The Control Of Long-Period Stacking Ordered Phase And Its Influence On The Mechanical Properties Of Mg-RE-Zn Series Alloys

As the lightest commercial metal structural materials, magnesium alloys with thepreferable properties such as high specific strength, high specific stiffness, and goodmachining properties and so on, had been applied in the area of aerospace, railtransportation, electrommunication, etc in a certain extent. However, due to the lack ofeffective strengthening and toughening phases, the strength and ductility of the alloyswere not so good. Therefore, even the mature commercial Mg-Zn alloy series were justused for the non-load bearing structural parts mainly. However, the Mg-RE-Zn alloyseries with addition of Rare-earth (RE) element possessed better solution-strengtheningand aging-strengthening effects. In addition, by controlling the ratio of element Zn andRE reasonably, long-period stacking ordered (LPSO) structure phases with the uniquecharacteristic, such as high hardness, high elastic modulus, high thermal stability, thecoherent interface between it and the Mg matrix, etc, can been introduced into thesealloys, and drastically improve their strength and ductility, and make them exhibitexcellent comprehensive mechanical properties.In this research, based on the Mg-Zn alloy series, the Mg-RE-Zn alloy series weredesigned and prepeared by addition of RE elements, which were the binary combinationof the elements of Y, Er, and Gd. So that, we obtained Mg-Er-Y-Zn alloy series,Mg-Gd-Y-Zn alloy series, and Mg-Gd-Er-Zn alloy series in order to investigate theinfluence of alloy elements, extrusion parameters and heat treatment on the structurecharacteristic, formation, evolution and morphology of LPSO phase and its influence onthe mechanical properties. Finally, by controlling the structure characteristic,morphology, distribution of LPSO phases, and the combination of LPSO phases andother phases, and so on, we hope to obtain high-strength wrought magnesium alloyswith excellent comprehensive mechanical properties.First, the influence of extrusion parapeters on the morphology of LPSO phases inMg-5Y-5Zn-0.6Zr (wt.%) alloy was investigated. The broken LPSO phases withdispersive distribution resulting from the increased extrusion ratio and the refinedrecrystallized grains even at high extrusion temperature enhanced the strengthdeastically and made the alloy obtain excellent comprehensive mechanical properties:ultimate tensile strength (UTS):412MPa, Tensile yield strength (TYS):380MPa, andelongation (δ):11.2%. Second, the influence of Zn content on the phase composition and the mechanicalproperties of Mg-9Er-6Y-xZn-0.6Zr(x=1～4, wt.%) alloy series was investigated. Withthe increase of Zn content, the body-centred cubic structure Mg24(Er, Y, Zn)5phaseswith granular shape decreased and the18R-type LPSO structure Mg12Zn(Y, Er) phaseswith block shape increased gradually. Moreover, the Mg12Zn(Y, Er) phase exhibitedbetter strengthening and toughening effects than Mg24(Er, Y, Zn)5phase. However, theabundant coarsened lamellar LPSO phases resulting from the excessive Zn contentdeteriorated the ductility of the alloys.Third, the influence of Zn content on the phase composition and the mechanicalproperties of Mg-9Gd-6Y-xZn-0.6Zr(x=1～4, wt.%) alloy series was investigated. Withthe increase of Zn content, the Mg5(Gd,Y,Zn) phases and lamellar LPSO phasesdecreased but the block-shaped Mg12Zn(Y, Gd) phases with14H-type LPSO structureincreased granularly. After the hot extrusion, the aboundant distorted lamellar LPSOphases in the Mg matrix made the alloy with Zn content of1.6wt.%obtain a UTS of400MPa and a TYS of330MPa but an lower elongation of3.2%. However, with theincrease of Zn content, the lamellar LPSO phases in the Mg matrix decreased and thelarger block-shaped LPSO phases increased, so that the strength emerged a decreasetendency but the ductility was improved slightly.Besides, the influence of heat treatment on the formation and evolution of LPSOphase and the mechanical properties of Mg-9Gd-xEr-1.6Zn-0.6Zr (x=1～4, wt.%) alloyseries was investigated. There were just (Mg,Zn)3(Gd,Er) phases but no LPSO phases inthe as-cast alloys with different Er content. Afer heat-treated between400℃~515℃, the(Mg,Zn)3(Gd,Er) phase could transform to the block-shaped Mg12Zn(Gd, Er) phaseswith14H-type LPSO structure and there were numerous lamellar LPSO phasesgenerating parallelly in the Mg matrix grains. After the hot extrusion, aboundantisometric crystal grains with the block-shaped LPSO phases as the main second phasesin the grain boundaries made alloys obtain better ductility. However, the numerouslamellar LPSO phases as the main second phases can refine the recrystallized grain sizeand introduce numerous slender strip-shaped crystal grains, which made the alloysobtain better strength but a slight decrease of elongation.At last, from the alloy series studied above, we selected the alloy with highstrength, added Mn element as the grain refinement element, and prepared theMg-9Gd-6Y-1.6Zn-0.6Mn (wt.%) alloy by extrusion. The aging at200℃for50h afterextrusion could make the alloy obtain UTS of496MPa, TYS of322MPa, and elongation of9.7%, and exhibit prominent comprehensive mechanical properties. Thereasons why the alloy aged obtained such high strength could be mainly attributed to theaboundant convexlens-shaped precipitate phases formed on prismatic planes of α-Mgmatrix and being fully coherent with α-Mg matrix as well as the numerous smallblock-shaped LPSO phases distributed in the grain boundaries.