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

Abstract

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 °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 °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 δ-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.