Progress in Fatigue Research of Nitrided Titanium Alloys

LI Cong; WANG Xin; ZHOU Libo; CHEN Wei; CHEN Jian; LI Wei; CHEN Wanglin

doi:10.16490/j.cnki.issn.1001-3660.2026.09.009

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Surface Technology ›› 2026, Vol. 55 ›› Issue (9) : 98-112. DOI: 10.16490/j.cnki.issn.1001-3660.2026.09.009

Surface and Interface Strengthening Technology

Progress in Fatigue Research of Nitrided Titanium Alloys

LI Cong¹, WANG Xin¹, ZHOU Libo¹, CHEN Wei¹, CHEN Jian¹, LI Wei¹, CHEN Wanglin^2,*

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Abstract

Titanium alloys possess high specific strength, low density and excellent corrosion resistance and are therefore extensively employed in aerospace, chemical, energy and biomedical engineering. Nevertheless, their inherently low surface hardness, poor wear resistance and high friction coefficient restrict application under high load and long-life conditions. Nitriding is a thermochemical treatment in which nitrogen atoms diffuse into the surface below the alloy transformation temperature to form a hard nitride layer, and it serves as an important means to upgrade surface performance. However, its effect on fatigue behaviour is twofold: on the one hand, the nitrided layer introduces residual compressive stress and a hardness gradient to help suppress crack initiation; on the other hand, the brittle nitride film, grain coarsening and interfacial stress concentration created during the process can act as fatigue crack nucleation sites and reduce fatigue life. To date, investigations of the fatigue performance of nitrided titanium alloys have focused on single processes or specific parameters, and a systematic summary and mechanistic analysis have been lacking. The work aims to review the effects of different nitriding routes on the fatigue behaviour of titanium alloys, clarify the intrinsic relationship between nitrided layer architecture and fatigue performance, and reveal the micro-mechanisms of fatigue crack initiation and propagation. A comprehensive literature survey was conducted on the effects of gas nitriding, plasma nitriding, hybrid nitriding and several novel nitriding techniques on the fatigue response of representative titanium alloys. The effects of nitriding on fatigue performance were strongly structure dependent. Conventional gas nitriding and high-temperature plasma nitriding produced a 2-10 µm surface compound layer of brittle TiN and Ti₂N with a hardness of 1 000-2 000HV yet very low fracture toughness. Consequently, the layer readily developed surface micro-cracks under cyclic loading and served as the dominant fatigue crack origin. Beneath it, a 20-100 µm thick nitrogen diffusion zone exhibited graded hardness and high residual compression and effectively retarded crack propagation. Low-temperature plasma nitriding refined the diffusion-zone grains and introduced higher compressive stresses, thereby markedly increasing the fatigue limit. Hybrid treatments such as shot peening followed by nitriding, nitriding plus post-heat treatment, and nitriding plus particle bombardment eliminated the brittle compound layer and further refined the microstructure and introduced additional residual compression, leading to substantial fatigue improvements. Emerging techniques including low-temperature plasma nitriding, pulsed laser nitriding and induction-heated gas nitriding demonstrated the potential to control compound layer thickness, reduce surface roughness and optimize residual stress distributions. The ultimate fatigue performance resulted from the competition between strengthening and embrittlement mechanisms, and the outcome was governed by the synergy between nitrided-layer architecture and loading conditions. The compound layer, although increased surface hardness, was highly brittle and generated interfacial stress concentrations that readily nucleated fatigue cracks and were therefore the main cause of fatigue degradation. The diffusion zone, through its graded hardness, residual compression and grain refinement, effectively suppressed crack propagation and was the key factor in fatigue enhancement. Low-temperature, short-time, energy-controlled nitriding processes inhibit compound layer growth while producing a deep graded diffusion zone and significantly improve fatigue performance. Hybrid treatments, by combining multiple strengthening mechanisms, can achieve large increases in fatigue strength. This review provides a theoretical basis for optimizing surface treatment strategies for titanium alloys by elucidating how nitriding-route-dependent layer characteristics, crack initiation and propagation behavior, microstructural evolution and residual stress distributions govern fatigue performance.

Key words

titanium alloy / nitriding / compound layer / nitrogen diffusion layer / fatigue crack

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LI Cong, WANG Xin, ZHOU Libo, CHEN Wei, CHEN Jian, LI Wei, CHEN Wanglin. Progress in Fatigue Research of Nitrided Titanium Alloys[J]. Surface Technology. 2026, 55(9): 98-112

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Funding

National Natural Science Foundation of China (52575167, 52375142); Key Project of Hunan Provincial Department of Education (25A0193); Key Project of Hunan Provincial Natural Science Foundation (2026JJ30062); Guangdong Provincial Natural Science Foundation (2025A1515010944, 2022A1515010210)