First-principles study of the(Cu_xNi_(1-x))_3 Sn precipitations with different structures in Cu–Ni–Sn alloys

The structural parameters, the formation energies, and the elastic and thermodynamic properties of the （CuxNi1-x）3Sn phase with different structures are studied by the virtual crystal approximation （VCA） and super-cell （SC） methods. The lattice constants, formation energies, and elastic constants obtained by SC and VCA are generally consistent with each other. It can be inferred that ttle VCA method is suitable for （CuxNil ,）3Sn ordered phase calculation. The calculated results show that the equilibrium structures of Cu3Sn and Ni3Sn are D0a and D019 respectively. （CuxNi1-x）3Sn D03 with various components are the mctastable phase at temperature of 0 K, just as D022 and L I2. With the temperature increase, tile free energy of the D03 is lower than those of D022 and L12, and D022 and L12 eventually turn into D03 in tile aging process. The （CuxNi1-x）3Sn D022 is first precipitated in a solid solution because its structure and cell volume are most similar to Ihose of a solid solution matrix. The LI2 and the D022 possess better mechanical stability than the D03. Also, they may play a more important role in the strengthening of Cu-Ni-Sn alloys. This study is valuable lk）r further research on Cu-Ni-Sn alloys.

Piezoelectricity in K_(1-x)Na_xNbO_3: First-principles calculation

The piezoelectric properties of K1-xNaxNbO3 are studied by using first-principles calculations within virtual crystal approximation. To understand the critical factors for the high piezoelectric response in K1-xNaxNbO3, the total energy, piezoelectric coefficient, elastic property, density of state, Born effective charge, and energy barrier on polarization rotation paths are systematically investigated. The morphotropic phase boundary in K1-xNaxNbO3 is predicted to occur at x = 0.521, which is in good agreement with the available experimental data. At the morphotropic phase boundary, the longitudinal piezoelectric coefficient d33 of orthorhombic K0.5Na0.5NbO3 reaches a maximum value. The rotated maximum of d＊33 is found to be along the 50° direction away from the spontaneous polarization （close to the [001] direction）. The moderate bulk and shear modulus are conducive to improving the piezoelectric response. By analyzing the energy barrier on polarization rotation paths, it is found that the polarization rotation of orthorhombic K0.5Na0.5NbO3 becomes easier compared with orthorhombic KNbO3, which proves that the high piezoelectric response is attributed to the flattening of the free energy at compositions close to the morphotropic phase boundary.

Concentration-dependent crystal structure, elastic constants and electronic structure of ZrxTi1-x alloys under high pressure

The physical properties of ZrxTi1-x（x=0.0, 0.33, 0.5, 0.67, 0.75 and 1.00） alloys were sinmlated by virtual crystal approximation （VCA） methods which is generally used for disordered solid solutions modeling. The elastic constant, electronic structure and thermal Equation of state （EOS） of disor- dered ZrxTi1-x alloys under pressure are investigated by plane-wave pseudo-potentia1 method. Our simulations reveal increasement of variations of the calculated equilibrium volumes and decrease- ment of Bulk modulus as a function of the alloy compositions. Lattice parameters a and c of alloys with differentZr concentrations decrease linearly with pressure increasing, but the c/avalues are increasing as pressure increases, indicating no phase transitions under pressure from 0 GPa to 100 GPa. The elastic constants and the Bulk modulus to the Shear modulus ratios （B/G） indicate good ductility of Zr, Zr0.33 Ti0.67 Zr0.5Ti0.5, Zr0.75Ti0.25 and Ti, but the Zr0.67Ti0.33 alloy is brittle under 0 K and 0 GPa. The metallic behavior of these alloys was also proved by analyzing partial and total DOS.