Nickel alloys are a type of metal composed of nickel and other elements. Monel is an alloy of nickel and copper, Invar is an alloy of nickel and iron, Kamacite is an alloy of nickel and iron-nickel, Constantan is an alloy of nickel and copper, and Nitinol is an alloy of nickel and titanium. These alloys are used in a variety of applications, such as aerospace engineering, medical devices, and jewelry.