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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.