging time prolonged, ?'appears and reached the peak hardness.During hot deformation, the true stress-strain curves of alloy is belong to dynamic recrystallization. Flow stress decreases with the temperature increase and strain rate decrease. The material constants of alloy are figured out as:Q=246.2987KJ/mol, n=6.189, ?=0.01309MPa-1, A=4.486×1016s-1. The following relationship between the flow stress, strain rate, deformation temperature and Z parameters was derived:?=4.486×1016[sinh(0.01309?)]6.189exp(-246.2987×103/RT)Extruded Mg-3Sm-1.5Gd-0.3Zn-0.5Zr alloy is fully recrystallized structure, grain size are small. The strength and ductility of the extruded alloy has been significantly improved, the yield strength, ultimate tensile strength and elongation are225MPa,302MPa and30.8%at room temperature, respectively. the extruded alloy directly at200?aging, there is a strong age hardening phenomenon, precipitates strengthening phase ?', thus increased the mechanical properties of the alloy.the yield strength reached267MPa, more than18.7%higher than as-extruded alloy.?" phase has D019structure (a=2aMg=0.64nm and c=cMg=0.52nm) with an orientation relationship of [2110]?"//[2110]?,(0110)?"//(0110)?.?'phase is known to have a base-centred orthorhombic unit cell a=0.64nm, b=2.223nm, c=0.521nm, an orientation relationship with the matrix in this paper was [001]?',||[0001]?,(020)?',||(1010)?.?1phase has a fcc structure with lattice parameters a=0.74nm. ? phase also has a fcc structure, lattice parameters a=2.23nm. ? phase habiting on the basal plane of the Mg matrix, hexagonal, a=0.55nm, c=0.52nm and orientation relationship is (1100)?'//(1120)?,[0001]?'//[0001]?.Mg-3Sm-1.5Gd-0.3Zn-0.5Zr alloy has excellent corrosion resistance and creep resistance, at a5%sodium chloride solution after72h immersion corrosion weight loss rate is0.32mg/cm2day. The steady creep rate is2.63×10-9s-1at200?,100MPa conditions, the creep amount is less than0.1%after100h. |
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