Iron–hydrogen alloy, also known as iron hydride, is an alloy of iron and hydrogen and other elements. Because of its lability when removed from a hydrogen atmosphere, it has no uses as a structural material. Iron is able to take on two crystalline forms (allotropic forms), body centered cubic (BCC) and face centered cubic (FCC), depending on its temperature. In the body-centred cubic arrangement, there is an iron atom in the centre of each cube, and in the face-centred cubic, there is one at the center of each of the six faces of the cube. It is the interaction of the allotropes of iron with the alloying elements that gives iron-hydrogen alloy its range of unique properties. In pure iron, the crystal structure has relatively little resistance to the iron atoms slipping past one another, and so pure iron is quite ductile, or soft and easily formed. In iron hydride, small amounts of hydrogen within the iron act as a softening agent that promote the movement of dislocations that are common in the crystal lattices of iron atoms. Other elements and inclusions act as hardening agents that prevent the movement of dislocations. The hydrogen in typical iron hydrides may contribute up to 13 ppm in its weight. Varying the amount of hydrogen, as well as controlling its chemical and physical makeup in the final iron hydride (either as a solute element, or as a precipitated phase), hastens the movement of those dislocations that make pure iron ductile, and thus controls and undermines its qualities. Varying the other alloying elements and controlling their chemical and physical makeup also controls, but enhances its qualities. These qualities include such things as the hardness, quenching behaviour, need for annealing, tempering behaviour, yield strength, and tensile strength of the resulting iron-hydrogen alloy. The retention of iron hydride's strength compared to pure iron is possible only by maintaining iron's ductility. At ordinary pressure, iron can incorporate a small amount of hydrogen into its crystal structure, and at extreme temperatures and pressures, such as might be found in the Earth's core, larger amounts of hydrogen can be incorporated. These substances are the subject of study in industrial metallurgy and planetary geology.
Material properties 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 mixtures of hydrogen and iron that make an iron hydride at atmospheric pressure, a small number of different metallurgical structures with different properties can form. At room temperature, the most stable form of pure iron is the body-centred cubic (BCC) structure called alpha-iron or α-iron. It is a fairly soft metal that can dissolve only a very small concentration of hydrogen, no more than 2 ppm at 25 °C (77 °F) and 3.6 ppm at 912 °C (1,674 °F). The inclusion of hydrogen in alpha iron is called ferritic iron hydride. At 910 °C (1,670 °F) pure iron transforms into a face-centred cubic (FCC) structure, called gamma-iron or γ-iron. The inclusion of hydrogen in gamma iron is called austenitic iron hydride. The more open FCC structure of austenitic iron can dissolve somewhat more hydrogen, as much as 9.0 ppm hydrogen at 1,394 °C (2,541 °F). At this temperature iron transforms into another BCC structure called delta-iron or δ-iron. It can dissolve even more hydrogen, as much as 13 ppm hydrogen at 1,538 °C (2,800 °F), which reflects the upper hydrogen content of iron hydride. When hydrogen moves out of solution with iron it reverts to elemental hydrogen (H2). When iron hydrides with more than 2 ppm hydrogen are cooled, the hydrogen no longer fits within the crystalline structures, resulting in an excess of hydrogen. The way for hydrogen to leave the crystalline phases is for it to precipitate out of solution as elemental hydrogen, leaving behind a surrounding phase of BCC iron called ferrite with a small proportion of hydrogen in solution. In a supersaturated composition (greater than 2 ppm hydrogen), the hydrogen will precipitate out as large inclusions of elemental hydrogen at the grain boundaries until the proportion of hydrogen in the grains has decreased to the saturated composition (2 ppm). The above assumes that the cooling process is very slow, allowing enough time for the hydrogen to migrate. As the rate of cooling is increased, the hydrogen will have less time to migrate to form elemental hydrogen at the grain boundaries; hence the elemental hydrogen is more widely dispersed and acts to prevent slip of defects within those grains, resulting in hardening of the iron hydride. At the very high cooling rates produced by quenching, the hydrogen has no time to migrate but is locked within the crystalline structure and forms martensic iron hydride. Martensic iron hydride is a highly strained and stressed, supersaturated form of hydrogen and iron and is exceedingly hard but brittle.
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