Behaviour of hydrogen and hydride formation in titanium, zirconium, and hafnium

Titanium, zirconium and hafnium of group 4 transition metals are important as the materials for aviation and space use and nuclear reactors. All are chemically active, and form oxides and nitrides by reacting with light elements. These metals and their alloys are excellent in their corrosion resistance because the surface is covered with stable oxides. However, when titanium and zirconium are used as structural materials, they absorb large quantity of hydrogen, but the solid solubility at normal temperature is small, accordingly, the separation of hydrides often becomes the problem. Metal hydrides were actively studied in 1960s due to the expectation that those can be used as the neutron moderator for nuclear reactors. The hydrogen embrittlement of Ti-Al-V and Ti-Al-Sn alloys has been researched. As for zirconium, the neutron absorption cross section is small, therefore, zircaloy has been used for fuel cladding tubes and pressure tubes. The phase diagrams of Ti-H, Zr-H and Hf-H binary systems are shown. The diffusion of hydrogen in the solid solution and the hydrides, and the separation of the hydrides are discussed. (K.I.)