Modern aircraft use a variety of materials for their construction. While composite structures are becoming more and more common, metal is still the primary structural material for aircraft. Because aircraft weight and strength are critical design factors, metals are chosen to match the exact needs of each part of the structure.
Aluminium is the primary metal used in aircraft structures. Aluminium is lightweight, corrosion-resistant, and strong for its weight. The internal "skeletons" and skins of most aircraft are composed of aluminium. These advantages are offset by a low melting point and the fact that aluminium is a comparatively soft metal; these characteristics prevent it from being used in areas of extremely high heat or stress.
Steel is used in areas of aircraft structure where high heat or significant forces exist. Heavy and prone to corrosion (rusting), steel is used sparingly in aircraft. Landing gear, engine mounts, jet engine exhausts, and heat shielding are primary applications for steel in aviation. Stainless steel offers a significant advantage in corrosion resistance over regular steel, and is therefore used more often in aircraft.
Titanium is an outstanding metal for aircraft construction. It is half the weight of steel but offers comparable strength (some titanium alloys far exceed the strength of steel). Titanium withstands heat and corrosion better than the highest-quality steel. Titanium is expensive to machine, and large quantities are not found on general civilian or commercial aircraft. Modern military fighter jets use titanium extensively; the SR-71 Blackbird structure is known for being almost completely composed of titanium. High-performance jet engines also use titanium for most of their internal components.