There have been extensive experimental observations of the anisotropic corrosion behavior of metals and alloys, and their mechanisms were assumed to be correlated with the so-called surface energy or the work function. However, to date, a specified mechanism or theory to interpret anisotropic corrosion behavior remains unclear. Here, we determine the anisotropic anodic dissolution of metals and alloys in corrosive environments by developing a formula to specify the relationship between the electrode potential (U) and the current density (I) by considering the basic parameters of our defined surface energy density (Esurf/ρ) and the work function (Φ). Therefore, we build an ab initio model to evaluate the anisotropic anodic dissolution behavior of metals and alloys using the inputs obtained within density functional theory. This theory is further validated in the case of variations in the crystallographic planes of Mg. Moreover, some selected alloying additions such as Ga, Cd, Hg, In, As, and Cr are theoretically elucidated to effectively reduce the anodic dissolution rates of the Mg matrix to some extent, in close agreement with available experimental observations. This model is capable of predicting the anisotropic anodic dissolution behavior, providing a promising perspective for designing better corrosion-resistant alloys.Download high-res image (334KB)Download full-size image

•The stability of tetragonal YB2C was ensured by the phonon spectrum study.•The robust interlayer interactions are caused by the hybridization of orbitals.•The hardness of tetragonal YB2C is relatively high.•The lowest ideal shear strength of tetragonal YB2C is 12.9 GPa in slip system of (001)[100].By means of the first-principles calculations, the lattice parameters, electronic structures, phonon dispersions, and mechanical properties of the rare earth metal borocarbide YB2C have been theoretically investigated. The dynamically stability of the layered tetragonal YB2C has been evidenced based on the frozen phonon method. We have found that the covalent bonding between B-2p, C-2p and Y-5d orbitals are responsible for the strong interlayer interactions based on the calculated electronic structures and ELF images. The estimated hardness of P42/mbc-YB2C is around 23.46 GPa which is comparable with the well-known ultra-incompressible oP6-OsB2. Additionally, the analysis of the ideal shear and tensile strength of YB2C reveals the importance of covalent bonds between Y and B/C layer which help to enhance the resistance under deformation.

The site preferences, fast and huge anisotropic diffusion mechanisms of Cu (Ag) in β–Sn as well as their reduction on the self-diffusivity of Sn, have been investigated using the first-principles and ab initio molecular dynamics methods. We have found that Cu prefers the interstitial site, whereas Ag is preferable in the substitutional site, which is mostly dominated by their different size factors. Electronic structure further evidences that the d–s hybridization between the solute and the host atom also contributes to the site preferences. It is also deduced that the fast diffusion of Cu (Ag) is mostly due to the interstitial diffusion mechanism and their diffusivity can be correlated with the amount of their respective interstitial solution. Their faster diffusion along the c-axis can be attributed to the extremely low migration energy barrier caused by the straight tunnel of considerable size with the screw axis symmetry of 2π/4 along the c-axis. Furthermore, it is found that during the process of diffusion the interstitially dissolved Cu (Ag) atoms would combine with the nearby Sn-vacancy and further annihilate the vacancy, thereby reducing the self-diffusion of Sn.

At P = 1 atm, the only stable compounds in the Na–Bi binary system are Na3Bi and NaBi, which have recently been discovered to exhibit intriguing electronic behaviour as a 3D topological Dirac semimetal and a topological metal, respectively. By means of first-principles calculations coupled with evolutionary structural searches, we have systematically investigated the phase stabilities, the crystal structures and the corresponding electronic properties of the binary Na–Bi system. At ambient pressure, our calculations have reproduced well the experimentally observed compositions and structures of Na3Bi and NaBi. At high pressures, we have found that Na3Bi is transformed from the ground-state hexagonal hP24 phase to a cubic cF16 phase above 0.8 GPa, confirming previous experiments, and then to a conventional band-insulating oC16 phase above 118 GPa. The cubic cF16 phase would exhibit novel topological band ordering similar to that in HgTe. The topological metal NaBi has also been found to undergo a structural phase transition from the ambient tetragonal tP2 to a cubic cP2 structure above 36 GPa. Four compounds never before reported, Na6Bi, Na4Bi, Na2Bi and NaBi2, with new compositions, have been predicted to be experimentally synthesizable over a wide range of pressures starting at 142.5 GPa, 105 GPa, 38 GPa and 171 GPa, respectively. Moreover, a common charge transfer from Na to Bi has been observed for all compounds, but substantial interstitial charge localization in Na atomic cages has been noticed only in two compounds, Na6Bi and Na4Bi, and may be associated with close-packed Na environments.

