The titanium market relies on the use of commercially-pure titanium, as well as on various alloys. While the first is often used by chemical process industries, its alloys are chosen for airframes and aircraft engines, among other components. This article highlights the differences between titanium and its alloys, as well as when the latter are specifically used.
Titanium, in its unalloyed form, is a lightweight and strong material. Its tensile strength is similar to the one of carbon steel, however its weight is half. Pure titanium is silver-colored, has a low density and a unique luster.
Titanium has excellent corrosion resistance, being the ideal choice for seawater applications (offshore and marine environments). The pure titanium wrought products are mainly used for their resistance to corrosion.
It is also worth mentioning that commercially-pure titanium can withstand the damage (corrosion) caused by other fluids, such as acid rain. For this reason, titanium is nowadays used innovatively for architectural applications. Titanium also has a high resistance to stress corrosion cracking, which makes it an even more interesting material to use.
From a green perspective, titanium is an environmentally-friendly material. Unlike other metals, it does not liberate toxic heavy metal ions (these occur through the corrosion process, which is not a problem with titanium). With regard to forming, titanium can be formed just as easily as stainless steel. Its thermal expansion and shrinkage are, however, higher than stainless steel.
In its pure form, titanium has four distinct grades, each with different properties to offer (corrosion resistance, formability/ductility, strength, etc.). For example, grade 1 titanium has the highest corrosion resistance and formability, but lower strength. Grade 4, by comparison, has the highest strength and only moderate formability.
Commercially-pure titanium is available in different forms, such as bars, cables, strands, coils and flat wire. It can be used for medical applications, including orthopedic implants, needles, sutures, dental implants and even eye glass frames. Titanium has a high strength-to-weight ratio, which means that it is highly resistant to damage and lightweight at the same time.
Titanium can be alloyed with different materials, such as aluminum and vanadium, the resulting alloys being used in the aerospace, chemical and energy fields. Other titanium alloys are made with molybdenum and iron, chromium, nickel, copper, cobalt. The mixture of titanium and various alloys results in increased tensile strength and toughness (even at extreme temperatures).
With their excellent mechanical properties, titanium alloys can be used for the most challenging applications. The components of gas turbine engines can be made with titanium alloys, as well as various airframe parts, for both commercial and military aircrafts.
Nuclear power plants and food processing plants rely on the use of titanium alloys. These are used for heat exchangers in oil refineries, marine components thanks to their high corrosion resistance and, given their biocompatibility, for medical prostheses.
Titanium is sometimes alloyed with palladium, the resulting alloy exhibiting improved resistance to corrosion and strength. Titanium-palladium alloys are used in applications that require excellent corrosion resistance. You will see them being used in chemical processing industries, as well as for storage applications.
Titanium alloys are suitable for environments in which corrosion is a challenge, where there is a permanent fluctuation between oxidizing and reducing. One of the most used titanium alloys is Ti-6Al-4V, which is an alpha-beta alloy. This alloy has a high level of tolerance, being suitable for a wide range of applications.
Alpha alloys contain neutral-alloying elements, as well as alpha stabilizers (aluminum, oxygen, etc.). These have a good strength and weldability, presenting oxidation resistance (even when used at elevated temperatures, this being the result of the aluminum content).
Both titanium and its alloys can be heat treated, in order to increase their overall strength, reduce the residual stress and even to optimize the fracture toughness. However, alpha alloys cannot be heat treated to enhance their mechanical properties, as they are single-phase alloys.
Near-alpha alloys contain a reduced amount of beta-phase stabilizers, which increase their overall ductility. The beta-phase stabilizers are added to a percentage of 1-2% (most commonly these are silicon, vanadium or molybdenum).
Alpha-beta alloys, as their name clearly suggests, are a combination of both beta and alpha stabilizers. These can be heat treated for added strength but it is worth mentioning that their formability will decrease in proportion to the newly-obtained strength (post-treatment).
Beta alloys have a high percentage of beta stabilizers, such as silicon, vanadium or molybdenum. These are treated with different solutions and aged, resulting in improved strength. Beta alloys have excellent formability and they can be easily welded. These are often seen in orthodontic applications, having replaced stainless steel.
Titanium or titanium alloys – selection for service
In deciding on the use of unalloyed commercially-pure titanium or one of its alloys, manufacturers will take into account basic factors such as strength and corrosion resistance. Mechanical properties, such as density, fatigue crack growth rate and fracture toughness will determine the alloy composition and the necessity for heat treatment.
Commercially-pure, unalloyed titanium is generally preferred for corrosion applications, as it as a lower strength. Such applications can include heat exchangers, tanks and reactor vessels, for various industries and fields, including power generation, chemical processing and desalination.
When it comes to high-performance applications, higher-strength titanium alloys are employed. These are used for the development of gas turbines and various aircraft structures, as well as submersibles and drilling equipment. Titanium alloys are nowadays used for the making of biomedical implants and bicycle parts (frames) as well.
Alloys such as Ti-6Al-4V and Ti-3Al-8V-6Cr-4Mo-4Zr are used for offshore drilling applications and geothermal piping. Other alloys, including Ti-6V-2Sn-2Zr-2Cr-2Mo+Si, Ti-10V-2Fe-3Al, Ti-6Al-2Sn-4Zr-2Mo+Si, can be employed for aerospace applications, as well as gas turbine engines.
Manufactures might decide to consider first corrosion resistance and not the strength or temperature resistance of a certain titanium alloy (corrosion resistance as major selection factor). When deciding whether a certain titanium alloy will be used for corrosion applications, economic considerations way a lot in the decision process.