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Twinning is a fundamental mechanism behind the simultaneous increase of strength and ductility in medium- and high-entropy alloys, but its operation is not yet well understood, which limits their exploitation. Since many high-entropy alloys showing outstanding mechanical properties are actually thermodynamically unstable at ambient and cryogenic conditions, the observed twinning challenges the existing phenomenological and theoretical plasticity models. Here, we adopt a transparent approach based on effective energy barriers in combination with first-principle calculations to shed light on the origin of twinning in high-entropy alloys. We demonstrate that twinning can be the primary deformation mode in metastable face-centered cubic alloys with a fraction that surpasses the previously established upper limit. The present advance in plasticity of metals opens opportunities for tailoring the mechanical response in engineering materials by optimizing metastable twinning in high-entropy alloys.

The formation energy of the interface between face-centered cubic (fcc) and hexagonal close packed (hcp) structures is a key parameter in determining the stacking fault energy (SFE) of fcc metals and alloys using thermodynamic calculations. It is often assumed that the contribution of the planar fault energy to the SFE has the same order of magnitude as the bulk part, and thus the lack of precise information about it can become the limiting factor in thermodynamic predictions. Here, we differentiate between the interfacial energy for the coherent fcc(1 1 1)/hcp(0 0 0 1) interface and the 'pseudo-interfacial energy' that enters the thermodynamic expression for the SFE. Using first-principles calculations, we determine the coherent and pseudo-interfacial energies for six elemental metals (Al, Ni, Cu, Ag, Pt, and Au) and three paramagnetic Fe-Cr-Ni alloys. Our results show that the two interfacial energies significantly differ from each other. We observe a strong chemistry dependence for both interfacial energies. The calculated pseudo-interfacial energies for the Fe-Cr-Ni steels agree well with the available literature data. We discuss the effects of strain on the description of planar faults via thermodynamic and ab initio approaches.

We demonstrate that the interface structure has a significant influence on the magnetic state of MnGa/Co films consisting of L1(0)-MnGa on face-centered-cubic Co(001) surface. We reveal an antiferromagnetic to ferromagnetic magnetization reversal as a function of the lateral lattice constant. The magnetization reversal mainly originates from localized states and weak hybridization at interface due to charge redistribution between muffin-tin spheres and interstitial region. The magnetic anisotropy energy of Mn/Co interface system is enhanced with increasing in-plane lattice constant, which is ascribed to the interface interactions and the above magnetization reversal.

Using the full potential linearized augmented plane wave method, we have investigated the magnetic properties of a hexagonal boron nitride (h-BN) monolayer induced by a 0.5 monolayer of oxygen (O) and fluorine (F). It has been observed that the F and O adlayers are adsorbed on the boron top site in the most stable structure. We find that both systems display ferromagnetic ground states. In F/BN, the calculated magnetic moments of F and N atoms are 0.18 and 0.44 μ(B), respectively. Also, the band structure of F/BN is very close to half-metallic. This result is fundamentally different from that found with the pseudo-potential method (Zhou et al 2010 Phys. Rev. B 81 085442). In O/BN, the calculated magnetic moments of O and N atoms are 0.91 and 0.4 μ(B), respectively. The calculated band structure shows a metallic state. Using mean-field theory, the estimated Curie temperature is about 185 K when we assume a three-dimensional interaction. Very interestingly, it has been found that the O/BN may show a ferromagnetic ground state far beyond room temperature. In addition, the calculated x-ray absorption spectroscopy and x-ray magnetic circular dichroism are presented.

Using the full potential linearized augmented plane wave (FLAPW) method, we have explored the magnetic properties of ultrathin C and N layers on a MgO(001) surface. It has been found that the free standing C layer has a ferromagnetic (FM) ordering with a magnetic moment of 0.82 μ(B), while the free standing N layer with the same coverage displays an antiferromagnetic (AFM) state with a magnetic moment of 1.68 μ(B). Through a structure optimization procedure, we have found that both C and N adatoms are adsorbed on the O-top position in the presence of a MgO(001) surface. The ultrathin 0.25 monolayer coverage of C/MgO(001) film becomes a magnetic semiconductor and shows the c(2 × 2) AFM ground state with a magnetic moment of 0.62 μ(B). Surprisingly, the ultrathin N/MgO(001) manifests a half metallic state with a magnetic moment of 0.47 μ(B). In addition, the induced spin polarization in the O atom is found and this hybridization significantly affects the density of states (DOS) behavior. This peculiar DOS feature results in the opposite magnetic ground states of C and N layers on a MgO(001) surface. Additionally, it is found that the N/MgO(001) has a perpendicular magnetocrystalline anisotropy energy of 63 µeV. We also present theoretically calculated x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) results.

Using the full potential linearized augmented plane wave (FLAPW) method, the magnetic properties of two-dimensional Ru and Rh monolayers (MLs) on a NiAl(001) surface have been investigated. It has been found that free standing one monolayer Ru and Rh films have ferromagnetic ground state with magnetic moments of 2.21 and 1.48 μ(B), respectively. The ferromagnetism is still observed even on a Ni terminated NiAl(001) surface, while no magnetic state is found on an Al terminated surface. The calculated magnetic moments of Ru and Rh atoms are 1.56 and 0.88 μ(B), respectively. In addition, an induced magnetic moment in surface Ni is observed. It has been found that the free standing Ru film has perpendicular magnetization to the film surface with a magnetocrystalline anisotropy (MCA) energy of 0.66 meV/atom, while an in-plane MCA energy of 0.37 meV/atom is achieved in Rh film. Very interestingly, we find that both Ru/NiAl(001) and Rh/NiAl(001) films have perpendicular magnetic anisotropy and the calculated MCA energies are 0.66 and 1.11 meV in Ru/NiAl(001) and Rh/NiAl(001), respectively. Along with the magnetic anisotropy, we have presented theoretically calculated x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) results.

Using the full-potential linearized augmented plane wave method, we have investigated the oxygen vacancy defect induced ferromagnetism in both rutile and anatase TiO(2). It has been found that the oxygen vacancy induces lattice distortion in rutile TiO(2), whereas there is no such meaningful change in the anatase structure. Interestingly, the lattice distorted rutile TiO(2) shows an oxygen vacancy induced ferromagnetic state with a magnetic moment of 0.22 µ(B) in the Ti atom neighboring the vacancy site, while only 0.06 µ(B) is observed in the Ti atom in anatase TiO(2). We attribute the sizable magnetic moment due to the oxygen vacancy in rutile TiO(2) to the charge redistribution owing to lattice distortion. Experimentally measured magnetic hysteresis curves for undoped rutile and anatase TiO(2) films clearly display ferromagnetic behavior at room temperature. The observed magnetic strength of the rutile sample turns out to be larger than that of the anatase sample, in accordance with the theoretical calculations.