Due to their high specific strength, low density, and excellent biocompatibility, high-strength titanium alloys have become critical structural materials in fields such as aerospace, high-end equipment manufacturing, and biomedicine. In core applications-such as aircraft engine blades and load-bearing components of spacecraft-these materials must withstand significant loads and extreme temperature fluctuations, placing higher demands on the structural stability and strength-ductility balance of high-strength titanium alloys. High-strength titanium alloys consist of α and β phases; the ratio, morphology, and content of these two phases are easily influenced by heat treatment processes. However, an increase in strength is often accompanied by a decrease in ductility, making it difficult to meet the comprehensive performance requirements of the high-end equipment manufacturing industry. Therefore, achieving a synergistic improvement in both strength and ductility of high-strength titanium alloys is a current research priority.
1 Annealing Treatment
Annealing of high-strength titanium alloys involves the use of appropriate heating and holding methods to relieve the internal stresses accumulated during processing. This process also improves the microstructure of the alloy, enhancing its uniformity and stability, thereby increasing its ductility. As the annealing temperature increases and the holding time extends, the lamellar primary α phase gradually spheroidizes, resulting in a decrease in strength and an increase in elongation. Furthermore, annealing plays a crucial role in optimizing the properties of welded joints in high-strength titanium alloys. Higher annealing temperatures lead to the recrystallization and expansion of the α and β phases at the weld, increasing the number of phase boundaries and significantly improving the plasticity and toughness of the joint.
Annealing is primarily used to stabilize the microstructure, relieve internal stresses, and improve ductility; it remains irreplaceable in applications requiring low loads and high ductility. However, excessively high annealing temperatures can easily lead to coarse grain size, which severely reduces mechanical properties. Furthermore, annealing does not produce a significant precipitation hardening effect, resulting in notably low strength and hardness in high-strength alloys. This drawback directly limits the use of high-strength titanium alloys in the annealed condition in high-strength structural applications.
2 Solution Treatment and Aging
Solution treatment involves heating the alloy within a temperature range selected based on its phase transformation points. The purpose is to ensure that alloying elements are fully dissolved in the α and β phases and to optimize the ratio of these phases, thereby adequately preparing the alloy for subsequent aging treatment. Research by Vrancken et al. found that after solution treatment in the two-phase region, the microstructure of Ti-6Al-4V alloy transforms into an α+β layered mixture while retaining columnar β grains; when the temperature exceeds the phase transformation point, the alloy completely transforms into the β phase, and both the microstructure and mechanical properties vary with changes in the solution treatment temperature.
Age Hardening
Age hardening is designed to activate the metastable β phase, promote the precipitation of fine secondary α phases within the β matrix, and enhance the overall properties of titanium alloys. The process is carried out at relatively low temperatures for extended periods to control the size, morphology, and content of the α phase.
3 Other Heat Treatment Processes
For high-strength titanium alloys, in addition to the commonly used heat treatment processes mentioned above-such as annealing and solution treatment followed by aging-other methods include thermal deformation treatment and surface treatment. Customized heat treatment can be performed for parts intended for special applications. Thermal deformation treatment combines plastic deformation with heat treatment; by controlling temperature and strain rate, it regulates the precipitation of phases or refines the grain structure to achieve an optimal balance between strength and ductility. Surface treatment is designed to optimize the surface properties of titanium alloys by forming a high-performance surface alloy layer, thereby improving wear resistance or enhancing oxidation resistance.
Summary
This paper comprehensively examines the effects of heat treatment on the microstructure and properties of high-strength titanium alloys: annealing can eliminate residual internal stresses and improve the ductility and toughness of titanium alloys, but its effect on strength enhancement is limited; solution aging can effectively increase the strength of titanium alloys, but results in a significant reduction in ductility; two-stage aging resolves the strength-ductility trade-off; Methods such as hot deformation and surface treatment can broaden the alloy's applications in specific scenarios by refining the grain structure or improving surface properties.
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