Key Takeaways & Executive Findings
- •• • 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.
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Abstract
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.
1. Introduction
Titanium alloys deliver high specific strength, low density, and excellent corrosion resistance, yet their low surface hardness, poor wear resistance, and high friction coefficient impose severe limitations under high-load, long-life conditions. Conventional gas nitriding and high-temperature plasma nitriding generate a 2–10 µm compound layer of brittle TiN and Ti2N with hardness of 1,000–2,000 HV but very low fracture toughness. This layer readily develops surface micro-cracks under cyclic loading and becomes the dominant fatigue crack origin, while the underlying 20–100 µm nitrogen diffusion zone provides graded hardness and residual compression. The net effect on fatigue life is a trade-off between compound-layer embrittlement and diffusion-layer strengthening, and industrial adoption has stalled because the brittle layer's thickness, hardness, and elastic modulus mismatch with the substrate can trigger early cracking at interfaces or surface defects.
This review systematically examines gas nitriding, plasma nitriding, hybrid nitriding, and novel nitriding techniques, clarifying the intrinsic relationship between nitrided layer architecture and fatigue performance. Low-temperature, short-duration, or energy-controlled processes such as plasma nitriding, low-temperature plasma nitriding, and pulsed laser nitriding 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) remove the brittle layer, refine grains, and introduce high residual compressive stress, further suppressing surface crack initiation. The analysis provides a theoretical basis for optimizing titanium alloy surface treatment processes, with the goal of suppressing brittle TiN, amplifying diffusion-layer compressive stress, and delivering efficient, green, transferable fatigue life extension for blades, fasteners, and other critical components.
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LI Cong, WANG Xin, ZHOU Libo, CHEN Wei, CHEN Jian, LI Wei, CHEN Wanglin (2026). Progress in Fatigue Research of Nitrided Titanium Alloys. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.09.009
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Frequently Asked Questions
What is the dominant fatigue failure mechanism in conventionally gas-nitrided titanium alloys, and how does the compound layer thickness influence crack initiation?
The dominant mechanism is brittle cracking of the 2–10 µm surface compound layer composed of TiN and Ti2N with hardness 1,000–2,000 HV and very low fracture toughness. Under cyclic loading, this layer readily develops surface micro-cracks that serve as the primary fatigue crack origin. When the compound layer is excessively thick, hard, or exhibits a large elastic modulus mismatch with the substrate, brittle cracking initiates at the interface or surface defects, reducing fatigue life. Beneath the compound layer, a 20–100 µm nitrogen diffusion zone provides graded hardness and residual compressive stress that can shield against crack propagation, but high-temperature long-duration nitriding causes grain coarsening and interfacial tensile stress concentration, weakening the strengthening contribution.
Which nitriding processes shift crack initiation to subsurface or internal defects, and what are the operational thresholds for achieving this shift?
Low-temperature, short-duration, or energy-controlled processes—specifically plasma nitriding, low-temperature plasma nitriding, and pulsed laser nitriding—tend to form thin compound layers and deep diffusion layers. These conditions push crack sources to subsurface or internal defects, extending fatigue life. In contrast, high-temperature, long-duration, or energy-uncontrolled processes such as conventional gas nitriding, continuous-wave laser nitriding, and induction heating nitriding generate thick, rough brittle layers that cause surface crack initiation and rapid propagation, degrading fatigue performance. The critical operational threshold is maintaining a thin compound layer (typically submicron to a few microns) while preserving a deep diffusion zone (20–100 µm) with high residual compressive stress.
How do hybrid treatments such as nitriding plus shot peening or heat treatment improve fatigue strength compared to single nitriding?
Hybrid treatments remove the brittle compound layer, refine grains, and introduce high residual compressive stress, which further suppresses surface crack initiation and achieves significant fatigue strength improvement. By combining nitriding with shot peening or heat treatment, the process enables submicron compound layer control, deep residual compressive stress fields, and synergistic grain refinement. This multi-mechanism approach addresses the trade-off between compound-layer embrittlement and diffusion-layer strengthening, delivering fatigue life extension that single nitriding cannot achieve, particularly for high-cycle, long-life service scenarios.
What are the industrial scalability and cost-parity challenges for adopting low-temperature plasma nitriding and pulsed laser nitriding over conventional gas nitriding?
Conventional gas nitriding is a mature, high-temperature, long-duration process that is energy-intensive and generates thick, rough brittle layers, leading to fatigue debit. Low-temperature plasma nitriding and pulsed laser nitriding are short-duration, low-energy-consumption processes that produce thin compound layers and deep diffusion layers, but their scalability requires precise control of energy input and reaction atmosphere. Cost parity against legacy gas nitriding depends on throughput, equipment capital, and the value of fatigue life extension in critical components such as turbine blades and fasteners. The review recommends phasing out high-temperature gas nitriding and prioritizing low-temperature plasma and pulsed laser processes, while promoting hybrid routes for submicron compound layer control and deep residual stress fields.
What are the recommended future research directions for nitrided titanium alloys in high-cycle, long-life service scenarios?
Future research should focus on suppressing brittle TiN and amplifying diffusion-layer compressive stress. Priority should be given to developing low-temperature plasma and pulsed laser processes as short-duration, low-energy-consumption alternatives, while gradually eliminating high-temperature gas nitriding. The hybrid route of nitriding plus shot peening or heat treatment should be promoted to achieve submicron compound layer control, deep residual compressive stress, and synergistic grain refinement. These strategies aim to provide efficient, green, and transferable fatigue life extension solutions for critical components such as blades and fasteners, addressing the dual effect of nitriding on fatigue performance.
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