What Are Titanium Alloy Types, Heat Treatments, and Applications?

Titanium and titanium alloys outperform aluminum and aluminum alloys in strength, heat resistance, and corrosion resistance. They are widely used in aviation, aerospace, chemical processing, missiles, and marine vessels. However, titanium becomes extremely reactive at high temperature. Melting, pouring, welding, and heat treatment must therefore be performed under vacuum or an inert atmosphere. These strict processing conditions and high costs limit broader use.

Commercially Pure Titanium

Titanium is a silver-white metal with a melting point of 1,680°C and a density of 4.5 g/cm³. It is heavier than aluminum but lighter than steel. Its strength is approximately six times that of aluminum, giving it high specific strength. Titanium also has high ductility, good heat resistance, and a low coefficient of thermal expansion, so thermal stress is relatively low during high-temperature service or hot working.

Titanium has excellent corrosion resistance in sulfuric acid, hydrochloric acid, nitric acid, sodium hydroxide and other alkaline solutions, humid environments, and seawater. It does not resist hydrofluoric acid. Titanium is stable in the atmosphere because a dense oxide film forms on its surface and preserves its metallic luster. Above 600°C, however, the film loses its protective effect.

Titanium has two allotropic forms. Below 882.5°C, the stable structure is hexagonal close-packed α-Ti. From 882.5°C to the melting point, the stable structure is body-centered cubic β-Ti.

Commercially pure titanium (CP titanium) is classified into different grades according to the allowable levels of interstitial and impurity elements. These impurities influence strength, ductility, and corrosion resistance. Common grades in ASTM systems include Grades 1, 2, 3, and 4, and the source identifies Grade 2 as one of the more widely used grades for general industrial applications. This statement describes the international CP titanium framework and does not establish a one-to-one correspondence with Chinese CP titanium grades.

CP titanium combines good corrosion resistance with moderate strength. It is suitable for parts that do not require very high strength but demand corrosion resistance, including aircraft skins and structures, chemical-processing equipment, reactors, heat exchangers, and seawater-desalination equipment.

CP titanium normally cannot be strengthened by conventional heat treatment. Cold working can increase its strength. Its principal heat treatments are recrystallization annealing and stress-relief annealing, which restore ductility, reduce residual stress, and improve dimensional stability.

Composition, Microstructure, Classification, and Designations

Alloying Elements

Alloying elements are added to titanium to increase strength. Elements dissolved in α-Ti form an α solid solution, while elements dissolved in β-Ti form a β solid solution. Aluminum, carbon, nitrogen, oxygen, and boron raise the α/β allotropic transformation temperature and are called α-stabilizing elements. Iron, molybdenum, magnesium, chromium, manganese, and vanadium lower the transformation temperature and are called β-stabilizing elements. Tin and zirconium have little effect on the transformation temperature and are treated as neutral elements.

Microstructural Classes

According to the service-condition, or annealed, microstructure, titanium alloys are divided into α alloys, β alloys, (α+β) alloys, and near-α alloys containing a small amount of β phase. Chinese alloy designations use TA, TB, and TC prefixes followed by a number for these three main alloy groups. By performance, titanium alloys can also be described as low-, medium-, or high-strength alloys.

Heat Treatment of Titanium Alloys

Stress-Relief Annealing

Stress-relief annealing removes internal stress produced by machining or welding. The source gives a typical temperature of 450 to 650°C, a holding time of 1 to 4 h, and air cooling.

Recrystallization Annealing

Recrystallization annealing removes work hardening. Typical temperatures are 550 to 690°C for pure titanium and 750 to 800°C for titanium alloys, with a holding time of 1 to 3 h followed by air cooling.

Quenching and Aging

Quenching and aging increase alloy strength and hardness. α titanium alloys are generally not quenched and aged. For β alloys and (α+β) alloys containing relatively high levels of stabilizing elements, the quenching temperature is usually selected in the upper part of the α+β two-phase region. The source gives 760 to 950°C, a holding time of 5 to 60 min, and water cooling.

After quenching, some α phase remains while the β phase becomes metastable. During aging, dispersed α precipitates from the metastable β phase, increasing strength and hardness. The quenched-and-aged Ti-6Al-4V microstructure contains blocky α+β and acicular α. Heating must prevent contamination, oxidation, and overheating. Once β grains have grown excessively, heat treatment cannot restore the original grain size.

Common Titanium Alloy Types and Applications

Alpha Titanium Alloys

α titanium alloys consist entirely of an α solid solution and therefore have good toughness and ductility. They can be cold worked into semifinished products such as sheet and bar. Their microstructure is stable at high temperature, with good oxidation resistance and hot strength. At 500 to 600°C, their strength is the highest among the three principal titanium-alloy classes, although their room-temperature strength is generally lower than that of β and (α+β) alloys. A representative alloy is Ti-5Al-2.5Sn, used for missile fuel tanks and supersonic-aircraft turbine casings.

Beta Titanium Alloys

Fully β titanium alloys are seldom used industrially because they have relatively high density, poor heat resistance, and low oxidation resistance. Above 700°C, they are readily contaminated by impurity gases in the atmosphere, and their production process is complex. However, the body-centered cubic structure provides good ductility, which led to the development of metastable β titanium alloys. A representative alloy is Ti-5Mo-5V-8Cr-3Al. It is generally used below 350°C for heavily loaded rotating parts such as compressor blades, shafts, and disks.

Alpha-Beta Titanium Alloys

(α+β) titanium alloys combine the advantages of α and β alloys. They offer good heat resistance and ductility, can be strengthened by heat treatment, and are comparatively straightforward to produce. Ti-6Al-4V is the most widely used alloy in this group. It is suitable for engine components requiring elevated-temperature strength below 400°C and for liquid-hydrogen fuel-tank components used at low temperature in rockets and missiles.

Titanium-alloy automotive engine turbochargers and precision-forged aircraft-engine blades are representative components.

Representative Compositions, Properties, and Uses