Phase evolution peculiarities and micromechanical behavior of nitrided 3D-printed Ti–6Al–4V titanium alloy

The kinetics of phase-structural transformations and the complex local mechanical response of the surface layers of Ti–6Al–4V titanium alloy manufactured by selective laser melting (SLM) were investigated during diffusion nitriding in a commercially pure nitrogen environment. Gas nitriding was performed in three isothermal regimes (750°C, 800°C, 850°C) with holding times of 5, 10, and 15 h. Quantitative X-ray diffraction analysis showed that at 850°C for 15 h, the total volume fraction of nitride phases exceeded 61%, with textured tetragonal nitride ε-Ti2N (50.21%) being the dominant phase. Using the instrumental microindentation method according to the Oliver–Farr algorithms (under a load of 0.5 N), a monotonic increase in the mechanical characteristics of the modified surface was observed. The maximum strengthening was obtained at 850°C, 15 h: the microhardness HIT increased in 2.7 times compared to the initial state (from 4.99 to 13.44 GPa), and the elastic modulus EIT reached 205.33 GPa. Analysis of the load-unload curves showed a regular redistribution of the energy components of the deformation contact: the work of plastic deformation Wplast decreased by 37% (to 0.212 mkJ), and the elastic recovery coefficient ηIT increased to 33.95%. In combination with the calculated nine-fold increase in the resistance to plastic deformation H3/Е2 to 0.0574 GPa, this justifies the high ability of the formed gradient structure to elastically relax contact forces without the risk of microcracking and tribodestruction of the nitride layer.