Titanium 101: Properties, Production and Alloy Families

Titanium occupies an unusual position among structural metals: it is the fourth most abundant element in the Earth's crust, yet the finished material costs more than almost any other engineering metal. This note explains where it comes from, why it costs what it costs, what the three alloy families actually are, and how the number on a drawing maps onto the material that arrives on the shop floor.

1. What titanium is

Titanium (Ti, atomic number 22) sits in group 4 of the periodic table, alongside zirconium and hafnium. Its atomic weight is 47.867. The metal is silver-grey, notably lighter than steel at a density around 4.51 g/cm³ — about 57 % of steel and roughly 1.6 times aluminium — and it is characterised above all by an extremely tenacious passive oxide film.

That oxide film (TiO₂) forms spontaneously in air, is chemically stable, and re-forms instantly when damaged. It is the reason titanium resists seawater, chlorides and oxidising acids so well. It is also the reason the metal is biologically inert — the film, not the metal, is what the body's tissue actually meets.

Physical constants

Commercial purity

The mechanical properties of titanium depend heavily on its interstitial content — carbon, nitrogen, hydrogen and especially oxygen. The purest commercially available material, iodide titanium, holds impurities below 0.1 % but is soft and highly ductile. At the other end, 99.5 % commercially pure titanium (industry grade) typically delivers:

2. How titanium is produced, and why it costs what it costs

Titanium is abundant. Titanium metal is expensive, and the two facts are not in conflict — the cost is in the extraction and melting, not the ore.

The feedstock is rutile (TiO₂) or ilmenite (FeTiO₃). Titanium cannot simply be reduced with carbon the way iron is, because at high temperature titanium reacts with carbon and with nitrogen and oxygen in the air. The industrial route is the Kroll process, developed in the 1940s:

1. Chlorination. Rutile is reacted with chlorine and carbon at high temperature to give titanium tetrachloride (TiCl₄), a volatile liquid that can be distilled to separate it from the other metal chlorides present.
2. Reduction. TiCl₄ is reduced with magnesium under an inert atmosphere to give titanium metal and magnesium chloride.
3. Vacuum distillation. The magnesium chloride and residual magnesium are removed by vacuum distillation, leaving a titanium sponge.
4. Melting. The sponge is compacted into an electrode and melted — normally by vacuum arc remelting (VAR), sometimes by electron beam or plasma arc melting — to give an ingot.

Industrial titanium production began in 1948. Two consequences of this route explain the price:

• It is a batch process with multiple high-temperature steps under atmosphere control. Every stage consumes energy: the Kroll route is an order of magnitude more energy-intensive per tonne than steelmaking.
• Contamination is the enemy. Oxygen and nitrogen pick-up at any stage raises hardness and destroys ductility, so melting has to be done in vacuum or inert atmosphere, and remelting is often repeated to homogenise the ingot. Post-consumer scrap can generally only be recycled into lower-grade product, or re-melted with careful sorting.

The result is that titanium's cost is a process cost, not a scarcity cost. It is also why titanium scrap value is comparatively high, and why the industry treats swarf and offcuts as a recoverable asset rather than waste.

3. The allotropic transformation — the single fact that explains titanium

Below 882 °C titanium has a close-packed hexagonal crystal structure, called alpha (α). Above 882 °C it has a body-centred cubic structure, called beta (β). The transition is reversible.

Everything about titanium alloys follows from this. Alloying elements are classified by what they do to the transformation temperature, and the phases present at room temperature determine the alloy's family:

Interstitials — the impurities that matter

Oxygen and nitrogen are the reason titanium welding needs shielding gas and why a titanium part left in a hot furnace without atmosphere control is ruined rather than merely discoloured. Hydrogen is the reason hydrogen-bearing cutting fluids and pickling baths are treated as process hazards — see the machining note.

4. The three alloy families

At room temperature a titanium alloy has one of three matrix structures. National designation systems differ — the US uses Grade numbers, China uses TA/TC/TB, Russia uses BT and Britain IMI — but the families are the same everywhere.

Alpha alloys (α) — TA series, commercially pure grades

Single-phase alpha solid solution. Not heat-treatable — strengthening comes from solid solution and from interstitial content only.

• Strengths: structurally stable at temperature; better wear resistance than pure titanium; strong oxidation resistance; retains strength and creep resistance to 500–600 °C; excellent weldability; excellent cryogenic toughness.
• Weaknesses: cannot be strengthened by heat treatment, so room-temperature strength is modest; machinability is the best of the three families.
• Used for: chemical and process plant, heat exchanger tube, welded assemblies, cryogenic vessels, and — where the alpha family includes near-alpha alloys — high-temperature compressor components.

Alpha-beta alloys (α+β) — TC series

Two-phase mixture of alpha and beta. Heat-treatable by solution treatment and ageing, giving a substantial strength increase over the annealed condition — commonly quoted at 50–100 % improvement after heat treatment.

