INFLUENCE OF NITRIDING ON MICROSTRUCTURE AND FATIGUE BEHAVIOR OF A SOLUTE-RICH BETA-TITANIUM ALLOY

The solute-rich beta titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr was subjecte d to 1500 bar nitrogen pressure at elevated temperatures (500-920 degr ees C), leading to a diffusion layer with a high surface hardness. Mic rostructural, crystallographic and compositional analyses indicate tha t TiN (delta) and Ti2N (epsilon) are formed at temperatures exceeding 815 degrees C. The increased concentration of nitrogen, which is a pot ent alpha-stabilizer in titanium, also causes alpha-Ti to form near th e surface. The nitriding treatment does not significantly alter the te nsile properties or fatigue limit in solution heat treated material. A subsequent ageing treatment of 72 h at 440 degrees C and 16 h at 500 degrees C reduces toughness significantly, allowing cracks induced by nitriding to propagate more easily into the bulk; Tensile ductility an d fatigue performance of aged nitrided Ti-3Al-8V-6Cr-4Mo-4Zr are thus significantly lower than in the untreated reference condition.