TB9 (Ti-38644) Titanium Alloy: Ti-3Al-8V-6Cr-4Mo-4Zr Properties and Applications
Mar 13, 2024
Titanium exists in two allotropic crystal structures. Below 882°C it has a hexagonal close-packed lattice, called alpha titanium; at and above 882°C it transforms to a body-centered cubic lattice, called beta titanium. The transformation temperature and the resulting microstructure can be controlled by adding alloying elements, which is the basis of titanium alloy design. The melting point of titanium is approximately 1668°C.
Titanium Alloy Classification
Wrought titanium alloys are grouped into three families by the phases present at room temperature: alpha alloys, beta alloys, and alpha-plus-beta alloys. Alpha alloys generally offer the best machinability, alpha-plus-beta alloys are intermediate, and beta alloys are the most difficult to machine but offer the highest achievable strength and deep hardenability. In the Chinese designation system used in GB/T 3620, alpha alloys are coded TA, beta alloys are coded TB, and alpha-plus-beta alloys are coded TC.
What Is TB9 (Ti-38644)?
TB9, also known as Ti-38644, is a metastable beta titanium alloy with the nominal composition Ti-3Al-8V-6Cr-4Mo-4Zr. It was developed in the 1960s by a U.S. titanium producer for applications requiring very high strength combined with corrosion resistance. The alloy carries a high content of beta-stabilizing elements, with a relatively high molybdenum level, which gives it excellent corrosion resistance in general media as well as resistance to salt-stress corrosion.
Chemical Composition
| Element | Nominal Content (%) |
|---|---|
| Aluminum, Al | 3 |
| Vanadium, V | 8 |
| Chromium, Cr | 6 |
| Molybdenum, Mo | 4 |
| Zirconium, Zr | 4 |
| Titanium, Ti | Balance |
Heat Treatment and Mechanical Properties
The large content of beta-stabilizing elements allows TB9 to respond to a wide range of heat treatments. By changing the solution and aging schedule, different strength-plasticity combinations can be produced from the same alloy. After solution treatment and aging, the tensile strength reaches 1200 to 1400 MPa with a fracture toughness of 58 to 66 MPa·m1/2, combining high tensile strength with good plasticity and elastic properties. The alloy also has deep hardenability, and the hardened cross-section can reach 225 mm, which means thick sections can be fully hardened by quenching without a strength drop in the core.
The high strength-to-modulus ratio and good spring characteristics of TB9 make it a suitable spring material where conventional steel springs are too heavy or where corrosion resistance is required.
Applications
Aviation and aerospace springs, where weight saving and high fatigue strength are required.
High-strength fasteners, including bolts and screws for airframe and engine mounting.
Torsion bars and foil honeycomb structures for airframe control systems.
Carrier-based aircraft launch components that must absorb high dynamic loads.
Oil-drilling tools and downhole components exposed to saline and corrosive environments.
Machining and Fabrication Notes
Beta titanium alloys are harder to machine than alpha or alpha-plus-beta alloys because of their high strength and tendency to work-harden. Use sharp tooling, low cutting speeds, positive rake angles, and generous coolant flow. Heat treatment should be performed in vacuum or with protective atmosphere to avoid oxygen pickup, and surface contamination layers must be removed before service, for example by pickling or machining.
Frequently Asked Questions
What does TB9 mean in titanium designations?
TB is the Chinese designation prefix for beta titanium alloys, and 9 is the alloy sequence number. The alternative name Ti-38644 reflects the nominal composition Ti-3Al-8V-6Cr-4Mo-4Zr.
Why is TB9 classified as a metastable beta alloy?
Because its high beta-stabilizer content retains the beta phase at room temperature after quenching from the beta field, and the alloy then precipitates fine alpha during aging to achieve high strength.
What strength can TB9 reach after heat treatment?
After solution treatment and aging, tensile strength is typically 1200 to 1400 MPa with fracture toughness of 58 to 66 MPa·m1/2, depending on the aging schedule.
Is TB9 suitable for corrosion service?
Yes. The high molybdenum content gives excellent general corrosion resistance and resistance to salt-stress corrosion, which is why the alloy is used in marine-adjacent and oil-drilling applications.
How thick a section can be fully hardened?
The alloy shows deep hardenability, with hardened cross-sections up to 225 mm achievable, which is far beyond the capability of most alpha-plus-beta alloys.
Why is beta titanium harder to machine than alpha titanium?
Beta alloys combine high strength with low thermal conductivity, which concentrates heat at the cutting edge and promotes work hardening. Lower speeds, positive geometry, and high coolant flow are required.
