Huazhi Fang

Effects of alloying element and temperature on the stacking fault energies of dilute Ni-base superalloys

A systematic study of stacking fault energy (γ(SF)) resulting from induced alias shear deformation has been performed by means of first-principles calculations for dilute Ni-base superalloys (Ni(23)X and Ni(71)X) for various alloying elements (X) as a function of temperature. Twenty-six alloying elements are considered, i.e., Al, Co, Cr, Cu, Fe, Hf, Ir, Mn, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Sc, Si, Ta, Tc, Ti, V, W, Y, Zn, and Zr. The temperature dependence of γ(SF) is computed using the proposed quasistatic approach based on a predicted γ(SF)-volume-temperature relationship. Besides γ(SF), equilibrium volume and the normalized stacking fault energy (Γ(SF) = γ(SF)/Gb, with G the shear modulus and b the Burgers vector) are also studied as a function of temperature for the 26 alloying elements. The following conclusions are obtained: all alloying elements X studied herein decrease the γ(SF) of fcc Ni, approximately the further the alloying element X is from Ni on the periodic table, the larger the decrease of γ(SF) for the dilute Ni-X alloy, and roughly the γ(SF) of Ni-X decreases with increasing equilibrium volume. In addition, the values of γ(SF) for all Ni-X systems decrease with increasing temperature (except for Ni-Cr at higher Cr content), and the largest decrease is observed for pure Ni. Similar to the case of the shear modulus, the variation of γ(SF) for Ni-X systems due to various alloying elements is traceable from the distribution of (magnetization) charge density: the spherical distribution of charge density around a Ni atom, especially a smaller sphere, results in a lower value of γ(SF) due to the facility of redistribution of charges. Computed stacking fault energies and the related properties are in favorable accord with available experimental and theoretical data.

Al-centered icosahedral ordering in Cu46Zr46Al8 bulk metallic glass

Icosahedral short-range order, of which Al atoms are caged in the center of icosahedra with Cu and Zr atoms being the vertices, has been evidenced in the Cu46Zr46Al8 glassy structure by ab initio molecular dynamics simulation. These Al-centered clusters distribute irregularly in the three-dimensional space and form a “backbone” structure of the Cu46Zr46Al8 glass alloy. It is suggested that this kind of local structural feature is attributed to the requirement of efficient dense packing and the chemical affinity between Zr–Zr, Zr–Al, and Cu–Zr atoms. Our calculated results are found to be in good agreement with the experimental data.

First-principles studies on vacancy-modified interstitial diffusion mechanism of oxygen in nickel, associated with large-scale atomic simulation techniques

This paper is concerned with the prediction of oxygen diffusivities in fcc nickel from first-principles calculations and large-scale atomic simulations. Considering only the interstitial octahedral to tetrahedral to octahedral minimum energy pathway for oxygen diffusion in fcc lattice, greatly underestimates the migration barrier and overestimates the diffusivities by several orders of magnitude. The results indicate that vacancies in the Ni-lattice significantly impact the migration barrier of oxygen in nickel. Incorporation of the effect of vacancies results in predicted diffusivities consistent with available experimental data. First-principles calculations show that at high temperatures the vacancy concentration is comparable to the oxygen solubility, and there is a strong binding energy and a redistribution of charge density between the oxygen atom and vacancy. Consequently, there is a strong attraction between the oxygen and vacancy in the Ni lattice, which impacts diffusion.

Icosahedral ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass

This paper presents a computational evidence of icosahedral short and medium range ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass using ab initio molecular dynamics simulation. It is found that 1551, 1541, and 1431 types of bond pairs are pronounced in both the liquid and glass states, resulting in icosahedral coordinate polyhedra at low temperatures. By linking the individual icosahedra through vertex-, edge-, face-, and intercross-shared atoms, icosahedral medium range ordering is formed. The predicted homogenized structure factor and pair correlation function of the glass structure have been confirmed to be in agreement with the experimental results.

Insight into structural, elastic, phonon, and thermodynamic properties of α-sulfur and energy-related sulfides: a comprehensive first-principles study

Earth-abundant and nontoxic sulfur (S) is emerging as a key element for developing new materials for sustainable energy. Knowledge gaps, however, still remain regarding the fundamental properties of sulfur especially on a theoretical level. Here, a comprehensive first-principles study has been performed to examine the predicted structural, elastic, phonon, thermodynamic, and optical properties of α-S8 (α-S) as well as energy-related sulfides. A variety of exchange–correlation (X–C) functionals and van der Waals corrections in terms of the D3 method have been tested to probe the capability of first-principles calculations. Comparison of predicted quantities with available experimental data indicates that (i) the structural information of α-S is described very well using an improved generalized gradient approximation of PBEsol; (ii) the band gap and dielectric tensor of α-S are calculated perfectly using a hybrid X–C functional of HSE06; (iii) the phonon and elastic properties of α-S are predicted reasonably well using for example the X–C functionals of LDA and PBEsol, and in particular the PBE + D3 and the PBEsol + D3 method; and (iv) the thermodynamic properties of α-S are computed accurately using the PBEsol + D3 method. Examinations using Li2S, CuS, ZnS, Cu2ZnSnS4 (CZTS), SnS, Sn2S3, SnS2, β-S8 (β-S), and γ-S8 (γ-S) validate further the crucial role of the van der Waals correction, thus suggesting the X–C functionals are PBEsol + D3 and PBE + D3 (and PBEsol in some cases) for sulfur as well as S-containing materials. We also examine the possibility by using the Debye model to predict thermodynamic properties of unusual materials, for example, α-S. In addition, the bonding characteristics and non-polar nature of α-S have been revealed quantitatively from phonon calculations and qualitatively from the differential charge density.

Low energy structures of lithium-ion battery materials Li(MnxNixCo1−2x)O2 revealed by first-principles calculations

A long-standing issue regarding the low energy structures for the partially disordered cathode materials Li(MnxNixCo1−2x)O2 has been probed by first-principles calculations. It is found that the transitional metals Mn, Ni, and Co in Li(MnxNixCo1−2x)O2 follow the maximum entropy probability distribution (MEPD), instead of the random distribution, according to the distributions of the minimal partial radial distribution functions and the correlation functions. Here, the MEPD is proposed to understand the low energy structures of the partially disordered lithium-ion battery materials.

First-principles calculations of interfacial and segregation energies in α-Cr2O3

The interfacial energies of three twin boundaries with low-index boundary planes: prismatic (101⁻0), basal O-terminated (0001), and basal Cr-terminated (0001), and the segregation energies of five doping elements (Ce, Hf, La, Y and Zr) have been calculated as a function of temperature. The static energies at 0 K were obtained through first-principles calculations and the energies at finite temperatures were derived based on the Debye model. The calculation results show that both the interfacial and segregation energies decrease as temperature increases and the segregation energies are found to be proportional to the ionic size mismatch and the interfacial energy. Our combined approaches suggest an efficient and less computationally intensive way to derive grain boundary energetics at finite temperatures.