The Classification System
Titanium alloys are classified by the phases present at room temperature: alpha, alpha-beta and beta. The classification is set by the alloying elements, which either stabilize the alpha phase, such as aluminum, tin and oxygen, or the beta phase, such as vanadium, molybdenum and chromium. The phase structure controls the strength, weldability and heat treatment response of each grade.
Commercially Pure and Alpha Alloys
Commercially pure grades 1 to 4 are single-phase alpha materials whose strength rises with oxygen content. They are fully weldable and corrosion resistant but limited in strength and temperature capability. The alpha alloy Grade 6, Ti-5Al-2.5Sn, adds aluminum and tin for strength and is used where toughness at cryogenic temperature is required.
Alpha-Beta Alloys
Alpha-beta alloys contain enough beta stabilizers to retain some beta phase, which allows strengthening by heat treatment. Grade 5, Ti-6Al-4V, is the dominant alloy of this family and accounts for the largest share of titanium production. Grade 9, Ti-3Al-2.5V, offers a weldable, formable middle ground, and Grade 23, the ELI version of Grade 5, serves medical and cryogenic applications.
Grade 5, Ti-6Al-4V, for structural and aerospace parts.
Grade 9, Ti-3Al-2.5V, for weldable tubing and thin sections.
Grade 23, Ti-6Al-4V ELI, for implants and cryogenic service.
Beta and Near-Beta Alloys
Beta alloys contain high levels of beta stabilizers and can be solution treated and aged to very high strength. Ti-10V-2Fe-3Al is used for high-strength forgings, and Ti-15V-3Cr-3Al-3Sn is formed as sheet and strip. These alloys respond to heat treatment with strength above 1200 MPa, but they are harder to weld and more expensive than alpha-beta grades.
Heat Treatment Response
Alpha alloys cannot be strengthened by heat treatment and are used annealed, while alpha-beta and beta alloys respond to solution treatment and aging. The response depends on the retained beta phase, which decomposes during aging to strengthen the alloy. The treatment cycle is defined by the alloy and the target property balance.
Selecting an Alloy Family
The selection starts with the service temperature, the strength requirement and the fabrication route. CP grades cover corrosion service below about 300 degrees Celsius, alpha-beta alloys cover structural strength to about 400 degrees, and beta alloys cover the highest strength forgings and sheet parts. Weldability and cost are weighed against the property needs of each application.
Frequently Asked Questions
Q: How are titanium alloys classified?
A: By the phases present at room temperature: alpha, alpha-beta and beta.
Q: What is the most widely used titanium alloy?
A: Grade 5, Ti-6Al-4V, the alpha-beta alloy that dominates structural titanium production.
Q: Which alloys are weldable?
A: Commercially pure grades and low-alloy grades such as Grade 9 weld readily, while high-strength beta alloys are harder to weld.
Q: What is the ELI version of Ti-6Al-4V?
A: Grade 23, with extra-low interstitials, which improves toughness for medical and cryogenic use.
Q: Which alloys are heat treatable?
A: Alpha-beta and beta alloys respond to solution treatment and aging, while alpha alloys do not.
Q: Where are beta alloys used?
A> In high-strength forgings and sheet parts where strength above 1200 MPa justifies the higher cost.
