Titanium is a “superhero” element with extraordinary properties. It is the ninth most abundant metal in Earth’s crust. In fact, small amounts of titanium occur in almost every igneous rock.
Commercially pure grades of titanium have great tensile strength and acceptable mechanical properties, but for most applications, titanium is alloyed with other chemical elements such as vanadium and aluminum.
Titanium alloys exhibit high toughness and ultimate tensile strength, even at extreme temperatures. They are lightweight and have outstanding corrosion resistance to seawater.
In this overview article, we have explained different types of titanium alloys (with examples), their properties, why they are important, and their real-world applications.
Four Main Categories of Titanium Alloys
The pure titanium atoms align in the solid state in either a hexagonal close-packed crystalline structure, known as the alpha phase, or a body-centered cubic structure, known as the beta phase.
Alloying elements, such as tin, when dissolved in titanium, do not alter the transformation temperature. However, elements such as oxygen and aluminum cause the transformation temperature to increase. These elements are known as alpha stabilizers.
And elements (usually transitional metals) that reduce the phase-transformation temperature are known as beta stabilizers. Thus, commercial titanium alloys can be classified as alpha, beta, and alpha-beta alloys. The latter may also include ‘near-alpha’ and ‘near-beta’ alloys based on their constituents.
1. Alpha Titanium Alloy
Titanium fan blades for high bypass aero-engines | Image Credit: Marina Pousheva / Shutterstock
Examples: Ti-8Al-1Mo-1V and Ti-5Al-2Sn-ELI
Alpha titanium alloys are made by mixing commercially pure titanium with alpha-stabilizer elements in a specific ratio. These alloys contain a significant amount of alpha-stabilizing elements (more than 5% by weight), such as oxygen or aluminum. Some of them also consist of neutral alloying elements such as tin.
They have reasonably good ductility, impressive toughness, low to medium strength, and are non-heat treatable. Moreover, they offer high oxidation resistance and high-temperature creep strength.
Ti-5Al-2.5Sn-ELI, for example, is wrought titanium alloy containing the base metal titanium, 5% aluminum, and 2.5% tin.
Another alpha titanium alloy, Ti-8Al-1Mo-1V, contains 7-8% aluminum and 1% molybdenum. It is mostly used to fan and compressor blades, seals, discs, and rings.
2. Near-Alpha Titanium Alloy
Example: Ti 1100, Ti-6Al-2Sn-4Zr-2Mo, and IMI 685.
Near-alpha titanium alloys usually contain aluminum, tin, and zirconium. They are also alloyed with the 1-2% of beta phase stabilizers such as molybdenum, vanadium, or silicon.
Most of these alloys have been manufactured and used for various high-temperature applications for aircraft engines. For example, IMI 679 in 1958 was used to make disc and blades of the RR Spey engine. In 1964, the RR Adour engine for Hawk airplane used IMI 685 for combined creep resistance, good weldability, and high fracture toughness.
It was preceded by IMI 829, which offered a better creep resistance and could operate in temperatures up to 540°C. IMI 829 was used in the compressor design for Boeing 757.
Enhanced near-alpha titanium alloys have been developed in the past few decades, such as Ti 1100 and TA15, that could properly operate at temperatures above 600°C.
The development of these alloys continues to receive attention with numerous studies focusing on oxidation and fatigue resistances while safeguarding the thermomechanical characteristics for high, energy-efficient thrust-to-weight ratios in aircraft.
3. Alpha and Beta Titanium Alloys
Ti-6Al-4V alloy bars
Examples: Ti-6Al-7Nb, Ti-6Al-4V-ELI, and Ti-6Al-4V
As the name suggests, these titanium alloys contain a combination of both alpha and beta stabilizers. Their strength levels are medium to high, and they can be heat treated. Weldability and creep resistance are lower than alpha and near-alpha alloys, primarily due to the presence of the beta phase.
Ti–6Al–4V is the most popular of all titanium alloys, representing over 50 percent of the total titanium market. Because of its excellent corrosion resistance and high strength-to-weight ratio, it is used in a broad range of applications, including the aerospace industry and biomechanical applications (prostheses and implants).
Another popular alloy named Ti-6Al-7Nb is used in implant devices for artificial hearts, artificial knee joints, cardiac valve prostheses, pacemakers, screws for fracture fixation, pace, artificial hip joints, and failed hard tissue.
4. Beta and Near-Beta Titanium Alloys
Boeing aircraft use beta titanium for several parts, including landing gear | Getty Images
Examples: Ti-8Mo-8V-2Fe-3Al, Ti-15-3, and Ti-13Zr-13Nb
The beta and near-beta alloys offer several advantages in terms of mechanical properties, processing, and inexpensive fabricated components compared to traditional titanium alloys.
They are created by adding beta-stabilizing elements to base titanium, which retain the beta phase when the metal is cooled quickly from the transus temperature. While many elements can be used as beta-stabilizers, only vanadium, niobium, chromium, iron, and molybdenum are used in significant amounts (usually 10 to 20 percent by weight).
