In aerospace, medical devices, high-end equipment manufacturing, and other fields, titanium alloy has become an indispensable key material due to its excellent strength, corrosion resistance, and lightweight properties. The excellent performance of titanium alloys is inseparable from the precise heat treatment process regulation and the complex structural transformations that occur during the process. Today, we will delve into the core knowledge of titanium alloy heat treatment and tissue transformation, and uncover the technical code behind this "space metal".
Mechanical transformation law in heat treatment of titanium alloys
The essence of heat treatment is to guide the orderly transformation of the internal structure of titanium alloy through the regulation of temperature and cooling speed. From heating to cooling to aging, the structure of titanium alloys undergoes a series of complex changes that directly determine the final properties of the material.
1. Heating Process: The "Trio" of Recovery, Recrystallization and Phase Transition
When heated, titanium alloys usually undergo crystal form transformation (transition between α phase and β phase) at the same time, and if it is a cold-deformed titanium alloy, it will also undergo recovery and recrystallization processes, which together shape the microstructure after heating.
(1) Restoration and recrystallization: repair the deformed structure and optimize the grain structure
After cold working, titanium alloy has a large number of defects caused by deformation (such as dislocation and vacancy), and after heating to a certain temperature, "recovery" will first occur: at 450~640°C (the recovery temperature is lower than the recrystallization temperature), part of the internal stress is eliminated through the slow movement of the vacancy and dislocation, but the grain shape of the material remains basically unchanged.
As the temperature continues to rise, "recrystallization" begins to occur: new non-distortion-free isoaxial grains gradually appear in the deformed structure, and these new grains will gradually replace the deformed grains, eventually reducing the hardness of the material and restoring its plasticity. The recrystallization characteristics of different types of titanium alloys are obviously different:
• α titanium alloy: limited cold deformation ability, difficult to refine grains through deformation and recrystallization;
• β titanium alloy: strong cold deformation ability, which can achieve a certain degree of grain refinement through deformation and recrystallization;
• α β duplex titanium alloy: With the help of deformation and recrystallization, it can not only refine the structure, but also further improve plasticity.
(2) α phase transition to β phase: the "temperature switch" of crystal form
When the heating temperature exceeds α→β phase transition point, titanium alloys initiate a crystal transition from α phase to β phase. Taking pure titanium as an example, its phase transition temperature is about 875±5°C. It is worth noting that the Burgers positional relationship remains unchanged throughout the α↔β phase transition, which provides an important basis for the tunable structure of titanium alloys.
2. Cooling Process: Speed determines tissue, and tissue determines performance
Cooling speed is a key factor affecting the final structure of titanium alloys, and under different cooling speeds, titanium alloys will form completely different microstructure morphology, which in turn shows significantly different properties.
(1) Slow cooling: orderly transition, forming a stable phase
When the titanium alloy slowly cools from the single-phase region to the two-phase region, the β phase gradually changes to the α phase, and the two strictly follow the Burgers orientation relationship: (110)β//(0001)α; [111]β//[11₂0]α. The structure formed by this orderly transition is highly stable, which is suitable for scenarios with high material stability requirements.
(2) Rapid cooling: induce metastable phase to pave the way for strengthening
Rapid cooling (such as water quenching) can disrupt the equilibrium transition process of titanium alloy structure, which may induce martensitic phase transitions, quenched ω phase formation, supersaturated α phase generation, and residual high-temperature β phase retention. The final transformation products (such as α′, α", ω, supercooled β phase, metastable β phase, supersaturated α phase) mainly depend on the content of β stable elements in the titanium alloy, which are the "core raw materials" for subsequent aging strengthening.
3. Aging transformation: metastable phase "transformation" to achieve performance leap
The metastable phase produced by rapid cooling is not stable, and will gradually change to an equilibrium phase during the aging process, accompanied by metastable phase decomposition, supersaturated α phase decomposition and other reactions. This process is the fundamental reason why titanium alloys can achieve strength and hardness improvement through heat treatment, and it is also a key link in the transformation of titanium alloys from "basic form" to "high-performance form".
4. Co-analysis and transformation: the "plastic killer" that needs to be wary of
The eutectic transition of titanium alloys is commonly found in alloys composed of stable elements of titanium and fast eutectic β alloys, which usually leads to a decrease in the plasticity of the material, which is not good for the processing and service performance of the material. However, by isothermal treatment of the tissue after eutectic transformation, it can be transformed into a non-lamellar tissue of the Bain size, which alleviates the problem of plasticity decline to a certain extent.
5. Stress-Induced Phase Transition: Unlock "Phase Change-Induced Plasticity"
The metastable β phase will transform into martensitic (e.g., hexagonal martensitic α′, orthorhombic martensitic α") under strain or stress, a process known as stress-induced phase transition. This transition can produce a "phase transition-induced plastic effect", which significantly improves the elongation and strain hardening rate of titanium alloys, and provides performance guarantee for the application of titanium alloys in scenarios under complex stresses (such as aerospace structural parts).
