Low-temperature plasma nitriding at 500°C on surface-nanocrystalline Ti–4Al–2V alloy

Abstract

Both surface nanocrystallization and high temperature favor nitrogen diffusion during plasma nitriding, but the surface nanostructure may be destroyed by high temperature to lose its diffusion-enhancing effect. In this paper, the combined effect of surface nanocrystallization by shot peening (SP) and nitriding temperature on the plasma nitriding of Ti–4Al–2V titanium alloy was studied. The results show that the interaction of nanocrystallization and temperature on plasma nitriding balanced best at 500 °C of the Ti–4Al–2V sample shot peened at both 0.2 MPa-10 min and 0.6 MPa-10 min. At this temperature, the surface nanocrystalline had maintained its thermal stability to some extent and played the greatest role in promoting nitrogen diffusion by more grain boundaries and a thickest nitrided layer was obtained on the SPed sample. The SPed sample nitrided at 500 °C exhibited better corrosion resistance and interface adhesion than the coarse-grained sample due to its denser/thicker nitrided layer and the retained gradient structure. In addition, the effect of surface roughness under different SP pressures on the corrosion resistance and adhesion of the nitrided layer was also concerned. A small surface roughness obtained under a low SP pressure was beneficial to improve the corrosion resistance and reflect the true adhesion of the nitrided layer.