Theoretical study of structural stability and elastic properties ofα-andβ-MgPd_(3)intermetallic compounds as well as their hydrides have been carried out based on density functional theory.The results indicateα-MgPd_(3)is more stable thanβphase with increased stability in their hydrides.The calculated elastic constants ofα-MgPd_(3)are overall larger thanβphase.After hydrogenation,the elastic constants are enlarged.And the elastic moduli exhibit similar tendency.The anisotropy ofα-MgPd_(3)is larger thanβphase,and the hydrides demonstrate larger anisotropy.Their ductility follows the order ofα-MgPd_(3)H_(0.5)<α-MgPd_(3)<β-MgPd_(3)H<β-MgPd_(3).Compared withβphase,higher Debye temperature ofα-MgPd_(3)implies stronger covalent interaction,and the Debye temperature of hydrides increases slightly.The electronic structures demonstrate that the Pd-Pd interaction is stronger than Pd-Mg,and Pd-H bonds play a significant role in the phase stability and elastic properties of hydrides.
The microstructure of the 18R-type long period stacking ordered （LPSO） phase in Mg 97 Y 2 Zn 1 alloy was investigated by the first principles calculation. The arrangement rule of Zn and Y atoms in the LPSO structure is determined theoretically. The calculation results reveal that the additive atoms are firstly located in the fault layers at the two ends of the 18R-type LPSO structure, and then extend to fault layers in the interior, which is in good agreement with the experimental observations. This feature also implies the microstructural relationship between 18R and other LPSO structures. The cohesive energy and the formation heat indicate the dependence of the stability of 18R LPSO structure on contents of Y and Zn atoms. The calculated electronic structures reveal the underlying mechanism of microstructure and the stability of 18R LPSO structure.
Abstract： With the substitution of part Mg in LaMg3 by Cu, the elastic constants CH and C12 increase while C44 decreases, implying an enhanced Poisson effect and smaller resistance to 〈001〉（100） shear. Furthermore, the bulk modulus B increases, while the shear modulus G, elastic modulus E and anisotropie ratio A are reduced. The calculated Debye temperature of LaCuMg2 is lower, implying the weaker interaction between atoms in LaCuMg2. Then, the stress-strain curves in entire range and the ideal strength at critical strain are studied. The present results show that the lowest ideal tensile strength for LaMg3 and LaCuMg2 is in the 〈100〉 direction. The ideal shear strength on the 〈 1 ^-1 0〉（110） slip system of LaMg3 is greater than LaCuMg2. The density of states and charge density distribution are further studied to understand the inherent mechanism of the mechanical properties.
First-principles calculations within generalized gradient approximation have been performed to investigate ideal strengths of anti-fluorite structured Mg_(2)X(X¼Si,Ge,Sn and Pb)compounds.The present calculations showed that the ideal tensile strengths of Mg_(2)X occur in the[111]directions while the ideal shear strengths appear in the(111)[11-2]systems.Both ideal tensile strength and shear strength of Mg_(2)X(X¼Si,Ge,Sn and Pb)decreased gradually with the increase of atomic number of X.The microscopic process and inherent mechanisms of mechanical properties were discussed from the evolution of electronic structures during strain.
