What is the tailhook on naval aircraft made out of?
They're made of alloy steels that have ultra-high strength, high fracture toughness, excellent fatigue resistance, and high resistance to stress corrosion cracking.

Based on one of the manufacturers—QuesTek—the steel's grade is AMS 6516.

The 6xxx grade is a chromium-vanadium steel:


  Chromium-vanadium steel refers to steel alloys incorporating carbon, manganese, phosphorus, sulfur, silicon, chromium, and vanadium. Some forms can be used as high-speed steel. Chromium and vanadium both make the steel more hardenable. Chromium also helps resist abrasion, oxidation, and corrosion. Chromium and carbon can both improve elasticity.


Elasticity is important because you wouldn't want a deformable tailhook.

Other applications for it are:


  Landing gear, rotorshafts, driveshafts, arresting tailhooks and hookshanks, actuators, armor, munitions, gun barrels, and blast-resistant or impact containment devices.