An investigation was made of the ordering of oxygen and nitrogen interstitials in hexagonal close-packed Zr, Hf, and Ti using the corresponding oxygen-oxygen and nitrogen-nitrogen interactions obtained in the state-of-the-art first-principles calculations. Two main contributions, chemical and strain induced, to the interstitial-interstitial interactions were obtained by different techniques. It was found that there was the strong repulsion between interstitial atoms at the nearest- and next-nearest-neighbor coordination shells, which was solely determined by the chemical interaction determined on a fixed ideal lattice, while both contributions were important for more distant coordination shells. The Monte Carlo simulations revealed the existence of 3 stoichiometric compositions, MeI1/6, MeI1/3, and MeI1/2, for the ground-state structures of interstitials, having different ordering types. The results for the structures of oxygen interstitials were in good agreement with existing experimental data for the Ti and Hf alloys. In the case of Zr-O interstitial alloys, the general type of ordering was correctly predicted, although the detailed structure was at variance the experimental observations. The ordering transition temperatures in some cases were overestimated by a factor of 2. Also predicted was the ordering types of nitrogen interstitials in hexagonal close-packed Ti, Zr and Hf, which was similar to those in the case of oxygen interstitials.

Oxygen and Nitrogen Interstitial Ordering in HCP Ti, Zr, and Hf: an ab initio Study. A.V.Ruban, V.I.Baykov, B.Johansson, V.V.Dmitriev, M.S.Blanter: Physical Review B, 2010, 82[13], 134110