1. Oxidation Resistance of Grade 5 Titanium Alloy at High Temperatures
Grade 5 titanium alloy exhibits good oxidation resistance at moderate temperatures, but its performance is temperature‑dependent.
At temperatures up to 300℃ – 350℃, the alloy forms a thin, dense, and adherent oxide layer mainly composed of TiO₂. This protective layer prevents further oxygen diffusion into the substrate, resulting in slow oxidation kinetics and stable performance during long‑term service.
When the temperature exceeds 400℃, oxidation accelerates noticeably. The oxide layer becomes thicker, less uniform, and may lose adhesion, leading to increased oxygen dissolution in the titanium matrix. Above 450℃ – 500℃, oxygen penetration causes oxygen embrittlement: oxygen atoms dissolve interstitially in the titanium lattice, increasing hardness but severely reducing ductility and impact toughness. This phenomenon is known as interstitial embrittlement.
Therefore, Grade 5 is not recommended for continuous use above 400℃ if structural integrity and ductility are required. For applications above 400℃, oxidation‑resistant coatings or higher‑temperature titanium alloys such as IMI 834 or Ti‑6242 are preferred.
2. Long‑Term Service: Strength and Corrosion Resistance Degradation
(1) Long‑Term Strength Degradation
During long‑term high‑temperature exposure, Grade 5 titanium alloy undergoes microstructural changes that affect mechanical properties.
In the range of 300℃ – 350℃, tensile strength and yield strength remain relatively stable for thousands of hours.
Prolonged holding between 400℃ – 550℃ causes alpha phase coarsening and thermal relaxation, leading to a gradual decrease in tensile strength and fatigue strength.
Oxygen diffusion and interstitial embrittlement further reduce ductility and fracture toughness.
For long‑term reliable performance, the industry generally accepts a continuous service limit of 315℃ for Grade 5. Beyond this temperature, strength degradation becomes significant and limits component lifetime.
(2) Long‑Term Corrosion Resistance
In most environments, including marine, neutral atmospheres, and mild chemical media, Grade 5 maintains excellent corrosion resistance after long‑term use.
The protective TiO₂ layer spontaneously reforms if damaged, providing persistent corrosion resistance.
As mentioned, deep oxygen penetration degrades surface toughness and can promote crack formation under stress.
At high temperatures and sustained stress, creep deformation may rupture the passive film locally, accelerating localized corrosion in aggressive media.
Under normal service conditions below 300℃, no meaningful degradation in corrosion resistance occurs over long periods.
Grade 5 titanium alloy has satisfactory oxidation resistance up to 350℃ – 400℃. Above 400℃, oxidation and oxygen embrittlement become limiting factors.
Long‑term high‑temperature exposure leads to gradual strength reduction due to microstructural coarsening and interstitial embrittlement, while corrosion resistance remains stable in typical service environments. For maximum reliability, continuous operating temperature should be limited to approximately 315℃.
