Titanium is prized for its exceptional combination of low density, high strength, and remarkable resistance to corrosion. This unique set of properties makes it indispensable across demanding industries, from jet engines and spacecraft to medical devices and chemical processing plants. Titanium is not a single material but a family of distinct compositions, each tailored for a specific performance profile. The differences in titanium grades are defined by controlled chemical composition and processing, which fundamentally alter the metal’s physical and mechanical behavior. Understanding this grading system ensures the material selected meets the exact requirements of its intended application.
The Core Classification System
The most recognized framework for titanium grades is established by international standards organizations. The American Society for Testing and Materials (ASTM) is the primary body defining these materials, recognizing over 40 distinct grades of titanium and titanium alloys. These classifications are formalized in standards like ASTM B348 for bars and billets and ASTM F67 for unalloyed forms. Each grade designation is based on strict limits for chemical composition, which dictates the resulting mechanical properties.
This classification system ensures material performance predictability in critical applications. The grade number provides a universal language for manufacturers and engineers to specify the required material, guaranteeing consistency. Grades 1 through 4 are reserved for commercially pure titanium, while subsequent higher numbers designate complex titanium alloys.
Commercially Pure Titanium Grades
Grades 1 through 4 represent commercially pure (CP) titanium, which is unalloyed but varies in purity. These grades are distinguished primarily by the amount of interstitial elements they contain, particularly oxygen and iron. A higher concentration of these elements increases the material’s strength but reduces its ductility and formability. These CP grades share a stable hexagonal close-packed (alpha) crystal structure at room temperature.
Grade 1 is the softest and most ductile, possessing the lowest strength and highest formability, making it ideal for sheet metal and plate heat exchangers. Grade 2 offers a balance of moderate strength, good ductility, and excellent corrosion resistance, often used for chemical processing equipment. Grade 3 offers higher strength than Grades 1 and 2 but is less commonly used.
Grade 4 is the strongest of the commercially pure grades, containing the highest allowable level of interstitial elements. This strength makes it suitable for high-strength pressure vessels, airframe components, and certain surgical hardware. Selecting among these four grades requires balancing strength requirements against the need for ease of fabrication and formability.
The Major Titanium Alloy Families
The majority of titanium grades are alloys, classified based on the metal’s stabilized microstructure. Alloying elements stabilize the alpha phase, the beta phase, or a combination of both, creating three primary alloy families.
Alpha Alloys
The addition of aluminum stabilizes the alpha phase, leading to Alpha alloys that offer excellent stability and creep resistance at elevated temperatures. These alloys, such as Ti-5Al-2.5Sn, are non-heat-treatable but maintain strength in high-heat environments like jet engine compressors.
Beta Alloys
Elements like vanadium, molybdenum, and iron stabilize the body-centered cubic beta phase, resulting in Beta alloys. Beta alloys are known for their exceptional strength, high hardenability, and capacity for deep hardening, often used in landing gear components. They are highly responsive to heat treatment, allowing for a wide range of mechanical properties.
Alpha-Beta Alloys
The Alpha-Beta family, including the famous Grade 5, is the most versatile and widely used group. These alloys contain both alpha and beta stabilizers, allowing them to be strengthened through heat treatment processes. This mixed-phase structure provides an optimal balance of high strength, lower density, and good fracture toughness for structural applications requiring a high strength-to-weight ratio.
Defining the Most Common Grades by Application
Grade 5 (Ti-6Al-4V) is the most commercially used alloy worldwide and the primary choice for aerospace and high-performance components. Its combination of strength and low density makes it the standard for airframe structures and turbine engine blades.
For medical applications, Grade 23 (Ti-6Al-4V ELI), a refined version of Grade 5, is the industry standard for implants. The ELI designation stands for Extra Low Interstitial, indicating reduced levels of oxygen, nitrogen, and iron. This enhances the alloy’s ductility and fracture toughness, which is necessary for long-term fatigue resistance in surgical devices like hip and knee replacements.
Another specialized grade is Grade 7, which is Grade 2 titanium with a small addition of palladium (up to 0.25%). This minor alloying change significantly boosts corrosion resistance, particularly in highly acidic and reducing environments. This makes Grade 7 valuable in the chemical processing industry for applications involving aggressive corrosive media.