313:, 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.
289:(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 (
555:
138:
41:
285:(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
433:, HPC). In these phases the packing of iron atoms is less dense than in pure iron. The HCP and FCC forms have the same iron lattice as in the pure iron forms, but have different number of hydrogen neighbors, and have different local magnetic moments. The hydrogen and iron atoms are electrically neutral for the bcc form.
770:. Hydrogen has been thought unlikely because of its volatility, but recent studies have uncovered plausible mechanisms for its incorporation and permanence in the core. It is estimated that hcp FeH would be stable under those conditions. Iron–hydrogen alloys could have been formed in a reaction of iron with water in
190:, 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.
352:
Quenching involves heating the iron-hydrogen alloy to create a different phase then quenching it in water or oil. This rapid cooling results in a hard but brittle martensitic structure. The iron-hydrogen alloy is then tempered, which is just a specialised type of annealing, to reduce brittleness. In
193:
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
815:
and water with metallic iron in a diamond cell at 2000 C. Okuchi argues that most of the hydrogen accreted to Earth should have dissolved into the primeval magma ocean; and if the pressure at the bottom of the magma was 7.5 GPa or more, then almost all of that hydrogen would have reacted with iron
336:
Annealing is the process of heating the iron-hydrogen alloy to a sufficiently high temperature to relieve local internal stresses. It does not create a general softening of the product but only locally relieves strains and stresses locked up within the material. Annealing goes through three phases:
221:
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
1192:
Shibazaki, Yuki; Eiji Ohtani; Hiroshi Fukui; Takeshi Sakai; Seiji Kamada; Daisuke
Ishikawa; Satoshi Tsutsui; Alfred Q.R. Baron; Naoya Nishitani; Naohisa Hirao; Kenichi Takemura (1 January 2012). "Sound velocity measurements in dhcp-FeH up to 70 GPa with inelastic X-ray scattering: Implications for
1085:
Olga
Narygina, Leonid S. Dubrovinsky, Catherine A. McCammon, Alexander Kurnosov, Innokenty Yu. Kantor, Vitali B. Prakapenka, and Natalia A. Dubrovinskaia (2011), "FeH at high pressures and implications for the composition of the Earth's core". Earth and Planetary Science Letters, volume 307, issue
589:
This phase is rapidly created at room temperature and 3.8 GPa from hydrogen and α-iron. The transformation entails an expansion by 17–20% in volume. The reaction is complex and may involve a metastable HCP intermediate form; at 9 GPa and 350 °C there are still noticeable amounts of unreacted
436:
At low temperatures the stable forms are BCC below 5 GPa and ε’ (DHCP) above 5 GPa at least up to 80 GPa; at higher temperatures γ (FCC) exists at least up to 20 GPa. The triple point ε'-γ-melt is predicted to be at 60 GPa and 2000 K. Theoretical calculations however predict that, at 300 K, the
843:
for accurate estimates. Narygina and others estimate 0.5–1.0% (weight) of hydrogen in the melt. Similar, but without extrapolations in pressure, theoretical estimates give a narrower range of concentrations 0.4-0.5% (weight), however, this results to too low mean atomic mass of the inner core
758:
is estimated at 330 GPa, still somewhat beyond the range of laboratory experiments. The density of the outer and inner cores can only be estimated by indirect means. The inner core was at first thought to be 10% less dense than pure iron at the predicted conditions, but this presumed “density
509:
state to be studied. At ordinary temperatures, rapid depressurization of FeH from 7.5 GPa (at 1.5 GPa/s) results in metallic iron containing many small hydrogen bubbles; with slow depressurization the hydrogen diffuses out of the metal. High pressure stability of different iron hydrides was
570:(DHCP) structure. It consists of layers of hexagonal packed iron atoms, offset in a pattern ABAC; which means that even-numbered layers are vertically aligned, while the odd-numbered ones alternate between the two possible relative alignments. The c axis of the
832:
sound waves in the solid inner core. A different study predicts 0.08–0.16% (weight) hydrogen in the inner core, while others proposed from 50% to 95% FeH (by mole count) If the core has this much hydrogen it would amount to ten times as much as in the oceans.
353:
this application the annealing (tempering) process transforms some of the dissolved hydrogen into elemental hydrogen and hence it reduces the internal stresses and defects. The result is a more ductile and fracture-resistant iron-hydrogen alloy.
179:, 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
194:
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
819:
Based on density and sound velocity measurements at room temperature and up to 70 GPa, extrapolated to core conditions, Shibazaki and others claim that the presence of 0.23 ± 0.06% hydrogen in weight (that is, a mean atomic composition of
428:
Since then, the pressure-temperature phase diagram of the iron-hydrogen system has been intensively investigated up to 70 GPa. Two additional stable crystalline forms have been observed, denoted “ε’” (the original DHCP form), “ε”
836:
The liquid outer core also appears to have density 5–10% lower than iron. Shibazaki and others estimate that it should have a somewhat higher proportion of hydrogen than the inner core, but there is not enough data about molten
304:
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
1273:
Surendra K. Saxena, Hanns-Peter
Liermann, and Guoyin Shen (2004), "Formation of iron hydride and high-magnetite at high pressure and temperature". Physics of the Earth and Planetary Interiors, volume 146, pages 313-317.
