Preferential site occupancy of alloying elements in TiAl-based phases

First principles calculations are used to study the preferential occupation of ternary alloying additions into the binary Ti-Al phases, namely $\gamma$-TiAl, $\alpha_2$-Ti$_3$Al, $\beta_{\mathrm{o}}$-TiAl, and B19-TiAl. While the early transition metals (TMs, group IVB, VB , and VIB elements) prefer to substitute for Ti atoms in the $\gamma$-, $\alpha_2$-, and B19-phases, they preferentially occupy Al sites in the $\beta_{\mathrm{o}}$-TiAl. Si is in this context an anomaly, as it prefers to sit on the Al sublattice for all four phases. B and C are shown to prefer octahedral Ti-rich interstitial positions instead of substitutional incorporation. The site preference energy is linked with the alloying-induced changes of energy of formation, hence alloying-related (de)stabilisation of the phases. We further show that the phase-stabilisation effect of early TMs on $\beta_{\mathrm{o}}$-phase has a different origin depending on their valency. Finally, an extensive comparison of our predictions with available theoretical and experimental data (which is, however, limited mostly to the $\gamma$-phase) shows a consistent picture.Comment: 7 figures, 1 tabl

Alloying-related trends from first principles: An application to the Ti--Al--X--N system

Tailoring and improving material properties by alloying is a long-known and used concept. Recent research has demonstrated the potential of ab initio calculations in understanding the material properties at the nanoscale. Here we present a systematic overview of alloying trends when early-transition metals (Y, Zr, Nb, Hf, Ta) are added in the Ti$_{1-x}$Al$_x$N system, routinely used as a protective hard coating. The alloy lattice parameters tend to be larger than the corresponding linearised Vegard's estimation, with the largest deviation more than 2.5% obtained for Y$_{0.5}$Al$_{0.5}$N. The chemical strengthening is most pronounced for Ta and Nb, although also causing smallest elastic distortions of the lattice due to their atomic radii being comparable with Ti and Al. This is further supported by the analysis of the electronic density of states. Finally, mixing enthalpy as a measure of the driving force for decomposition into the stable constituents, is enhanced by adding Y, Zr and Nb, suggesting that the onset of spinodal decomposition will appear in these cases for lower thermal loads than for Hf and Ta alloyed Ti$_{1-x}$Al$_x$N.Comment: 9 pages, 6 figure

Ab initio study of the alloying effect of transition metals on structure, stability and ductility of CrN

The alloying effect on the lattice parameters, isostructural mixing enthalpies and ductility of the ternary nitride systems Cr1-xTMxN (TM=Sc, Y; Ti, Zr, Hf; V, Nb, Ta; Mo, W) in the cubic B1 structure has been investigated using first-principles calculations. Maximum mixing enthalpy due to large lattice mismatch in Cr1-xYxN solid solution shows a strong preference for phase separation, while Cr1-xTaxN exhibits a negative mixing enthalpy in the whole compositional range with respect to cubic B1 structured CrN and TaN, thus being unlikely to decompose spinodally. The near-to-zero mixing enthalpies of Cr1-xScxN and Cr1-xVxN are ascribed to the mutually counteracted electronic and lattice mismatch effects. Additions of small amounts of V, Nb, Ta, Mo or W into CrN coatings increase its ductility.Comment: 19 pages, 3 figure

Ab initio study of point defects in NiTi-based alloys

Changes in temperature or stress state may induce reversible B2$\leftrightarrow$(R)$\leftrightarrow$ B19' martensitic transformations and associated shape memory effects in close-to-stoichiometric nickel-titanium (NiTi) alloys. Recent experimental studies confirmed a considerable impact of the hydrogen-rich aging atmosphere on the subsequent B2 austenite $\leftrightarrow$ B19' martensite transformation path. In this paper, we employ Density Functional Theory to study properties of Ar, He, and H interstitials in B2 austenite and B19' martensite phases. We show that H interstitials exhibit negative formation energies, while Ar and He interstitials yield positive values. Our theoretical analysis of slightly Ni-rich Ni--Ti alloys with the austenite B2 structure shows that a slight over-stoichiometry towards Ni-rich compositions in a range 51--52\,\text{at.%} is energetically favorable. The same conclusion holds for H-doped NiTi with the H content up to \approx6\,\text{at.%}. In agreement with experimental data we predict H atoms to have a strong impact on the martensitic phase transformation in NiTi by altering the mutual thermodynamic stability of the high-temperature cubic B2 and the low-temperature monoclinic B19' phase of NiTi. Hydrogen atoms are predicted to form stable interstitial defects. As this is not the case for He and Ar, mixtures of hydrogen and the two inert gases can be used in annealing experiments to control H partial pressure when studying the martensitic transformations in NiTi in various atmospheres.Comment: 7 pages, 7 figure

Stability and elasticity of metastable solid solutions and superlattices in the MoN-TaN system: a first-principles study

Employing ab initio calculations, we discuss chemical, mechanical, and dynamical stability of MoN-TaN solid solutions together with cubic-like MoN/TaN superlattices, as another materials design concept. Hexagonal-type structures based on low-energy modifications of MoN and TaN are the most stable ones over the whole composition range. Despite being metastable, disordered cubic polymorphs are energetically significantly preferred over their ordered counterparts. An in-depth analysis of atomic environments in terms of bond lengths and angles reveals that the chemical disorder results in (partially) broken symmetry, i.e., the disordered cubic structure relaxes towards a hexagonal NiAs-type phase, the ground state of MoN. Surprisingly, also the superlattice architecture is clearly favored over the ordered cubic solid solution. We show that the bi-axial coherency stresses in superlattices break the cubic symmetry beyond simple tetragonal distortions and lead to a new tetragonal $\zeta$-phase (space group P4/nmm), which exhibits a more negative formation energy than the symmetry-stabilized cubic structures of MoN and TaN. Unlike cubic TaN, the $\zeta\text{-TaN}$ is elastically and vibrationally stable, while $\zeta$-MoN is stabilized only by the superlattice structure. To map compositional trends in elasticity, we establish mechanical stability of various Mo$_{1-x}$Ta$_x$N systems and find the closest high-symmetry approximants of the corresponding elastic tensors. According to the estimated polycrystalline moduli, the hexagonal polymorphs are predicted to be extremely hard, however, less ductile than the cubic phases and superlattices. The trends in stability based on energetics and elasticity are corroborated by density of electronic states

Point-defect engineering of MoN/TaN superlattice films: A first-principles and experimental study

Superlattice architecture represents an effective strategy to improve performance of hard protective coatings. Our model system, MoN/TaN, combines materials well-known for their high ductility as well as a strong driving force for vacancies. In this work, we reveal and interpret peculiar structure-stability-elasticity relations for MoN/TaN combining modelling and experimental approaches. Chemistry of the most stable structural variants depending on various deposition conditions is predicted by Density Functional Theory calculations using the concept of chemical potential. Importantly, no stability region exists for the defect-free superlattice. The X-ray Diffraction and Energy-dispersive $\text{X-ray}$ Spectroscopy experiments show that MoN/TaN superlattices consist of distorted fcc building blocks and contain non-metallic vacancies in MoN layers, which perfectly agrees with our theoretical model for these particular deposition conditions. The vibrational spectra analysis together with the close overlap between the experimental indentation modulus and the calculated Young's modulus points towards MoN$_{0.5}$/TaN as the most likely chemistry of our coatings