Hafnium is a heavy, steel-gray metal in the reactive metals group that is very closely related to zirconium, and forms a continuous solid-solution at all concentrations of zirconium and hafnium. Hafnium occurs naturally with zirconium at a ratio of approximately 1:50, and is produced exclusively as a co-product of nuclear-grade zirconium. It is used in a variety of applications where few substitutes are available.

Hafnium and zirconium have nearly opposite nuclear properties; thus with its relatively high thermal neutron cross-section, hafnium's biggest application is as control rod material in nuclear reactors. In this application, the presence of a few percent of zirconium (< 4.5%) does not adversely affect its performance.

When used in relatively small amounts, hafnium strengthens the grain boundaries of nickel-based super-alloys and high temperature alloys, improving the creep ductility and rupture lifetime. It is extensively used in a number of alloys that find use in the hot end of jet engines. Hafnium has an extremely high affinity for oxygen, nitrogen, and carbon, and is one of the most effective solid-solution strengtheners via dispersion strengthening.

Like the other reactive metals, hafnium is HCP (Hexagonal Close-packed) at room temperature and anisotropic. Even more than zirconium or titanium, the mechanical properties of hafnium are affected by interstitial element contamination. In many applications, a very low interstitial content may be necessary to obtain the required ductility or other properties. For such critical applications, the iodide crystal bar process is used by ATI Wah Chang to produce hafnium with a typical oxygen concentration of 60 ppm or less.

Hafnium is also used extensively as a tip insert for plasma welding tips. With a low work function and a high melting point, hafnium has been found to extend the life of plasma welding tips, and to give better performance.