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Iron–hydrogen alloy
An iron–hydrogen alloy, also known as an iron hydride,〔 is an alloy of iron and hydrogen formed when hydrogen atoms dissolve in the crystal structure of metallic iron. This article deals with two broad classes of the iron-hydrogen system: solid iron at ordinary pressure that can incorporate a small amount of hydrogen into its crystal structure; and iron-hydrogen phases that exist at extreme temperatures and pressures, such as might be found in the Earth's core. These substances are the subject of study in industrial metallurgy and planetary geology.
== Low pressure phases ==
Iron is commonly found in the Earth's crust in the form of an ore, usually an iron oxide, such as magnetite, hematite, etc. Iron is smelted from iron ore by a number of chemical processes. One such process, known as hydrogen roasting, is more commonly applied to metals such as tungsten and molybdenum, but can be used to produce iron-hydrogen alloys.
In the narrow range of concentrations that make up iron hydride at atmospheric pressure, mixtures of hydrogen and iron can form a small number of different structures. At room temperature, the most stable form of iron is the body-centred cubic (BCC) structure, α-ferrite. It is a fairly soft metal that can dissolve only a very small concentration of hydrogen, no more than 0.0004 wt% at , and only 0.0002% at . If iron hydride is at iron hydride-making temperatures it transforms into a face-centred cubic (FCC) structure, called austenite or γ-iron. It is also soft and metallic and can dissolve somewhat more hydrogen, as much as 0.0009% hydrogen at . If iron hydride is at temperatures higher than it transforms into a different BCC structure, called δ-iron. It can dissolve even more hydrogen, as much as 0.001% hydrogen at , which reflects the upper hydrogen content of iron hydride.
Hydrogen and other elements within iron act as softening agents that promote the movement of dislocations that naturally exist in the iron atom crystal lattices. Varying the amount of alloying elements -- their formation in the iron hydride either as solute elements, or as precipitated phases -- enables the movement of those dislocations that make iron so ductile and weak, and thus controls qualities such as the hardness, ductility, and tensile strength of the resulting iron hydride. Iron hydride's ductility compared to pure iron is only possible at the expense of strength, of which elemental iron has a relative deficiency. However, iron hydride is metastable and subject to the rapid formation of a precipitated phase of gaseous dihydrogen, a process known as hydrogen embrittlement. Thus, the material properties of iron hydride are time-sensitive. One alloying element intentionally added to modify the characteristics of iron hydride is titanium.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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