titanium alloys
Titanium is the material of jet engines, hip implants, and high-end bike frames, the metal you reach for when you need steel-like strength at nearly half the weight, plus corrosion resistance that shrugs off seawater and body fluids. It is strong, light, and nearly inert. Its only real drawback is that it is expensive and fussy to make, which keeps it out of everyday use.
Titanium's density is about 4.5 g/cm^3, heavier than aluminum but far lighter than steel, and its strong alloys rival steel's strength, so its strength-to-weight (specific strength) is outstanding. Like iron, titanium is allotropic: it is hexagonal-close-packed alpha at room temperature and body-centered-cubic beta when hot, and alloying elements that stabilize one phase or the other are used to tune properties. The workhorse Ti-6Al-4V (6 percent aluminum, 4 percent vanadium) is an alpha-beta alloy that can be heat-treated for strength. Titanium's corrosion resistance, like stainless steel's, comes from a tenacious, self-healing titanium-oxide passive film, which is also why it is biocompatible: the body tolerates it, so it is ideal for implants.
You find titanium wherever performance justifies the price: aircraft structures and engine fan blades, chemical plant and marine hardware, medical implants, and premium sports gear. The honest limits are cost and manufacturing. Titanium is reactive when hot; it grabs oxygen, nitrogen, and hydrogen and turns brittle, so it must be melted and welded under vacuum or inert gas, and it is hard to machine (it work-hardens and runs hot). Those difficulties, not the ore (titanium is abundant), are why it costs many times more than steel or aluminum.
Ti-6Al-4V reaches a yield strength near 880 MPa at a density of 4.4 g/cm^3, giving a specific strength (strength divided by density) that beats most steels, which is why a jet engine's fan blades and a fighter's airframe are titanium despite the cost.
Titanium's selling point is specific strength: steel-class strength at roughly 55 percent of steel's weight, plus seawater and body-fluid corrosion resistance.
Titanium is not rare, the expense is in extracting and processing it, because it is so reactive when hot that it must be melted and welded under vacuum or inert gas. And it is stiffer than aluminum but only about half as stiff as steel, so it is not a drop-in replacement where rigidity rules.