| As the world’s lightest metallic structural materials in the21st century, magnesiumalloys have been widely used in all areas of life wit high specific stiffness and strength, goodtoughness, excellent damping and electromagnetic shielding performance, high recyclabilityand so on. But poor strength and high-temperature creep properties, weak corrosionresistance are the restrictions of their application. Faced with a relative shortage of resourcesand energy today, our country should embark on these questions and take advantage of theresource superiority to accelerate the development and application of magnesium alloysowing the world’s richest magnesium resources. Alloying is the basic and common method toimprove the properties of magnesium alloys. Therefore, it is necessary to understand themechanism of the alloy phase (solid solution, intermetallic compounds) in magnesium alloy.Alloying elements studied in this paper are Ca, Sr, Ba, Bi and Sb.Using the first-principles plane-wave pseudopotential method, the place holder positionsof the alloying element X (X=Ca, Sr, Ba) doped Mg2Si are investigated. The enthalpy offormation and the cohesive energy demonstrate that Mg7Si4X is easier to form a stablecompound with the strong alloying capability. The elastic modulis and electronic structures ofMg7Si4X show that: Mg7Si4Ca and Mg7Si4Sr are brittle phases; Mg7Si4Ba is a ductile phasewith the best plasticity. The doped elements make Mg2Si transited from brittle to ductilegradually. Besides, the bond feature and structural stability of the doped alloy phases areanalysed from the electronic structure.The structural, elastic, thermodynamic and electronic properties of Mg-Sr compoundsMg2Sr，Mg17Sr2and Mg23Sr6have been calculated. The cohesive energy shows that with theincrease of Sr concentration, structural stability of Mg-Sr compounds gradually increases. The calculated elastic modulus show that Mg2Sr and Mg23Sr6are both essentially brittle,Mg17Sr2behaves in a ductile manner. Mg17Sr2has the best plasticity, followed by Mg2Sr andMg23Sr6. The sequence of Vickers hardness is: Mg23Sr6Mg2Sr Mg17Sr2. A smaller degreeof deviation from the isotropic occurs with increasing Sr content. The thermodynamicproperty shows that at the range of298to1000K the thermal stability of these compoundsgradually decreases with the Sr content increasing. From the electronic structure, the strengthof the covalent bond, ionic and metallic bond of three intermetallic compounds is compared.The effects of the pressure on structural, elastic, thermodynamic and electronicproperties of the Mg3X2(X=Sb, Bi) compounds with hexagonal D52structure are investigatedanalyzed. The heat of formation and cohesive energy show that Mg3Sb2and Mg3Bi2compounds are elastically stable at zero temperature and pressure. The P-V equation of states,bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio ν and Debyetemperature ΘDas a function of pressure are obtained and discussed in detail. The values of Bincrease with increasing pressure, while the values of G, E andΘDincrease first and thengradually decrease. The ratio of G/B and ν indicate that Mg3Sb2and Mg3Bi2are ductility andhave a good capability of plasticity. In addition, the ductility and plasticity can be improved asthe pressure increases. Finally, further analysis on the pressure dependence of the electronicbehavior is performed. |
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