**4. Conclusions**

The process of high-temperature interaction of titanium with gaseous medium (nitrogen or oxygen) was modelled at temperatures T<Tα↔β and T>Tα↔β considering the surface processes and structural phase transformations.

The kinetics of surface processes is reflected by the mass balance equation, which takes into account the interaction of an external flux of impurities to the surface and its chemisorption with diffusion dissolution and segregation on defects as a result of a chemical interaction with titanium atoms.

The kinetics of diffusion saturation of α-titanium by nitrogen under rarefied atmosphere (1 Pa) in the temperature range of 750-850 0 C was investigated experimentally and analytically. The influence of time and temperature parameters on the depth of the nitrided layer and a change of its microhardness was estimated.

It was shown the role of these interstitial elements as α-stabilizers in forming the diffusion zone which contains three layers based on α-phase, α+β-phases and β-phase.

It was received the solution of the formulated task as for diffusion of nitrogen or oxygen in such a heterogeneous medium taking into account the motion of interfaces.

The constants of parabolic growth of layers were calculated. It allowed to foresee the kinetics of their growth and distribution of interstitial elements (nitrogen or oxygen) in diffusion zone.

The adequacy of the proposed model representations was confirmed by the results of experi‐ mental investigations on nitriding of titanium at Т=950 0 C. The microstructural evolution (after processing times of 1 and 5 h) of the diffusion zone which is caused by the structural phase transformations during diffusion of nitrogen was examined experimentally.
