Titanium hydride

the stable titanium hydride

Titanium hydride normally refers to the inorganic compound |TiH2 and related nonstoichiometric materials. It is commercially available as a stable grey/black powder, which is used as an additive in the production of Alnico sintered magnets, in the sintering of powdered metals, the production of metal foam, the production of powdered titanium metal and in pyrotechnics. it is an alloy of titanium, hydrogen, and possibly other elements. When hydrogen is the main alloying element, its content in the titanium hydride is between 0.02% and 4.0% by weight. Alloying elements intentionally added to modify the characteristics of titanium hydride include gallium, iron, vanadium, and aluminium.

|TiH1.95 is unaffected by water and air. It is slowly attacked by strong acids and is degraded by hydrofluoric and hot sulfuric acids. It reacts rapidly with oxidizing agents, this reactivity leading to the use of titanium hydride in pyrotechnics.

As |TiH {x}| approaches stoichiometry, it adopts a distorted body centered tetragonal structure, termed the ε form with an axial ratio of less than 1. This composition is very unstable with respect to partial thermal decomposition, unless maintained under a pure hydrogen atmosphere. Otherwise, the composition rapidly decomposes at room temperature until an approximate composition of |TiH1.74 is reached. This composition adopts the fluorite structure, and is termed the δ form, and only very slowly thermally decomposing at room temperature until an approximate composition of |TiH1.47 is reached, at which point, inclusions of the hexagonal close packed α form, which is the same form as pure titanium, begin to appear. The evolution of the dihydride from titanium metal and hydrogen has been examined in some detail. α Titanium has a hexagonal close packed (hcp) structure at room temperature. Hydrogen initially occupies tetrahedral interstitial sites in the titanium. As the H/Ti ratio approaches 2, the material adopts the β form to a face centred cubic (fcc), δ form, the H atoms eventually filling all the tetrahedral sites to give the limiting stoichiometry of |TiH2. The various phases are described in the table below. + Temperature approx. 500 °C, taken from illustrationPhase Weight % HAtomic % H|TiH {x}|Metal latticeα0 – 0.20 – 8|TiH0 – |TiH0.1hcpα & β0.2 – 1.18 – 34|TiH0.1 – |TiH0.5β1.1 – 1.834 – 47|TiH0.5 – |TiH0.9bccβ & δ1.8 – 2.547 – 57|TiH0.9 – |TiH1.32δ2.7 – 4.157 – 67|TiH1.32 – |TiH2fcc
If titanium hydride contains 4.0% hydrogen at less than around 40 °C then it transforms into a body centred tetragonal (bct) structure called ε titanium. When α titanium hydride with a hydrogen content of 0.02 0.06% is quenched rapidly, it forms into γ titanium hydride, as the atoms "freeze" in place when the cell structure changes from hcp to fcc. γ Titanium takes a body centred tetragonal (bct) structure. Moreover, there is no compositional change so the atoms generally retain their same neighbours.

The absorption of hydrogen and the formation of titanium hydride are a source of damage to titanium and titanium alloys. This hydrogen embrittlement process is of particular concern when titanium and alloys are used as structural materials, as in nuclear reactors. Hydrogen embrittlement manifests as a reduction in ductility and eventually spalling of titanium surfaces. The effect of hydrogen is to a large extent determined by the composition, metallurgical history and handling of the Ti and Ti alloy. CP titanium (commercially pure: ≤99.55% Ti content) is more susceptible to hydrogen attack than pure α titanium. Embrittlement, observed as a reduction in ductility and caused by the formation of a solid solution of hydrogen, can occur in CP titanium at concentrations as low as 30 40 ppm. Hydride formation has been linked to the presence of iron in the surface of a Ti alloy. Hydride particles are observed in specimens of Ti and Ti alloys that have been welded, and because of this welding is often carried out under an inert gas shield to reduce the possibility of hydride formation. which offers a degree of protection to hydrogen entering the bulk. There are many types of heat treating processes available to titanium hydride. The most common are annealing and quenching. Annealing is the process of heating the titanium hydride to a sufficiently high temperature to soften it. This process occurs through three phases: recovery, recrystallization, and grain growth. The temperature required to anneal titanium hydride depends on the type of annealing. Annealing must be done under a hydrogen atmosphere to prevent outgassing.