Enhanced toughness of nitrided layers formed on Ti-6Al-4V alloy via surface mechanical attrition pre-treatment

This study focused on developing a beneficial nitriding process for use on Ti-6Al-4V alloy surfaces, involving surface mechanical attrition treatment (SMAT), which does not contaminate the existing nanocrystalline layer. The average grain size of this layer was refined to 10 nm with random crystallographic orientation. Differential scanning calorimetry confirmed that the recrystallization temperature of the Ti-6Al-4V nanocrystalline layer was 550 °C. The phase compositions, microstructure, hardness and toughness of the nitrided layer were studied by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and hardness testing. The nitrided layer of the SMAT sample was composed of nanoscale TiN, Ti2N and Ti with high nitrogen supersaturation. The nitriding kinetics was enhanced by the nanocrystalline layer at relatively low temperatures (600 °C). Compared to coarse-grained nitrided samples, the SMAT nitrided sample exhibited similar surface hardness but improved surface toughness, which may be attributed to the formation of a nanostructured nitrided layer.