Typically, the strength and fatigue resistance of these alloys is greater than the alpha titanium alloys. They are also heat treatable and weldable. However, they still have limited applications due to their low creep resistance at high temperatures.
The yield strength of most beta and near-beta titanium alloys lies between 1150 and 1300 MPa. In contrast, alpha alloys’ strength ranges between 750 and 1000 MPa. This higher strength comes from greater solid solution hardening as well as precipitation hardening.
Ti–13 V–11Cr–3Al was the first beta titanium alloy to be used in commercial quantities. It was used in the airframe of SR-71 Blackbird military aircraft, which was designed to fly at Mach 2.5.
Ti-10V-2Fe-3Al is the near-beta titanium alloy that features a great combination of strength, fracture toughness, and ductility. It is used to build airframe and landing gear components.
Titanium Alloy Grades
As per the American Society for Testing and Materials (ASTM) International standard, there are four grades of commercially pure titanium (Grade 1, 2, 3, 4). Their tensile and yield strength increases with the grade number.
Titanium alloys have more than 30 grades. Each has different properties and applications. The most popular ones are:
Grade 5: is the most commonly used alloy that can withstand temperatures up to 430°C and various environmental factors, including seawater. It contains the base element titanium, 6% aluminum, 4% vanadium, 0.25% iron, and 0.2% oxygen. It is also often referred to as Ti-6Al-4V or Ti 6-4.
Grade 6: alloy features intermediate strength, excellent weld fabricability, microstructure stability, oxidation resistance, and improved high-temperature strength (480 °C) compared to pure titanium. It is widely used in chemical processing equipment, gas turbine casings and rings.
Grade 12: contains 0.8% nickel, 0.3% molybdenum, small traces of carbon and oxygen. It provides the same excellent properties as pure titanium grades along with improved corrosion resistance. The alloy is mostly used in chemical processing, heat exchangers, pumps, and valves.
Grade 12 titanium tubes
Grade 23: is similar to grade 5 but has better ductility and fracture toughness. It consists of 6% aluminum, 4% vanadium, 0.25% iron, 0.13% oxygen, and small traces of carbon, nitrogen, and hydrogen. It is also known as Ti 6Al 4V ELI alloy, where ELI stands for extra-low interstitial. The alloy is mostly used in medical implants.
Grade 38: is made of 4% aluminum, 3% vanadium, 1.5% iron, 0.2% oxygen, and small traces of carbon, nitrogen, and hydrogen. Developed in the 1990s, the alloy is also known as Ti-4Al-2.5V. Its mechanical properties are quite similar to Grade 5 titanium, and can be used in higher temperature applications up to 320 °C.
Titanium alloys are mostly used in aircraft, spacecraft, naval ships, armor plating, and missiles. But they have tons of other applications in various fields.
For example, tanker trucks that carry sodium chromate and sodium hypochlorite use titanium alloy because it is strong, lightweight, and resistant to corrosion. It is also used in motorcycle or automobile racing, where weight reduction is important while maintaining rigidity and high strength.
The alloy is used in several sporting goods, including bicycle frames, football helmet grills, lacrosse stick shafts, golf clubs, hockey, and tennis rackets.
Titanium alloys are used to build lightweight, flexible eyeglass frames and luxury wristwatches. Because of their compelling texture, they are widely used in jewelry such as necklaces, earrings, cufflinks, and tie pins.
Titanium is also a bio-compatible element (non-toxic and is not rejected by the human body). It is used in various medical applications, such as surgical implements and implants. For example:
- Hip balls and sockets (for joint replacement) can stay in place for up to two decades.
- Dental implants can remain in place for more than three decades.
Titanium dental implants
Titanium surgical instruments are lightweight (40% lighter than stainless steel), non-ferrous, non-magnetic, and corrosion-resistant. They resist corrosion even in the presence of oxidizing acids, saltwater, chlorides, and organic and industrial chemicals. And since titanium instruments are non-magnetic, they can be safely used within the MRI machine.
Other than these, titanium alloys are used in dozens of industrial purposes, such as hydrometallurgical autoclaves, purified terephthalic acid plants for polyester production, and flue gas desulphurization for pollution control. Each grade is tailored (in terms of composition, strength, and ductility) to particular operating conditions.
The titanium alloy market is projected to register a CAGR (compound annual growth rate) of 3.5% during 2019-2026. It is expected to reach $7 billion by 2026. Alpha and near alpha titanium alloys are likely to account for over 50% of global revenue.
The major factors that will accelerate this market are titanium’s excellent physical and mechanical properties, as well as its extensive usage in a broad range of end-user applications, especially in the aerospace and automotive industry.
Innovative product development will act as an opportunity for the market during the forecast period. However, the high costs of titanium alloys may act as a restraint for the market.
The Asia-pacific region is expected to account for the largest share (over 35%) of the titanium alloy market. China will continue dominating the market during the forecast period, owing to the ever-growing demand from high-end aircraft, chemical, and medical sectors.