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.

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.

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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.