Titanium

Titanium (Ti, atomic number 22) is a Group 4 transition metal with an HCP crystal structure (α-Ti, stable below 882 °C), a melting point of 1,668 °C, and a density of 4.51 g/cm³ — giving it the highest strength-to-density ratio of any structural metal and the best corrosion resistance of any non-precious engineering metal. Titanium is relatively abundant in Earth's crust (~0.57 wt%, ninth most abundant element), occurring primarily in ilmenite (FeTiO₃) and rutile (TiO₂); however, the energy-intensive Kroll process (reduction of TiCl₄ with magnesium at ~850 °C in an argon atmosphere, followed by vacuum distillation) makes Ti metal expensive relative to its abundance, with global production of ~250,000 tonnes/year. Ti forms a self-repairing passive TiO₂ oxide layer (2–7 nm) that provides exceptional resistance to corrosion in seawater, chlorine-containing environments, body fluids, and most acids — surpassed only by platinum-group metals in chemical inertness. The α→β allotropic transformation at 882 °C (to BCC β-Ti) is the basis of titanium alloy metallurgy: α-stabilizers (Al, O, N) raise the β-transus, β-stabilizers (V, Mo, Cr, Fe) lower it, and controlled α+β or β microstructures are developed by alloy design and thermomechanical processing to achieve targeted property combinations.

Aerospace accounts for ~50% of titanium consumption, exploiting Ti's exceptional strength-to-weight ratio (~250 kN·m/kg for Ti-6Al-4V vs. ~180 kN·m/kg for 4340 steel), fatigue resistance, and compatibility with carbon fiber composites in airframe structures where galvanic corrosion with aluminum is a concern. Boeing 787 and Airbus A350 each contain ~130–140 tonnes of titanium (fan blades, compressor discs, landing gear beams, fuselage frames, fasteners); GE90 and Trent XWB turbofan engines use Ti fan blades and compressor discs in their cold sections (Ti is replaced by Ni superalloys above ~600 °C where Ti loses strength and is susceptible to titanium fires). Medical and dental applications consume ~5% of Ti production but represent its most value-added use: Ti-6Al-4V ELI (Extra Low Interstitials, ASTM F136) is the standard alloy for orthopedic implants (femoral stems, tibial trays, spinal cages) and Ti-6Al-7Nb is preferred for fracture fixation; commercially pure Ti (ASTM Grade 4) is used for dental implants where higher fatigue strength from cold-worked CP Ti is sufficient.

Titanium dioxide (TiO₂) — produced from Ti ore by the sulfate or chloride process — is by far the most important Ti compound, with ~7 million tonnes/year production as a white pigment (paint, paper, plastics) representing over 95% of total Ti ore consumption, dwarfing Ti metal production by ~30-fold. Photocatalytic TiO₂ (anatase phase, bandgap 3.2 eV, active under UV) is used for self-cleaning glass (Pilkington Activ), photocatalytic air purification, and hydrogen production by water splitting; this application drives research into visible-light-active TiO₂ via doping (N, C, S) and heterojunction composites. In emerging applications, Ti is the substrate for anodized TiO₂ nanotube arrays used in dye-sensitized solar cells, biosensors, and drug delivery coatings on implants; TiN hard coatings (PVD at 500–600 °C) provide gold-colored wear-resistant surfaces on cutting tools, medical instruments, and decorative components; and Ti-based shape-memory alloys (Nitinol, Ni-50Ti) are the basis of minimally invasive surgical devices, orthodontic wires, and stents.