Titanium Alloys – Definition, Types and Applications
Like other alloys, titanium alloys consist of a mixture of chemical elements. They are characterized by a very high tensile strength, toughness and corrosion resistance. In Addition, titanium alloys are able to resist extremely high temperatures of up to 760° C (1400 °C) and are density low compared to steel and other superalloys. Therefore, titanium alloys are comparably light. However, on the other hand, these advantages are offset by high production and material costs.
Pure titanium exist in form of β-phase at temperatures above 885 °C (1625 °F) and in form of α-phase at temperatures below 885 °C. The alloying elements may either stabilize the α- or β-phase. Alloys that stabilize α-phase include aluminium, oxygen and nitrogen. Molybdenum, tungsten and tantalum stabilize β-phase.
Types of titanium alloys
Titanium alloys are classified into three different categories. These are differentiated according to the phase composition.
Unalloyed grades or alpha alloys
Commercially pure or unalloyed titanium is characterized by a titanium content of more than 99 %. The main alloying element is oxygen, which determines the strength. A higher content of oxygen means that the strength and hardness increases as well. Alpha alloys are usually only consisting of α-phase. Due to impurities, however, small amounts of β-phase are possible.
Unalloyed titanium grades show great mechanical properties, such as very good corrosion resistance and high ductility and formability. However, the strength is relatively low compared to other titanium alloy grades. Further, alpha alloys can not be heat treated to increase strength.
Examples of unalloyed grades are ASTM grade 1, 2, 3 and 4.
Near alpha alloys
In contrast to alpha alloys which entirely consist of α-phase, near alpha alloys contain a small amount of ductile β-phase. To stabilize α-phase, alloys such as aluminium are added. Moreover, alloys such as molybdenum or vanadium are used as β-phase stabilizers. The content of these is about 1-2 %.
Near alpha alloys show good mechanical properties, suc as high toughness, good creep resistance and weldability. However, the mechanical strength is only moderate and increases with the aluminum content.
Examples of near alpha alloys include Ti-6Al-2Sn-4Zr-2Mo and Ti-5.5Al-3.5Sn-3Zr-1Nb.
Alpha-beta alloys
Alpha-beta alloys consist mainly of Ti-(4-6)Al combined with contents between 4 % and 5 % of β-stabilizer elements. These include elements such as tungsten, molybdenum, vanadium and aluminum. Therefore, alpha-beta alloys consist of a mixture of α and β phases.
Alpha-beta alloys can be heat treated. This results in a significant increase in strength, especially when precipitation hardening is applied. However, the heat treatment leads to a decrease in ductility.
Overall, alpha-beta alloys show high tensile and fatigue strength. Also, they are characterized by good hot formability and acceptable creep resistance.
Examples of alpha-beta alloys include Ti-6Al-4V (Grade 5), which makes up half of the total production of titanium alloys.
Beta Titanium alloys
Beta titanium alloys are rich in β-phase. This is ensured by adding enough β-phase stabilizers such as molybdenum and vanadium. In this way, it is possible to maintain the β-phase after quenching.
Like alpha-beta alloys, beta alloys can be heat and solution treated. Therefore, they can possess high strength and great formability. However, the fatigue strength and ductility are low.
Examples of beta titanium alloys include Ti-10V-2Fe-3Al, Ti-13V-11Cr-3Al and Ti-15-3.
Applications
Titanium alloys are applied in a variety of industries, which include the following.
Aerospace applications
Commercially pure titanium and titanium alloys show high strength, great creep and corrosion resistance combined with light weight. These mechanical properties make the material perfectly suited for use in industries that rely on high standards, such as the aerospace industry.
Titanium is used in the construction of jet engines and airframes. The alloy Ti-6Al-4V is particularly common, accounting for almost 50% of all alloys applied in the aircraft industry, which is the largest consumer of titanium.
Marine industry
Due to the excellent corrosion resistance, titanium has been widely applied in the marine industry. Building of vessels, especially for oil drilling platforms and the desalination of water are areas of application.
Medical applications
Titanium not only has a high corrosion resistance, but is also biocompatible. In combination with its ability to join particularly well with human bones, it is becoming an important material in the biomedical applications. It is used for the following applications:
- Joint replacements
- Dental implants
- Surgical devices
- Pharmaceutical equipment
Automotive industry
In the automotive industry, titanium is not yet used in mass production because of its high cost. However, it is often applied in racing and high-performance cars, especially in engine parts and exhaust and silencer systems.
