TEM analysis of surface layer of Ti-6Al-4V ELI alloy after slide burnishing and low-temperature gas nitriding

摘要

Gas nitriding is one of the more effective ways of improving wear and corrosion resistance of titanium alloys, but this treatment performed at a high temperature degrades their high specific strengths. Therefore, we have subjected a Ti-6Al-4V alloy to a hybrid multi-stage treatment including turning with slide burnishing followed by gas nitriding at temperature reduced down to 540 degrees C. The mechanical treatment introduces a large number of defects in the form of dislocations and grain boundaries-opening additional paths for nitrogen diffusion. High resolution electron microscopy shows that after such mechanical treatment the surface is covered with an amorphous tribo-film. Below the surface, to a depth of some 100 nm, a layer of elongated dislocation cells is seen. The latter was backed by a 500 nm thick layer of strongly defected laths. Gas nitriding at 540 degrees C caused at first a crystallization of the tribo-layer (after 8 h), producing a mixture of nitrides and oxides. Next, below a tribo-layer, a dense layer of the 8-TiN phase is formed, with a thickness of 5 nm and 50 nm after 16 h and 24 h, respectively. Application of slide burnishing and low-temperature gas nitriding causes an increase in surface hardness by 5-10 % without compromising the strength of the core material.