1. Fatigue Strength of Titanium Alloys
Key Characteristics of Titanium Alloy Fatigue Strength
For annealed Ti-6Al-4V (the most ubiquitous titanium alloy), the room-temperature fatigue strength (at 10⁷ cycles, R = -1, where R is the stress ratio of minimum to maximum stress) ranges from 300–400 MPa, with some heat-treated variants reaching 450–500 MPa. This is significantly higher than that of 304 stainless steel (≈170 MPa) and 6061-T6 aluminum (≈90 MPa) under the same test conditions, making Ti-6Al-4V ideal for high-cycle fatigue (HCF) applications.
For high-strength titanium alloys (e.g., Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-2.5Sn), fatigue strength can exceed 500 MPa in the solution-treated and aged (STA) state, as the fine precipitated phases in their microstructure impede dislocation movement and crack initiation.
Dual-phase (α+β) alloys (e.g., Ti-6Al-4V): Their balanced α/β microstructure provides optimal fatigue resistance. The α-phase contributes to strength and crack propagation resistance, while the β-phase enhances ductility and inhibits crack initiation. Over-aging or excessive cold working, however, can coarsen α-phase particles or introduce residual stresses, reducing fatigue strength by 10–20%.
α alloys (e.g., Ti-5Al-2.5Sn): These alloys have excellent low-cycle fatigue (LCF) performance due to their stable HCP α-phase microstructure, with LCF life (at Δσ/2 = 500 MPa) exceeding 10⁴ cycles. They are widely used in low-temperature aerospace components.
β alloys (e.g., Ti-10V-2Fe-3Al): With a fully BCC β-phase structure, these alloys offer high fatigue crack growth resistance (da/dN ≈ 10⁻⁸ m/cycle at ΔK = 20 MPa·m¹/²) and are suitable for components under dynamic, high-load conditions (e.g., helicopter rotor shafts).
Corrosive environment fatigue (CAF): In seawater or chloride-containing media, titanium alloys maintain far better fatigue performance than steel or aluminum, as their passive oxide film prevents corrosion-induced crack initiation. Ti-6Al-4V's fatigue strength in seawater only decreases by 5–10% (to ≈350 MPa at 10⁷ cycles), whereas 304 stainless steel experiences a 50% drop due to pitting corrosion.
Surface condition sensitivity: Surface defects (e.g., machining marks, microcracks) and hydrogen contamination are major fatigue failure triggers. Shot peening or anodizing can improve fatigue strength by 20–30% by introducing compressive residual stresses and enhancing surface passivation. Conversely, hydrogen embrittlement can reduce fatigue life by up to 50% by promoting intergranular crack growth at low temperatures.
At cryogenic temperatures (e.g., -196°C), Ti-6Al-4V's fatigue strength increases to 450–500 MPa due to enhanced atomic bonding and reduced dislocation mobility, with no ductile-to-brittle transition in fatigue behavior.
At elevated temperatures (up to 300°C), its fatigue strength remains above 250 MPa (10⁷ cycles), but above 400°C, oxidation and grain boundary softening cause a rapid decline (losing 30–40% of room-temperature fatigue strength at 500°C).
2. Creep Properties of Titanium Alloys
Key Characteristics of Titanium Alloy Creep Performance
α+β alloys (e.g., Ti-6Al-4V): Their maximum long-term creep service temperature is 300–350°C. At 300°C and 200 MPa stress, the steady-state creep rate is ≤10⁻⁸ s⁻¹, and creep deformation is less than 0.1% after 10,000 hours of exposure-sufficient for aircraft engine compressor blades and structural components in subsonic aircraft. Above 400°C, the creep rate accelerates sharply (exceeding 10⁻⁶ s⁻¹ at 450°C/200 MPa) due to α-phase coarsening and grain boundary sliding.
α alloys (e.g., Ti-5Al-2.5Sn, Ti-6Al-2Sn-4Zr-2Mo): These alloys have the highest creep resistance among titanium alloys, with a long-term service temperature of 400–500°C. For example, Ti-6Al-2Sn-4Zr-2Mo at 450°C and 250 MPa has a steady-state creep rate of ≤5×10⁻⁹ s⁻¹ and a rupture life exceeding 100,000 hours, making it suitable for high-temperature aerospace engine parts.
β alloys: Their creep resistance is lower than α and α+β alloys, with a maximum service temperature of 300–350°C, as the BCC β-phase has higher atomic mobility and is prone to creep deformation under long-term stress.
At low temperatures (<400°C) and high stresses, creep is dominated by dislocation climb and glide in the α-phase, with the β-phase acting as a barrier to dislocation movement (enhancing creep resistance in dual-phase alloys).
At high temperatures (>450°C), grain boundary sliding and diffusion creep become dominant. α alloys with fine, uniformly distributed grains and solid-solution-strengthened elements (Al, Sn, Zr) resist grain boundary sliding effectively, hence their superior high-temperature creep performance.
Heat treatment plays a critical role: Solution treatment followed by aging (STA) for α+β alloys precipitates fine α-phase particles in the β-matrix, which pin dislocations and reduce creep rates by 50–70% compared to the annealed state.
In oxidizing atmospheres, the formation of a dense TiO₂-Al₂O₃ passive film on titanium alloys (especially those with high Al content) inhibits oxygen diffusion and reduces creep embrittlement. However, at temperatures above 550°C, the oxide film becomes porous, allowing oxygen to penetrate the matrix and form a brittle "alpha case," which accelerates creep fracture.
In hydrogen-containing environments, hydrogen absorption increases creep rate by promoting dislocation mobility and intergranular cracking, limiting the creep service life of titanium alloys in such atmospheres by 20–30%.
