Zhiqin Wen

A first-principles study on interfacial properties of Ni(001)/Ni3Nb(001)

Abstract The Ni (001) surface, Ni 3 Nb (001) surface and Ni (001)/Ni 3 Nb (001) interfaces were studied using the first-principles pseudopotential plane-wave method. The adhesion work, thermal stability and electronic structure of Ni/Ni 3 Nb (001) interfaces were calculated to expound the influence of atom termination and stacking sequence on the interface strength and stability. Simulated results indicate that Ni and Ni 3 Nb (001) surface models with more than eight atomic layers exhibit bulk-like interior. The (Ni+Nb)-terminated interface with hollow site stacking has the largest cohesive strength and critical stress for crack propagation and the best thermal stability among the four models. This interfacial Ni and the first nearest neighbor Nb atoms form covalent bonds across the interface region, which are mainly contributed by Nb 4d and Ni 3d valence electrons. By comparison, the thermal stability of Ni/Ni 3 Nb (001) interfaces is worse than Ni/Ni 3 Al (001) interface, implying that the former is harder to form. But the Ni/Ni 3 Nb interface can improve the mechanical properties of Ni-based superalloys.

First-Principles Investigation of Mechanical and Thermodynamic Properties of Nickel Silicides at Finite Temperature

First-principles calculations are performed to investigate lattice parameters, elastic constants and 3D directional Young’s modulus E of nickel silicides (i.e., β-Ni3Si, δ-Ni2Si, θ-Ni2Si, ε-NiSi, and θ-Ni2Si), and thermodynamic properties, such as the Debye temperature, heat capacity, volumetric thermal expansion coefficient, at finite temperature are also explored in combination with the quasi-harmonic Debye model. The calculated results are in a good agreement with available experimental and theoretical values. The five compounds demonstrate elastic anisotropy. The dependence on the direction of stiffness is the greatest for δ-Ni2Si and θ-Ni2Si, when the stress is applied, while that for β-Ni3Si is minimal. The bulk modulus B reduces with increasing temperature, implying that the resistance to volume deformation will weaken with temperature, and the capacity gradually descend for the compound sequence of β-Ni3Si > δ-Ni2Si > θ-Ni2Si > ε-NiSi > θ-Ni2Si. The temperature dependence of the Debye temperature ΘD is related to the change of lattice parameters, and ΘD gradually decreases for the compound sequence of ε-NiSi > β-Ni3Si > δ-Ni2Si > θ-Ni2Si > θ-Ni2Si. The volumetric thermal expansion coefficient αV, isochoric heat capacity and isobaric heat capacity Cp of nickel silicides are proportional to T3 at low temperature, subsequently, αV and Cp show modest linear change at high temperature, whereas Cv obeys the Dulong-Petit limit. In addition, β-Ni3Si has the largest capability to store or release heat at high temperature. From the perspective of solid state physics, the thermodynamic properties at finite temperature can be used to guide further experimental works and design of novel nickel–silicon alloys.

Computation of stability, elasticity and thermodynamics in equiatomic AlCrFeNi medium-entropy alloys

We investigated the phase stability, elastic and thermodynamic properties of equimolar medium-entropy alloys (MEAs) AlCrFeNi by performing first-principles calculations in combination with quasi-harmonic Debye–Grüneisen model. Both body-centered cubic (BCC) and face-centered cubic structures in ferromagnetic (FM) and non-magnetic states are described using the special quasirandom structures technique. All the considered MEAs can form single-phase solid solutions and are dynamically stable, and FM BCC AlCrFeNi is the most stable. The elastic moduli including bulk modulus B, shear modulus G and Young’s modulus E of AlCrFeNi are calculated by first-principles and estimated by using the rule of mixtures (ROM) from their pure components. The lattice constants a of first-principles calculations are well reproduced by ROM. The obtained B and G of the two methods are close to equality lines with a minor scatter. The relevant free energies’ contributions including structural, configurational, vibrational and electronic excitations are taken into account to calculate the equilibrium lattice constants a, volumetric thermal expansion coefficient α, Debye temperature ΘD, constant volume heat capacity Cv, vibrational entropy Svib, electronic entropy Selec, vibrational Helmholtz free energies Fvib and electronic Helmholtz free energies Felec of AlCrFeNi MEAs at finite temperature. The thermodynamic properties strongly depend on crystal structures and magnetic states, and FM BCC AlCrFeNi shows the largest Svib and α, and the lowest Fvib among the considered MEAs. Finally, electronic density of states is analyzed to clarify the physical origin of AlCrFeNi MEAs with different crystal structures and magnetic states.