Dejiang Li

Predictions of the structural, electronic and thermodynamic properties of the anti-fluorite-type Mg2Sn under pressure from first principles

Using first-principles calculations based on density functional theory, the structural, electronic and thermodynamic properties of Mg2Sn in anti-fluorite structure under hydrostatic pressure are investigated. Our results for the equilibrium structural parameters are consistent with the previous experimental and theoretical data. The dependences of elastic constants, polycrystalline elastic moduli, Poissons ratio and the anisotropy factor on pressure have been investigated. It is found that pressure has a significant effect on elastic properties due to variations in interatomic distances. In addition, the variations in density of states with applied pressure are determined to reveal the bonding characteristics. Finally, the dependences of bulk modulus and thermodynamic properties of Mg2Sn on pressure and temperature are investigated with the quasi-harmonic Debye model, and the results of thermodynamic properties are consistent with the experimental report.

Dry Sliding Wear Behavior of Mg-Zn-Gd Alloy before and after Cryogenic Treatment

The dry sliding wear behavior of Mg-1.5Zn-0.15Gd alloy before and after cryogenic treatment was studied by wear tests using a ball-on-disk configuration at room temperature. The wear tests were performed at speeds ranging from 0.05 to 0.7 m/s under three different applied loads (5, 7, and 10 N). The worn surfaces and wear debris were analyzed using a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS). The results show that the wear rate of alloy decreases with increasing sliding speed. Two different wear mechanisms were identified under different sliding speeds. During low-speed sliding at 0.05 m/s, abrasive wear occurs with a high wear rate. When the sliding speed increases to 0.5 m/s, the wear mechanism transforms into oxidative wear with a low friction coefficient and wear rate. The wear resistance of the alloy is improved after cryogenic treatment. Such improvement is minor at high sliding speeds due to the formation of an oxidation layer on the wear surfaces, which weakens the hard-phase support effect during wearing. A wear map of each mechanism as a function of load and sliding speed is provided.

Predictions of mechanical and thermodynamic properties of Mg17Al12 and Mg2Sn from first-principles calculations

Using first-principles calculations, we predict mechanical and thermodynamic properties of both Mg17Al12 and Mg2Sn precipitates in Mg–Al–Sn alloys. The elastic properties including the polycrystalline bulk modulus, shear modulus, Young’s modulus, Lame’s coefficients and Poisson’s ratio of both Mg17Al12 and Mg2Sn phases are determined with the Voigt–Reuss–Hill approximation. Our results of equilibrium lattice constants agree closely with previous experimental and other theoretical results. The ductility and brittleness of the two phases are characterized with the estimation from Cauchy pressure and the value of B/G. Mechanical anisotropy is characterized by the anisotropic factors and direction-dependent Young’s modulus. The higher Debye temperature of Mg17Al12 phase means that it has a higher thermal conductivity and strength of chemical bonding relative to Mg2Sn. The anisotropic sound velocities also indicate the elastic anisotropies of both phase structures. Additionally, density of states and Mulliken population analysis are performed to reveal the bonding nature of both phases. The calculations associated with phonon properties indicate the dynamical stability of both phase structures. The temperature dependences of thermodynamic properties of the two phases are predicted via the quasi-harmonic approximation.

A first-principles study on structural stability and mechanical properties of polar intermetallic phases CaZn2 and SrZn2

Structural stability and electronic properties of polar intermetallic CaZn2 and SrZn2 in both CeCu2-type and MgZn2-type structures have been investigated using first-principles method. The calculated equilibrium lattice parameters agree closely with the available experimental and other theoretical results. In terms of formation enthalpy, it is discovered that the present compounds with CeCu2-type structure are energetically more stable than that with MgZn2-type. They are all mechanically stable according to the criteria of elastic stability. In particular, we have investigated the pressure effect on the compressive behaviour and structural stability of each compound. Subsequently, the bulk modulus, shear modulus, Young’s modulus, theoretical hardness, Poisson’s ratio and Debye temperature in the ground state can be estimated using Voigt–Reuss–Hill homogenization method. Mechanical anisotropy is characterized by the anisotropic factors and direction-dependent Young’s modulus. Finally, the electronic structures are determined to reveal the bonding characteristics of considered phases.

Microstructure Evolution and Mechanical Behavior of Mg-10Gd-3Y-0.4Zr Alloy Processed by ECAP at High Temperature

Microstructure evolution, texture development and their influence on mechanical properties of Mg-10Gd-3Y-0.4Zr (wt.%) alloy processed by equal channel angular press (ECAP) at 450 °C were studied. The samples were prepared by ECAP from one to four passes. The results show that recrystallization preferentially occurred near the grain boundary, and the percentage of high-angle grain boundaries increases with the increase of passes. After four passes, the microstructure is still inhomogeneous, the fine and coarse grain zones exist simultaneously. It is attributed to the higher extrusion temperature; the grain grows and aggregates easily, which is confirmed through comparative experiment. The strength increases obviously after one pass, unexpectedly, it reduces slightly after two passes, the elongation is dramatically increased from 6.7% to 20.3%, which is due to the basal plane rotation from ED to the direction of flow strain and formation of a typical shear texture. The change of elongation and mechanical properties of this alloy during ECAP was discussed in terms of texture and the effect of the fine grain.