First-principle Calculation, Microstructures And Properties Of Mg-RE Alloys

The addition of rare earth (RE) elements can significantly improve the tensileproperties, creep and castingperformance of magnesium alloywithout jeopardizingtheadvantages of magnesium such as low density, high specific strength, excellent rigidityand high modulus of elasticity. Samarium(Sm) is more concerned by researchersrecentlyforitcanleadtoconsiderableimprovementswithrelativelylowcost.However,the researches into Mg-Sm alloys are quite few up to now.First-principle calculations were used to investigate the elastic properties andelectronstructuresofMgandREatoms.AndMg-xSm-Zn-Zr（x=2,4,6,8wt.%）alloyswere researched systematically for their microstructure, mechanic properties, fracturebehavior and aging precipitation.The mechanical properties of Mg-RE（RE=Y, La, Sm）were calculated. The resultsshow that the alloy becomes more ductile when one Mg is replaced by Y. And Smmakes the solution harder and more fragile, while La has little effect. This result can beexplained by the calculated electron structures. For the f electron of Sm forms a narrowbut high peak in the density of states, the bonding of Mg-Sm shows more directionalcharacteristics, which is more like covalent bonding than metallurgy bonding. Whilethe p electron of Y weakens the electron hybrid of Mg-Y so that Mg-Y alloys trend tobe more ductile.The exploratory calculations of stacking faults of pure Mg show that ABC andABAC have a higher Young’s modulus E and lower poisson’s ratio v, indicating thatABC and ABAC are harder and more fragile than pure Mg. For ABACAB, E dropswhile v rises, showing that it is softer than stackingAB.The cast alloy mainly consists of α-Mg matrix and Mg41Sm5phase distributed at thegrain boundaries, and theamount of second phase increases with the rise of Smcontent.After the solution treatment (T4), the second phase disappeared, but small amount ofcuboid-like phase were found inside the grain, which is the solid solution of Sm. Sm is very effective in the aging strengthening process. The alloys with a higher Sm contenthave a higher peak hardness and a shorter peak aging time. Higher aging temperaturecan also lead to a shorter peak aging time while lower peak hardness.With theSmcontentincreases, the yield strength of castalloys (Fstate) areobviouslyimproved, but the tensile strength become lower because of the quick drop of theelongation. The elongations of solution (T4) and aging (T6) alloy become lower, whilethe yield strength and tensile strength increased at first and then decreased. Theoptimized component is: Mg-4Sm-0.4Zn-0.3Zr.Cast alloy of Mg-6Sm-0.4Zn-0.3Zr shows intergranular fracture, with secondarycrack at the grain boundaries, especially in the eutectic organization. T4alloys showscleavage fracture with secondary cracks inside the grains; T6alloy showed mixedfracture characteristics. The amount of secondary cracks is significantly reduced.The decomposition of S.S.S.S.in Mg-6Sm-0.4Zn-0.3Zr cast alloy during isothermalagingat225℃isasfollows:β"(D019)phaseappearswithin0.5h;at4h,β"growlarger,meanwhile β’ appears; After16h-260h, β" gradually disappear while β’ constantlygrows until none β" phases are left. Nothing new are formed, so the main precipitatedphase remains β’ phase.