Progress in Fatigue Research of Nitrided Titanium Alloys
Titanium alloys are extensively employed in aerospace, chemical, energy, and biomedical engineering owing to their high specific strength, low density, and excellent corrosion resistance. However, their inherently low surface hardness, poor wear resistance, and high friction coefficient restrict application under high-load, long-life conditions. Nitriding, a thermochemical treatment in which nitrogen atoms diffuse into the surface below the alloy transformation temperature to form a hard nitride layer, serves as an important means to upgrade surface performance. Its effect on fatigue behaviour is twofold: the nitrided layer introduces residual compressive stress and a hardness gradient that suppress crack initiation, while the brittle nitride film, grain coarsening, and interfacial stress concentration created during processing can act as fatigue crack nucleation sites and reduce fatigue life. This review systematically examines the effects of gas nitriding, plasma nitriding, hybrid nitriding, and several novel nitriding techniques on the fatigue response of representative titanium alloys. Conventional gas nitriding and high-temperature plasma nitriding produce a 2–10 µm surface compound layer of brittle TiN and Ti2N with hardness of 1,000–2,000 HV yet very low fracture toughness, which readily develops surface micro-cracks under cyclic loading and serves as the dominant fatigue crack origin. Beneath it, a 20–100 µm thick nitrogen diffusion zone exhibits graded hardness and high residual compression. Low-temperature, short-duration, or energy-controlled processes such as plasma nitriding, low-temperature plasma nitriding, and pulsed laser nitriding tend to form thin compound layers and deep diffusion layers, pushing crack sources to subsurface or internal defects and extending fatigue life. Hybrid treatments (nitriding followed by shot peening or heat treatment) further suppress surface crack initiation by removing the brittle layer, refining grains, and introducing high residual compressive stress, achieving significant fatigue strength improvement. The review clarifies the intrinsic relationship between nitrided layer architecture and fatigue performance, and reveals the micro-mechanisms of fatigue crack initiation and propagation, providing a theoretical basis for optimizing titanium alloy surface treatment processes.