Detailed explanation of heat treatment process and microstructure transformation of titanium alloy

A、 Overview of Heat Treatment Process for Titanium Alloy
      During the processing of titanium alloys, heat treatment is often required to improve their mechanical properties and microstructure. The common heat treatment processes for titanium alloys mainly include stress relief annealing, complete annealing, solution treatment, and aging treatment.

1. Stress relief annealing
The main purpose of stress relief annealing is to eliminate the internal stress generated in titanium alloys during cold working, cold deformation, and welding processes. This process is widely used in titanium alloy materials after processes such as hot forging and rolling, casting, cold deformation processing, cutting, cutting, and welding. The selection of annealing temperature and time is crucial for the effectiveness of stress relief annealing. Typically, recrystallization temperature is used for annealing, utilizing the material's recovery process to eliminate stress.
2. Complete annealing
Complete annealing aims to obtain a recrystallized structure and improve the plasticity of titanium alloys, hence it is also known as recrystallization annealing. Most alpha titanium alloys and alpha+beta duplex titanium alloys are used in a fully annealed state. For alpha titanium alloys, the annealing temperature is usually 120-200 ℃ below the phase transition point to avoid grain coarsening and insufficient plasticity. The annealing process of near alpha titanium alloys and alpha+beta duplex titanium alloys is more complex, involving recrystallization and changes in alpha and beta phases. The complete annealing of metastable beta titanium alloys is usually carried out by solid solution treatment.
3. Solid solution and aging treatment
The purpose of solid solution treatment is to obtain metastable phases that can be strengthened by aging, such as α 'martensite, α "martensite, or metastable β phase. These metastable phases will generate small equilibrium phases during decomposition, thereby producing precipitation strengthening effects and improving the hardness and strength of titanium alloys. The solid solution temperature is usually 40-100 ℃ lower than the alpha+beta/beta phase transition point. Time hardening plays a significant role in titanium alloys with high content of beta stabilizing elements, but its effect is weaker in near alpha alloys and alpha+beta two-phase titanium alloys with lower content of beta stabilizing elements.

B、Microstructure changes during heat treatment of titanium alloys
1. Organizational changes during heating process
During the heating process, titanium alloys typically undergo a change in crystal structure, including a transition between the alpha and beta phases. Cold deformed titanium alloys also undergo recovery and recrystallization processes. The recovery process eliminates the second type of internal stress generated during deformation through vacancy and dislocation motion, while the recrystallization process produces new undistorted equiaxed grains to replace deformed grains and restore the plasticity of the material.
2. Organizational changes during the cooling process
Titanium alloys also undergo structural changes during the cooling process. When slowly cooled, the beta phase transitions to the alpha phase, and the two maintain a specific orientation relationship. Rapid cooling may lead to the formation of martensitic transformation, quenched ω phase, supersaturated α phase, and residual high-temperature β phase structure. The types of these transformation products depend on the content of β - stable elements.

3. Time conversion
The metastable phase generated by rapid cooling will transform into an equilibrium phase during the aging process, accompanied by the decomposition of the metastable phase and the decomposition of the supersaturated alpha phase. This is the main reason why titanium alloys can be strengthened by heat treatment.
4. Co analysis transformation
The eutectoid transformation of titanium alloys often occurs in alloys containing stable elements such as titanium and fast eutectoid beta alloys, which typically leads to a decrease in material plasticity. By isothermal treatment, a bainitic non lamellar structure can be obtained to improve the properties of the material.
5. Stress induced phase transition
The metastable β phase can transform into martensite under strain or stress, including hexagonal martensite α 'and rhombohedral martensite α'. This process can generate phase transformation induced plasticity effects, increasing the elongation and strain hardening rate of titanium alloys.

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