1. Rolled State (As-Rolled Titanium)
Microstructural features: The microstructure is characterized by elongated, flattened grains (along the rolling direction) with a high density of dislocations, and residual internal stresses are present throughout the material. For α+β titanium alloys (e.g., Grade 5 Ti-6Al-4V), the α-phase grains are aligned in the rolling direction, forming a distinct fibrous texture.
Mechanical properties:
Strength: Significantly elevated due to work hardening-tensile strength and yield strength are 15–30% higher than those of the annealed state (e.g., cold-rolled Grade 2 titanium has a tensile strength of ~550 MPa, compared to ~345 MPa for annealed Grade 2).
Toughness and ductility: Reduced dramatically; elongation and reduction of area drop by 40–60% relative to the annealed state, and impact toughness decreases sharply. The material becomes prone to cracking under bending or impact loads due to residual stresses and dislocation tangling.
Formability: Poor, as work hardening raises the material's flow stress, making further plastic deformation difficult without intermediate annealing.
Application scenarios: Suitable for components requiring high strength but minimal post-processing (e.g., simple structural brackets, non-critical fasteners), but not for parts subject to dynamic or complex loads.
2. Annealed State (Annealed Titanium)
Microstructural features: The deformed, elongated grains recrystallize into equiaxed, fine-grained structures; dislocations are annihilated, and residual stresses are fully relieved. For α+β alloys, the microstructure consists of uniformly distributed equiaxed α-phase grains and a small amount of β-phase, with no directional texture.
Mechanical properties:
Strength: Moderate and balanced-lower than the rolled state but higher than the cast state. The strength is stable across the material's cross-section and along all directions (no anisotropy).
Toughness and ductility: Optimal among the three states; elongation can reach 20–30% for commercially pure titanium, and fracture toughness (KIC) is 20–40% higher than that of the rolled state. The material exhibits good resistance to impact and fatigue under cyclic loads.
Formability and machinability: Excellent; the soft, recrystallized microstructure allows for deep drawing, bending, and other forming processes, and machinability is improved due to reduced hardness.
Application scenarios: The most widely used state for general-purpose components, such as aerospace fuselage panels, marine pipeline systems, medical device casings, and chemical industry heat exchangers, where balanced strength, ductility, and corrosion resistance are required.
3. Solution-Annealed & Aged State (Solution-Treated and Aged, STA Titanium)
Solution annealing: Heating above the α/β transus (for α+β alloys, ~920–950°C) to form a uniform β-phase microstructure, then quenching to retain a metastable supersaturated α+β (or martensitic α') structure at room temperature.
Aging: Reheating to a lower temperature (450–550°C) to precipitate fine, dispersed secondary α-phase particles within the β matrix.
Microstructural features: The microstructure consists of a β-phase matrix with densely distributed fine secondary α precipitates, which act as dislocation barriers. For Ti-6Al-4V, the precipitates are needle-like or lath-shaped, with sizes ranging from 0.1–1 μm.
Mechanical properties:
Strength: The highest among the three states-yield strength can exceed 1000 MPa for STA Grade 5 (compared to ~860 MPa for annealed Grade 5), a 15–25% increase, due to precipitation strengthening.
Toughness and ductility: Moderate, lower than the annealed state but higher than the rolled state; elongation is typically 8–12% for STA Grade 5, and fracture toughness is slightly reduced due to the fine precipitate structure, but fatigue strength is enhanced (by 20–30%) compared to the annealed state.
High-temperature performance: Improved creep resistance, as the precipitates inhibit dislocation movement at elevated temperatures (up to 350°C for Grade 5), making it suitable for high-temperature structural applications.
Application scenarios: Used for high-performance, high-load components, such as aerospace engine compressor blades, aircraft landing gear, high-strength fasteners, and offshore drilling tool components, where maximum strength and fatigue resistance are critical.