The constitution of the binary system Ir-B has been established between 10 and 70 at.% boron for temperatures above 700°C based on differential scanning calorimetry, electron probe microanalysis, and isothermal low temperature annealing experiments (≤1000°C). Four binary phases have been found, namely Ir4B5+x, Ir5B4+x and the high and low temperature modification of Ir4B3-x. X-ray structure analyses were performed on single crystals of Ir4B5+x (x = 0, Ir4B5 type; space group C2/m; a = 1.05200(2), b = 0.289564(6) and c = 0.60958(1) nm, β = 91.156(2)°), Ir5B4+x (x=0, Ir5B4 type; space group I41/a; a = 0.62777(1) and b = 1.02599(2) nm) and on the low temperature modification of Ir4B3-x (x=0, IrB0.9 type; space group Cmc21; a = 0.27728(1), b = 0.75742(2) and c = 0.73152(2) nm). The high temperature modification of Ir4B3-x (WC type; space group \(P\overline 6 m2\); a = 0.28137(5) and c = 0.2828(1) nm) has been confirmed by X-ray powder diffraction. By means of the first-principle calculations, in combination with the evolutionary structural search algorithm, the compositions, structures and enthalpies of the Ir-B system have been investigated theoretically. Confirming the experimental observations on Ir4B5, Ir5B4 and Ir4B3, we have investigated several metastable phases at other stoichiometries, such as IrB, IrB2 and Ir3B2. We also proposed three thermodynamically and dynamically stable new structures of oF28-Ir4B3, oC8-IrB and mC10-Ir3B2, which may be synthesized under certain conditions.本文通过 差示扫描量热法、电子探针以及等温低温退火处理, 系统地研究了铱硼二元体系在元素含量10%~70%;范围内 化 合物的结构及组成, 并成功的合成及表征了四种化合物: Ir4B5+x, Ir5B4+x;以及Ir4B3;−129:832x的高温和低温相. 运用单晶衍射法确定了Ir4B5+x (x = 0, Ir4B5;型; 空间群C2/ma = 1.05200(2), b = 0.289564(6), c = 0.60958(1) nm, β = 91.156(2)°), Ir5B4+x (x = 0, Ir5B4型; 空间群I41/aa = 0.62777(1), b = 1.02599(2) nm);以及Ir4B3−x低温相 (x = 0, IrB0.9型; 空间群Cmc21a = 0.27728(1), b = 0.75742(2), c = 0.73152(2) nm)的晶格常数及结构. 运用粉末衍射法确定了Ir4B3−x高温相(WC;型; 空间群\(P\overline 6 m2\)a = 0.28137(5), c = 0.2828(1) nm)的晶格常数及结构. 通过 第一性原理计算结合结构演化搜索方法, 从理论上研究了该体系的组成, 结构以及相稳定性. 除了验证实验合成的Ir4B5, Ir5B4;以及Ir4B3;, 本文还预测了三个可能在一定条件下合成的新稳定结构: oF28-Ir4B3, oC8-IrB;以及mC10-Ir3B2.

In combination with variable-composition evolutionary algorithm calculations and first-principles calculations, we have systematically searched for all the stable compounds and their crystal structures in the extensively investigated binary Mn–B system. Our results have uncovered four viable ground-state compounds, with Mn2B, MnB, and MnB4, and previously never reported MnB3 and two metastable compounds, MnB2 and Mn3B4. Our calculations demonstrate that the early characterized mC10 structure of MnB4 showed dynamic instability with large imaginary phonon frequencies and, instead, a new mP20 structure is predicted to be stable both dynamically and thermodynamically, with a considerable energy gain and no imaginary phonon frequencies. The new MnB3 compound crystallizes in the monoclinic mC16 structure which lies 3.2 meV per atom below the MnB (oP8) ↔ MnB4 (mP20) tie-line at T = 0 K. Furthermore, these proposed phases have been verified by our annealed samples after arc-melting synthesis and corresponding powder XRD measurements.

By means of first-principles calculations, we have systematically investigated the structural, elastic, vibrational, thermal and electronic properties of the ground-state phase for the intermetallic compound U2Mo. Our results reveal that the previously synthesized I4/mmm structure of U2Mo is a metastable phase and unstable, neither thermodynamically nor vibrationally at the ground state. In combination with the evolutionary structural searches, our first-principles calculations suggest a new ground-state Pmmn phase, which has been confirmed theoretically to be stable, both thermodynamically and vibrationally. Moreover, through the DFT + D technique we have discussed the influence of van der Waals interactions on the structural, elastic and vibrational properties, revealing a weak effect in pure U and Mo solids and U2Mo alloy. The analysis of the electronic band structures evidences its electronic stabilities with the appearance of a deep valley in the density of states at the Fermi level. Moreover, we have investigated further the temperature-dependent structural, thermal expansion and elastic properties of our proposed Pmmn ground-state phase. These results are expected to stimulate further experimental investigations of the ground-state phase of U2Mo.

