Titanium Alloy: Six Major Properties Engineers Should Know
Sep 11, 2025
What Defines a Titanium Alloy
A titanium alloy is titanium alloyed with elements such as aluminum, vanadium, molybdenum, tin, and zirconium to adjust strength, creep resistance, and corrosion behavior. Impurity control is central: oxygen, nitrogen, carbon, and hydrogen are interstitial elements whose content, specified as maximums in ASTM B265 and GB/T 3621, moves the balance between strength and ductility. Six properties dominate engineering decisions, and each carries a design consequence.
1. High Specific Strength
Titanium alloys have a density of about 4.43-4.51 g/cm3, roughly 60% of steel, while high-strength grades exceed the tensile strength of many structural steels. Ti-6Al-4V in the annealed condition shows minimum tensile strength of 895 MPa and yield of 828 MPa per ASTM B265. The strength-to-density ratio sits above aluminum alloys and most steels, which is why titanium is used for aircraft engine parts, frames, skins, fasteners, and landing gear where every kilogram is budgeted.
2. Thermal Strength
Titanium alloys retain useful strength at temperatures where aluminum alloys soften. Alpha-beta alloys such as Ti-6Al-4V are used up to about 350-400°C in long-term service, and near-alpha alloys such as Ti-6Al-2Sn-4Zr-2Mo operate at 450-540°C. Aluminum alloys lose their advantage above 150°C, so compressor and exhaust components in engines move to titanium or nickel alloys as the operating temperature climbs.
3. Corrosion Resistance
Titanium forms a dense, self-repairing oxide film that passivates the surface in oxidizing, neutral, and mildly reducing media. It resists seawater, chlorides, nitric acid, organic acids, and alkalis better than stainless steel in most chloride environments, and it does not suffer the pitting and crevice attack that limits austenitic stainless steel in marine service. The film regenerates within milliseconds when damaged, which is why titanium equipment in chlor-alkali and desalination plants runs for decades. The exception is reducing media such as hot hydrochloric or sulfuric acid, where palladium-bearing (Grade 7) or molybdenum-nickel (Grade 12) alloys are specified.
4. Low-Temperature Toughness
Unlike carbon steel, titanium does not become brittle at low temperature. Alpha alloys with low interstitial content, such as Ti-5Al-2.5Sn, retain ductility and toughness down to -253°C, and titanium is a standard material for cryogenic vessels, LNG equipment, and rocket propellant tanks. Designers use the same alloy family at -253°C and at 500°C, which simplifies qualification.
5. High Chemical Reactivity
Titanium is chemically active at elevated temperature: above about 600°C it absorbs oxygen and nitrogen from air, forming a hard, brittle alpha case; hydrogen pickup causes hydride embrittlement. Consequences for fabrication are concrete: hot working and welding need controlled atmospheres or vacuum, machining must remove the alpha case before service, and grinding sparks of titanium can ignite. These constraints, not the material price, drive much of titanium part cost.
6. Low Thermal Conductivity and Elastic Modulus
Titanium conducts heat at about 15-17 W/(m·K) at room temperature, roughly one-fifth of iron and one-fourteenth of aluminum, and its elastic modulus is about half that of steel. The low conductivity concentrates cutting heat at the tool edge, demanding carbide tooling and coolant; the low modulus means higher springback in forming and lower stiffness in slender components. Designers compensate with section geometry rather than assuming steel-like behavior.
| Property | Typical value | Design consequence |
|---|---|---|
| Density | 4.43-4.51 g/cm3 | Weight saving versus steel |
| Melting point | 1668°C (pure Ti) | High-temperature melting and welding practice |
| Thermal conductivity | ~15-17 W/(m·K) | Heat concentration in cutting, cooling design |
| Elastic modulus | ~105-116 GPa | Springback, stiffness compensation |
| Tensile strength (Ti-6Al-4V annealed) | 895 MPa min | Structural load capability |
FAQ
What is the melting point of titanium?
Pure titanium melts at 1668°C. Alloying elements shift the solidus slightly; Ti-6Al-4V melts over a range around 1600-1660°C, so melting and welding practice is defined well below these temperatures.
Why is titanium expensive to machine?
Its low thermal conductivity traps heat at the cutting edge, it work-hardens, and it reacts with tool materials at high temperature. This forces slow cutting speeds, rigid machines, flood coolant, and frequent tool changes.
Does titanium corrode in seawater?
No. The passive oxide film makes titanium effectively immune to seawater corrosion, including pitting and crevice attack at ambient temperature, which is why it is standard for marine heat exchangers and offshore piping.
What is the alpha case in titanium?
The alpha case is an oxygen- and nitrogen-enriched hard surface layer formed when titanium is heated in air above roughly 600°C. It is brittle and must be removed by machining or chemical milling before the part enters service.
Which standards specify titanium alloy properties?
The common specifications are ASTM B265 (sheet, strip, and plate), ASTM B348 (bar and billet), ASTM B338 and B861 (tube and pipe), GB/T 3621 (Chinese plate and sheet), and ISO 5832 for implant alloys, each with chemistry and tensile tables for every grade.
