What is TC21 Titanium Alloy? What are the Features and Applications

TC21 is a high-strength, high-toughness, and high-damage-tolerance チタン合金. It is a crucial material used in advanced aerospace applications.

Overvie of TC21 Titanium Alloy

Key Characteristics of TC21 Titanium Alloy

1. High Strength: TC21 titanium alloy has a strength greater than 1100 MPa.
2. High Toughness: It has a fracture toughness (K_IC) ranging from 70 to 90 MPa·m^1/2.
3. Damage Tolerance: The alloy exhibits a low crack growth rate (da/dN), which enhances its damage tolerance.
4. Weldability: TC21 is a weldable alpha + beta two-phase titanium alloy.
5. Composition: The nominal composition of TC21 is Ti-6Al-2Zr-2Sn-3Mo-1Cr-2Nb-0.1Si.

Comparison to Other Alloys

TC21 was designed to exceed the fracture toughness, crack growth resistance, and thermal stability of the TC4 alloy while having higher strength. It aims to match or surpass the properties of the American Ti-62222S alloy.

Main Applications of TC21 Titanium Alloy

TC21 alloy is primarily used in aerospace applications, particularly in structural components of advanced fighter jets. For instance, 3% of the structural parts of the U.S. F22 fighter jet use the Ti-6-22-22S alloy, highlighting the importance of titanium alloys in aircraft structures.

In China, the application and technology of titanium alloys still lag behind developed countries. During the “15th Five-Year Plan,” China launched a research program to develop high-strength, high-toughness, high-damage-tolerance titanium alloys to align with the international trend of damage tolerance design. This led to the successful development of the TC21 alloy.

Processing and Manufacturing

TC21 titanium alloy is available mainly in the form of bars, forgings, and thick plates. The processing of such titanium alloy parts typically requires plastic deformation methods. Due to the high specific strength of titanium alloys, their cold plastic deformation ability is poor. Therefore, high-temperature thermoplastic deformation processes, such as forging, are usually required for plastic deformation processing, and TC21 is no exception.

How Special the TC21 is?

Microstructure and Performance of TC21 Titanium Alloy

The performance of an alloy is significantly influenced by alloying elements, processing techniques, and its microstructure. The microstructure, under specific alloy compositions, becomes the primary factor affecting its properties.

Microstructural Variations

During the deformation process of dual-phase titanium alloys like TC21, various microstructures can emerge depending on the processing methods and conditions. These microstructures include Widmanstätten structure, basketweave structure, duplex structure, and equiaxed structure, each exhibiting unique shapes, sizes, structures, and performance characteristics.

• Widmanstätten Structure: Known for good high-temperature creep resistance but poor plasticity, thermal stability, and fatigue performance.
• Basketweave and Duplex Structures: Offer good overall mechanical properties. Basketweave structure excels in creep resistance, fracture toughness, and fatigue crack propagation but may lag in fatigue performance and thermal stability compared to duplex structure.
• Equiaxed Structure: Provides the best plasticity and thermal stability but generally lower creep resistance.

Study on Basketweave Structure

In aerospace industries, designing for failure-safe based on damage tolerance criteria is crucial. Materials with damage-tolerant features should ideally possess:

1. High fracture toughness (K_IC).
2. Low crack propagation rate (da/dN).
3. High crack propagation threshold (Kth).

Understanding the relationship between material damage and critical conditions is essential for accurately estimating fatigue life. The basketweave structure of TC21 titanium alloy, achieved through specific heat treatment regimes, demonstrates superior strength, plasticity, toughness, and crack propagation resistance compared to other microstructures. Its fracture toughness and thermal stability are maintained at levels not lower than TC4 alloy, with strength surpassing TC4 alloy by a significant margin and comparable to the American Ti-62222S alloy.

Importance of Microstructural Parameters for Damage Tolerance

To enhance the damage tolerance of materials like TC21 titanium alloy, it is crucial to establish mathematical models that correlate microstructural parameters, especially fracture toughness, with performance. These efforts are essential for advancing material development and optimizing production processes in aerospace applications.

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