Thermomechanical Processing Technology For β-Titanium Alloys

β-titanium alloys have became one of the research hotspots in titanium alloy materials due to their excellent hot and cold workability, adjustable mechanical properties, and stability over a wide temperature range.Combined plastic deformation with heat treatment, thermomechanical processing (TMP) can effectively optimize the microstructure of β-titanium alloys and achieve precise regulation of mechanical properties.It can provide key technical support for the high-performance applications of β-titanium alloys.

 

 

Analysis of the Properties of Beta Titanium Alloys

 

 

The core lies in the synergistic effect of "deformation-induced microstructural evolution" and "heat treatment-controlled precipitation phases", which precisely regulates the behavior of crystal defects during deformation and the phase transformation/precipitation process during heat treatment to optimize the material's microstructure and properties.

 

1.1 Deformation-Induced Enrichment of Crystal Defects and Grain Refinement

Plastic deformation generates a large number of dislocations in β-titanium alloys. With the increase of deformation amount, dislocation slip and entanglement form substructures, which are further refined into equiaxed subgrains or recrystallized grains through dynamic recovery/recrystallization. Fine grains can improve strength through grain boundary strengthening and reduce stress concentration to enhance toughness (fine-grain strengthening effect). The deformation temperature determines the microstructure morphology: deformation in the β-phase region tends to obtain uniform and fine β grains, while deformation in the α+β dual-phase region forms a complex refined dual-phase structure.

 

1.2Synergistic Regulation of Phase Transformation and Precipitation Phases

By controlling the cooling rate and aging process, the transformation of β-phase to α-phase and ω-phase is regulated:

The α-phase is the main strengthening phase. Crystal defects introduced by deformation provide nucleation sites, enabling it to precipitate in a dispersed and fine form, which hinders dislocation movement to achieve precipitation strengthening. Low-temperature aging forms acicular/lamellar α-phase, while high-temperature aging forms spherical α-phase (balancing strength and toughness).

Although the ω-phase significantly improves strength, it sharply reduces toughness, so it is necessary to avoid or inhibit it by controlling the cooling rate and alloy composition.

 

1.3 Stress Relaxation and Optimization of Microstructural Stability

The heating process of heat treatment promotes atomic diffusion, realizing dislocation annihilation and residual stress elimination, which avoids deformation and cracking during subsequent processing/service. It stabilizes the deformation-induced fine-grain structure, improves its thermal stability, and prevents grain growth in high-temperature service. This effect allows the material's processing performance, dimensional stability, and service life, making it suitable for high-temperature and high-stress working conditions such as aerospace.

 

 

2.1 Core Process Routes

Deformation in β-phase region + Aging: Heat to the β-phase region (50-150℃ above the β-transus temperature), deform, then rapidly cool to room temperature, and perform aging treatment. This process obtains uniformly refined β grains and dispersed α-phases, and is suitable for high-strength and high-toughness structural components.

Deformation in α+β dual-phase region + Aging: Heat to the α+β dual-phase region (between the β-transus temperature and room temperature), deform to refine the structure through the dual-phase interface, and age after cooling. It has both high strength and excellent fatigue performance, and is suitable for fatigue-loaded components such as aeroengine blades.

For alloys with special requirements, composite processes such as deformation-step aging and isothermal thermomechanical processing can be adopted to optimize performance.

 

2.2 Key Process Parameter Control

1. Deformation Temperature (Main Parameter)

β-phase region: Controlled at β-transus +50℃~β-transus +100℃ to ensure dynamic recrystallization and grain refinement;

α+β dual-phase region: β-transus -50℃~β-transus -100℃, retaining 10%-30% α-phase to refine the structure through dual-phase synergy;

Key point: Excessively high temperature leads to grain coarsening, while excessively low temperature increases deformation resistance and tends to cause cracking.

 

2. Deformation Amount and Rate

Deformation amount: 30%-70%. Excessively large deformation is prone to cracking, while excessively small deformation is difficult to refine the structure;

Deformation rate: Medium-low speed (0.1-10 s⁻¹) to avoid grain growth caused by adiabatic heating; for difficult-to-deform alloys, the rate can be reduced or stepwise deformation can be adopted.

3. Cooling Rate and Aging Parameters

 

Cooling: Rapid cooling (water cooling/oil cooling) to obtain a supersaturated β solid solution, laying the foundation for aging strengthening; excessively slow cooling will reduce strength;

Aging: Low temperature (350-450℃, 1-4h) forms fine acicular α-phases with significant strengthening effect; medium-high temperature (450-600℃, 4-8h) obtains spherical/short rod-like α-phases, balancing strength and toughness; air cooling after aging is sufficient to avoid residual stress.

 

 

 

Detailed Phase Diagram of Titanium Alloy Phase Composition vs. Concentration of β-Stabilizing Elements and Temperature

 

 

Comparison Dimension	High-Stability β-Titanium Alloys	Medium-Stability β-Titanium Alloys	Low-Stability β-Titanium Alloys
Representative Alloys	Ti-15V-3Cr-3Sn-3Al, Ti-10V-2Fe-3Al	Ti-6Al-4V ELI, Ti-5Al-5Mo-5V-3Cr	Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-2Al-1.5Mn
Core Characteristics	High content of β-stabilizing elements, maintaining stable β-phase at room temperature, and α-phase is difficult to precipitate	Moderate content of β-stabilizing elements, having both good deformability and phase transformation activity, most widely used	Low content of β-stabilizing elements, poor β-phase stability, and prone to β→α phase transformation at room temperature
Response Mechanism to TMP	Deformation in β-phase region achieves dynamic recrystallization (fine grains), and aging at 500-650℃ precipitates a small amount of dispersed α-phases and TiAl compounds, with synergistic strengthening of "deformation + aging"	Deformation in α+β dual-phase region crushes α-phases and enriches β-phase dislocations; after rapid cooling + aging, a large number of dispersed acicular/lamellar α-phases precipitate, with synergistic fine-grain strengthening and precipitation strengthening	Crystal defects introduced by deformation accelerate phase transformation, and a large number of α-phases can precipitate by air cooling without additional aging treatment