Influence of nitriding on microstructure and fatigue behaviour of a solute-rich beta titanium alloy

The solute-rich beta titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr was subjected to 1500 bar nitrogen pressure at elevated temperatures (500-920°C), leading to a diffusion layer with a high surface hardness. Microstructural, crystallographic and compositional analyses indicate that TiN (δ) and Ti₂N (ε) are formed at temperatures exceeding 815°C. The increased concentration of nitrogen, which is a potent α-stabilizer in titanium, also causes α-Ti to form near the surface. The nitriding treatment does not significantly alter the tensile properties or fatigue limit in solution heat treated material. A subsequent ageing treatment of 72 h at 440°C and 16 h at 500°C reduces toughness significantly, allowing cracks induced by nitriding to propagate more easily into the bulk. Tensile ductility and fatigue performance of aged nitrided Ti-3Al-8V-6Cr-4Mo-4Zr are thus significantly lower than in the untreated reference condition.