Progress in Fatigue Research of Nitrided Titanium Alloys

• • Compound layers of 2–10 µm thickness with hardness 1,000–2,000 HV and low fracture toughness act as dominant fatigue crack origins under cyclic loading; industrial adoption of conventional gas nitriding must therefore balance wear resistance against a quantified fatigue debit, particularly for aerospace components requiring >10^7 cycles. • • Nitrogen diffusion zones of 20–100 µm provide graded hardness, grain refinement, and residual compressive stress that shield against crack propagation; however, high-temperature long-duration nitriding induces grain coarsening and interfacial tensile stress concentration, negating the strengthening contribution and reducing fatigue life by up to an order of magnitude. • • Low-temperature, short-duration, or energy-controlled processes (plasma nitriding, low-temperature plasma nitriding, pulsed laser nitriding) produce thin compound layers and deep diffusion layers, shifting crack initiation to subsurface or internal defects and extending fatigue life; these routes are recommended for high-cycle, long-life service scenarios such as turbine blades and fasteners. • • Hybrid treatments combining nitriding with shot peening or heat treatment remove the brittle compound layer, refine grains, and introduce high residual compressive stress, achieving significant fatigue strength improvement; this approach enables submicron compound layer control, deep residual stress fields, and synergistic grain refinement, offering a scalable, green, and transferable solution for critical components.