• Strengths: the best all-round balance of the three; stable structure; good toughness, ductility and hot formability; good hot working; retain strength to 400–500 °C long term.
• Weaknesses: thermal stability is lower than the alpha family, so sustained high-temperature service is a compromise; machinability is intermediate.
• Used for: the bulk of structural titanium — compressor discs and blades, airframe structure, fasteners, medical implants, and general engineering parts.
• The flagship: Ti-6Al-4V, developed 1954, accounting for the majority of all titanium alloy used. Most other titanium alloys can be read as a modification of it.

Beta alloys (β) — TB series

Single-phase beta solid solution. High strength even without heat treatment, and further strengthened by quenching and ageing; room-temperature strength can reach 1372–1666 MPa in the strongest grades.

• Strengths: highest achievable strength of the three, with excellent cold and hot formability — several beta alloys cold-work better than commercially pure titanium; forgeable and weldable; better toughness at high strength than alpha-beta alloys.
• Weaknesses: poorer thermal stability, so unsuited to sustained high-temperature service; the most difficult family to machine.
• Used for: high-strength fasteners and springs, cold-formed parts, airframe structure, and — in the medical field — implant applications where a low modulus is wanted.

Other grades worth knowing

• Near-alpha alloys. Alpha alloys with a small beta addition (typically 1–2 %) for improved creep resistance at high temperature — the mainstay of compressor applications at 500 °C and above.
• Titanium aluminides. Intermetallic compounds rather than alloys: Ti₃Al (α₂) with a service temperature around 816 °C, and TiAl (γ) around 982 °C. Density is about half that of nickel-base superalloys, with strong oxidation and creep resistance — the reason they are candidates for high-temperature aerostructure.
• Nickel-titanium alloys. Nitinol and related compositions exhibit shape memory and superelasticity. Used for vascular stents, thrombus filters, spinal correction rods, orthodontic archwire, bone plates, intramedullary pins and heart-valve components.

5. Reading the grade callout

The number on a drawing carries three pieces of information, and they are worth separating because they are specified by different documents.

1. The designation — a name in a national or industry system. GR5, TC4, Ti-6Al-4V, BT6 and IMI 318 all describe the same alpha-beta alloy.
2. The governing specification — the standard the material must actually meet. This is what sets chemistry limits, mechanical property floors and permitted test methods. A designation without a specification is not a purchase order.
3. The condition — annealed, stress-relieved, solution-treated-and-aged, mill-annealed. The same alloy in two conditions can have markedly different properties, so the heat treatment callout is a material decision, not a production detail.

Extra-low interstitial (ELI)

Appending ELI to a designation denotes tighter limits on oxygen, nitrogen and hydrogen, and lower iron. The result is lower strength with significantly better toughness and ductility and better weldability. Grade 5 becomes Grade 23; TC4 becomes TC4 ELI. The variants are separate mill streams with separate certificates, not a tolerance band on the same product.

ELI is specified where toughness governs (damage-tolerance design), where service is cryogenic, or where the part is a surgical implant. It is not justified by difficulty or tolerance alone. See the TC4 and TC4 ELI comparison.

6. What titanium does better than other metals

Where titanium is the wrong choice

• Reducing acids and chromium salt media. Titanium's resistance is poor against these. The mechanism that protects the metal in oxidising conditions is the mechanism that fails here.
• Crevice corrosion in hot chloride service. The base metal resists the bulk fluid and then corrodes inside a tight gap. This is designable against — see the crevice corrosion note — but it has to be designed against.
• Very light cuts, hydrogen-bearing coolant, chloride-bearing coolant. A process constraint rather than a material one, but it has to be respected.
• Electrical and thermal conduction. Low conductivity on both counts.
• Cost-sensitive, high-volume, non-structural parts. Aluminium or zinc will do the job for a fraction of the price.

7. Where titanium is used

• Aerospace. The original application and still the defining one. Engine compressor components — fan and compressor blades, discs, casings, bearing housings, intermediate cases — plus airframe structure, skins, fasteners and landing gear. Modern military aircraft run to 20–25 % titanium by structural weight; the number rises sharply with Mach number, because the point of titanium there is to replace steel and save weight.
• Chemical and process plant. Heat exchangers, condensers, process piping, pumps and valves, particularly in seawater, brine, wet chlorine and oxidising acid service.
• Medical. Implants, bone plates and screws, joint replacements, spinal instrumentation, dental implants, surgical instruments, and nitinol devices from stents to orthodontic wire.
• Desalination and marine. Seawater handling, offshore and shipboard systems.
• Consumer and premium goods. Eyewear frames, watches, bicycles, sports equipment — applications that trade on the weight, strength and finish rather than on temperature capability.
• Energy and cryogenics. LNG handling, cryogenic process equipment, pressure vessels and storage tanks.

8. The practical summary

Three things decide whether a titanium part is economic, and all three are settled before a single chip is cut:

1. The grade — which fixes the family, and therefore the machinability, the heat treatment route and the achievable strength.
2. The specification it is called out to — which fixes the chemistry, the property floors and the test basis.
3. The condition — annealed, stress-relieved, solution-treated-and-aged — which can change the properties as much as the grade does.

Get those three right and the material is predictable. Get them wrong and the material will still be made exactly to the drawing — which is the expensive way to discover that the drawing was wrong.