Structural characterisation of hardening of Ti–Al–V alloys after nitridation by plasma immersion ion implantation

The surface hardness of the titanium alloy Ti–6Al–4V, widely used as construction material, can be improved by nitrogen implantation. The paper reports on a complex study of the structural mechanisms of hardening and its loss at higher temperatures with different methods (grazing incidence X-ray diffraction (GIXRD), in situ high-temperature diffraction with synchrotron radiation, elastic recoil detection analysis (ERDA), hardness measurements and surface inspection by scanning electron microscopy (SEM)). Plasma immersion ion implantation (PIII) produces a nitrogen profile directly below the surface. The formation of small TiN crystallites is detected, but the hardness increase is only about 40%. Annealing leads to the reduction of the TiN phase and the formation of Ti2N deeper in the material. The in situ X-ray diffraction (XRD) study of the phase formation gives information about the temperature dependence of these changes. The hardness increases after annealing by a factor of 2.5 compared to the unimplanted state. It is explained by precipitation hardening due to the formation of titanium nitrides in different depths of the material.