•For one H atom, oxygen-vacancy (O:V) pair is a stronger trap compared with vacancy.•The multi-(O:V) pairs complex significantly forbids the swelling of H clusters.•The growth of H cluster is correlated with the available supplied charges.•The addition of Ti does not show an obvious effect in the interaction with H atom.Through first-principles calculations, we systematically investigated the hydrogen interactions with the oxygen-vacancy (O:V) pairs complex in bcc Fe matrix (mimic oxygen-enriched nanoclusters (NCs) of ODS steels) in comparison with the vacancy-alone defects. The results uncovered that the presence of the (O:V) pairs in oxygen-enriched NCs play a crucial role in prohibiting the growth and swelling of the hydrogen cluster but strongly trap a few hydrogen atoms around each cluster of vacancies. As accompanied with a high density of dispersed NCs in ODS steels, this fact significantly elevates the tolerance of the critical hydrogen concentration of ODS steels as compared with traditional steels. The underlying mechanism to pin the growth of hydrogen cluster has been elucidated to be strongly correlated with the viable charges transfer from the nearby Fe atoms around vacancies. This is the key to determine the trapped concentration and the distribution of hydrogen atoms in ODS steels.

•We have proposed charge transfer associated strain destabilization mechanism.•H trapping at vacancies is coupled with high pre-existing charges interstitials.•When trapped, H is usually negatively charged from its surrounding metal atoms.•Instability of H cluster around vacancy is determined by deficit charges supplying.•The sharp strain energy increment corresponds to the shortage of supplied charges.Interaction between hydrogen (H) and metals is central to many materials problems of scientific and technological importance. H segregation or trapping at lattice defects plays a crucial role in determining the properties of these materials. Through first-principles simulations, we propose a unified mechanism involving charge transfer associated strain destabilization to understand H segregation behavior at vacancies. We discover that H prefers to occupy interstitials with high pre-existing charge densities and the availability of such interstitials sets the limit on H trapping capacity at a vacancy. Once the maximum H capacity is reached, the dominant charge donors switch from the nearest-neighbor (NN) to the next-nearest-neighbor (NNN) metal atoms. Accompanying with this long-range charge transfer, the sharply increased reorganization energy would occur, leading to the instability of the H-vacancy complex. The physical picture unveiled here appears universal across the BCC series and is believed to be relevant to other metals/defects as well.

Using first-principles local density functional approach, we have calculated the ground-state structural phase stabilities and enthalpies of formation of thirty-six binary transition-metal refractory TM and TM3 compounds formed by Group IV elements T (T = Ti, Zr, Hf) and platinum group elements M (M = Ru, Rh, Pd, Os, Ir, Pt). We compared our results with the available experimental data and found good agreement between theory and experiment in both the trends of structural stabilities and the magnitudes of formation enthalpies. Moreover, based on our calculated results, an empirical relationship between cohesive energies (ΔE) and melting temperatures (Tm) was derived as Tm = 0.0292ΔE/kB (where kB is the Boltzmann constant) for both TM and TM3 compounds.Graphical abstractHighlights► Structural phase stabilities of thirty-six refractory compounds have been reported. ► Enthalpies of formation of thirty-six refractory compounds have been derived. ► A relationship between cohesive energies and melting temperatures was derived.

Though extensively studied, hardness, defined as the resistance of a material to deformation, still remains a challenging issue for a formal theoretical description due to its inherent mechanical complexity. The widely applied Teter’s empirical correlation between hardness and shear modulus has been considered to be not always valid for a large variety of materials. The main reason is that shear modulus only responses to elastic deformation whereas the hardness links both elastic and permanent plastic properties. We found that the intrinsic correlation between hardness and elasticity of materials correctly predicts Vickers hardness for a wide variety of crystalline materials as well as bulk metallic glasses (BMGs). Our results suggest that, if a material is intrinsically brittle (such as BMGs that fail in the elastic regime), its Vickers hardness linearly correlates with the shear modulus (Hv = 0.151G  ). This correlation also provides a robust theoretical evidence on the famous empirical correlation observed by Teter in 1998. On the other hand, our results demonstrate that the hardness of polycrystalline materials can be correlated with the product of the squared Pugh’s modulus ratio and the shear modulus (Hv=2(k2G)0.585−3Hv=2(k2G)0.585−3 where k = G/B is Pugh’s modulus ratio). Our work combines those aspects that were previously argued strongly, and, most importantly, is capable to correctly predict the hardness of all hard compounds known included in several pervious models.One sentence for highlighted figure: Vickers hardness has been theoretically evidenced to correlate successfully with shear modulus for various bulk metallic glasses (BMGs, left panel) and with a product of the squared Pugh’s modulus ratio and shear modulus for a wide variety of polycrystalline materials (including all superhard materials known, right panel).Highlights► This work derived a theoretical formula of Vickers hardness linearly correlated with shear modulus for various bulk metallic glasses. ► This work derived a theoretical formula to predict Vickers hardness of a wide variety of polycrystalline materials. ► This work generalized the hardness formula through a thorough comparison for BMGs and polycrystalline materials. ► This work validated the powerful prediction of the proposed hardness formula. ► This work highlighted a comparison of several semi-empirical hardness models with the currently proposed formula.

