Titanium Hydride - Material Properties

Material Properties

Titanium is found in the Earth's crust only in the form of an ore, usually a titanium oxide, such as anatase, brookite etc. Titanium is extracted from titanium ore by removing the oxygen and reacting the ore with chlorine. This process is known as the Kroll process.

The Kroll process results in a volatile chemical compound (tetrachloridotitanium). Titanium is further extracted from tetrachloridotitanium by removing the chlorine and reacting the tetrachloridotitanium with hydrogen. This process is known as hydrogen reduction, and results in an alloy (titanium hydride).

The density of titanium hydride varies based on the alloying constituents, but for pure titanium hydride it ranges between 3.76 and 4.51 g cm−3.

Even in the narrow range of concentrations that make up titanium hydride, mixtures of hydrogen and titanium can form a number of different structures, with very different properties. Understanding such properties is essential to making quality titanium hydride. At room temperature, the most stable form of titanium is the hexagonal close-packed (HCP) structure α-titanium. It is a fairly hard metal that can dissolve only a small concentration of hydrogen, no more than 0.20 wt% at 464 °C (867 °F), and only 0.02% at 25 °C (77 °F). If titanium hydride contains more than 0.20% hydrogen at titanium hydride-making temperatures it transforms into a body-centred cubic (BCC) structure called β-titanium. It can dissolve considerably more hydrogen, more than 2.1% hydrogen at 636 °C (1,177 °F). If titanium hydride contains more than 2.1% at 636 °C (1,177 °F) then it transforms into a face-centred cubic (FCC) structure called δ-titanium. It can dissolve even more hydrogen, as much as 4.0% hydrogen 37 °C (99 °F), which reflects the upper hydrogen content of titanium hydride. If titanium hydride contains 4.0% hydrogen at less than 37 °C (99 °F) then it transforms into a body-centred tetragonal (BCT) structure called ε-titanium.

When titanium hydrides with less than 1.3% hydrogen, known as hypoeutectoid titanium hydride are cooled, the β-titanium phase of the mixture attempts to revert to the α-titanium phase, resulting in an excess of hydrogen. One way for hydrogen to leave the β-titanium phase is for the titanium to partially transform into δ-titanium, leaving behind titanium that is low enough in hydrogen to take the form of α-titanium, resulting in an α-titanium matrix with δ-titanium inclusions.

γ-Titanium is a metastable phase. When titanium hydride is in an alpha phase with a hydrogen content of 0.02-0.06% and then quenched rapidly, it forms into γ-titanium, as the atoms "freeze" in place when the cell structure changes from HCP to FCC. γ-Titanium takes a BCT structure. Moreover, there is no compositional change so the atoms generally retain their same neighbours.

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.