Using an ab initio density functional approach, we report on the ground-state phase stabilities, enthalpies of formation, electronic, and elastic properties of the Ti–Pd alloy system. The calculated enthalpies of formation are in excellent agreement with available calorimetric data. We found a linear dependence between the calculated enthalpies of formation of several intermetallic structures and the Pd-concentration, indicating that each of these compounds has a very limited composition range. The elastic constants for many of these Ti–Pd intermetallics were calculated and analyzed. The B2 TiPd phase is found to be mechanically unstable with respect to the transformation into the monoclinic B19′ structure. A series of hydrides, Ti2PdHx (x = 1, 1.5, 2, 3, 4), have been investigated in terms of electronic structure, enthalpies of hydrogen absorption, and site preference of H atoms. Our results illustrate the physical mechanism for hydrogen absorption in term of the charge transfer, and explain why TiPd2 does not form a stable hydride.

At P = 1 atm, the only stable compounds in the Na–Bi binary system are Na3Bi and NaBi, which have recently been discovered to exhibit intriguing electronic behaviour as a 3D topological Dirac semimetal and a topological metal, respectively. By means of first-principles calculations coupled with evolutionary structural searches, we have systematically investigated the phase stabilities, the crystal structures and the corresponding electronic properties of the binary Na–Bi system. At ambient pressure, our calculations have reproduced well the experimentally observed compositions and structures of Na3Bi and NaBi. At high pressures, we have found that Na3Bi is transformed from the ground-state hexagonal hP24 phase to a cubic cF16 phase above 0.8 GPa, confirming previous experiments, and then to a conventional band-insulating oC16 phase above 118 GPa. The cubic cF16 phase would exhibit novel topological band ordering similar to that in HgTe. The topological metal NaBi has also been found to undergo a structural phase transition from the ambient tetragonal tP2 to a cubic cP2 structure above 36 GPa. Four compounds never before reported, Na6Bi, Na4Bi, Na2Bi and NaBi2, with new compositions, have been predicted to be experimentally synthesizable over a wide range of pressures starting at 142.5 GPa, 105 GPa, 38 GPa and 171 GPa, respectively. Moreover, a common charge transfer from Na to Bi has been observed for all compounds, but substantial interstitial charge localization in Na atomic cages has been noticed only in two compounds, Na6Bi and Na4Bi, and may be associated with close-packed Na environments.

In combination with variable-composition evolutionary algorithm calculations and first-principles calculations, we have systematically searched for all the stable compounds and their crystal structures in the extensively investigated binary Mn–B system. Our results have uncovered four viable ground-state compounds, with Mn2B, MnB, and MnB4, and previously never reported MnB3 and two metastable compounds, MnB2 and Mn3B4. Our calculations demonstrate that the early characterized mC10 structure of MnB4 showed dynamic instability with large imaginary phonon frequencies and, instead, a new mP20 structure is predicted to be stable both dynamically and thermodynamically, with a considerable energy gain and no imaginary phonon frequencies. The new MnB3 compound crystallizes in the monoclinic mC16 structure which lies 3.2 meV per atom below the MnB (oP8) ↔ MnB4 (mP20) tie-line at T = 0 K. Furthermore, these proposed phases have been verified by our annealed samples after arc-melting synthesis and corresponding powder XRD measurements.

By means of first-principles calculations, we have systematically investigated the structural, elastic, vibrational, thermal and electronic properties of the ground-state phase for the intermetallic compound U2Mo. Our results reveal that the previously synthesized I4/mmm structure of U2Mo is a metastable phase and unstable, neither thermodynamically nor vibrationally at the ground state. In combination with the evolutionary structural searches, our first-principles calculations suggest a new ground-state Pmmn phase, which has been confirmed theoretically to be stable, both thermodynamically and vibrationally. Moreover, through the DFT + D technique we have discussed the influence of van der Waals interactions on the structural, elastic and vibrational properties, revealing a weak effect in pure U and Mo solids and U2Mo alloy. The analysis of the electronic band structures evidences its electronic stabilities with the appearance of a deep valley in the density of states at the Fermi level. Moreover, we have investigated further the temperature-dependent structural, thermal expansion and elastic properties of our proposed Pmmn ground-state phase. These results are expected to stimulate further experimental investigations of the ground-state phase of U2Mo.