510:
systematically studied using density-functional calculations and evolutionary crystal structure prediction by
Bazhanova et al., who found that at pressures and temperatures of the Earth's inner core only FeH,
816:
to form the hydride, which then would have sunk to the core where it would be stabilized by the increased pressure. Moreover, it appears that at those pressures iron binds hydrogen in preference to carbon.
1235:
Sakamaki, K; Takahashi, E.; Nakajima, Y.; Nishihara, Y.; Funakoshi, K.; Suzuki, T.; Fukai, Y. (May 2009). "Melting phase relation of FeHx up to 20GPa: Implication for the temperature of the Earth's core".
404:
per 10 s at 5 GPa) to form a crystalline solid with formula close to FeH. This reaction, in which the iron expands significantly, was first inferred from the unexpected deformation of steel gaskets in
277:
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
654:(HCP) form of FeH also exists at lower pressure hydrogen, also described by M. Yamakata and others in 1992. This is called the ε phase (no prime). The hcp phase is not ferromagnetic, probably
628:
of this compound is 121 ± 19 GPa, substantially lower than iron's 160 GPa. This difference means that at 3.5 GPa FeH has 51% less volume than the mixture of hydrogen and iron that forms it.
582:
between the layers. The hydrogen layers come in vertically aligned pairs, bracketing the B and C layers and shifted like them. For each hydrogen added the unit cell expands by 1.8
590:α-Fe in the solid. The same form is obtained from by reacting hydrogen with the higher-pressure HCP form of iron (ε-Fe) at 1073 K and 20 GPa for 20 min; and also from α-iron and
505:) have been the subject of theoretical studies. These compounds dissociate spontaneously at ordinary pressures, but at very low temperatures they will survive long enough in a
976:
Takahiro
Matsuoka, Naohisa Hirao, Yasuo Ohishi, Katsuya Shimizu, Akihiko Machida and Katsutoshi Aoki (), "Structural and electrical transport properties of FeH
579:
218:
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.
567:
558:
The double hexagonal close packed (dhcp) structure with ABAC alignment of FeH. Each sphere is an iron atom. Hydrogen are located in the interstices.
563:
410:
349:. The temperature required to anneal a particular iron hydride depends on the type of annealing to be achieved and the alloying constituents.
617:
is higher than that of iron, and decreases down to a minimum at 8 GPa. Above 13 GPa the resistivity increases with pressure. The material is
186:
In pure iron, the crystal structure has relatively little resistance to the iron atoms slipping past one another, and so pure iron is quite
754:
sound waves (the existence of the latter implying that it is a solid). The pressure at the boundary between the inner core and the liquid
105:
642:
The DHCP form of iron hydride can be preserved in a metastable form at ambient pressures by first lowering the temperature below 100 K.
77:
844:(43.8-46.5) and hydrogen seems to be less likely than other elements (S, Si, C, O) to be the main light alloying element in the core.
84:
58:
914:
J.V. Badding, R.J. Hemley, and H.K. Mao (1991), "High-pressure chemistry of hydrogen in metals: in situ study of iron hydride."
1139:
1148:
Hydrogen in Matter: A Collection from the Papers
Presented at the Second International Symposium on Hydrogen in Matter (ISOHIM)
673:
The hcp form of FeH can be preserved in a metastable form at ambient pressures by first lowering the temperature below 100 K.
606:
This phase is stable at room temperature at least up to 80 GPa, but turns into the γ form between 1073 and 1173 K and 20 GPa.
172:
and other elements. Because of its lability when removed from a hydrogen atmosphere, it has no uses as a structural material.
91:
373:(HCP) structure. In an atmosphere of hydrogen at ambient temperature, α-Fe retains its structure up to 3.5 GPa (35,000
73:
124:
329:. Heat treatment is effective on compositions above the saturated composition of 2 ppm hydrogen, aiding to prevent
1048:
Takuo Okuchi (1997), "Hydrogen partitioning into molten iron at high pressure: implications for Earth's core."
342:
62:
621:
at the lowest pressure range, but the ferromagnetism begins to decrease at 20 GPa and disappears at 32 GPa t.
422:
374:
658:. This appears to be the most stable form in a wide pressure range. It seems to have a composition between
195:
1015:
V. E. Antonov, K. Cornell, V.K. Fedotov, A. I. Kolesnikov E.G. Ponyatovsky, V.I. Shiryaev, H. Wipf (1998)
146:
873:
586:(0.0018 nm). This phase was denoted ε’, after the similar structure that iron assumes above 14 GPa.
98:
149:
under a hydrogen atmosphere, wherein the temperature-dependent hydrogen solubility controls resistance.
321:
There are many types of heat treating processes available to iron-hydride alloy. The most common are
215:
980:
under high pressures and low temperatures". High
Pressure Research, volume 31, issue 1, pages 64–67
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681:
These high pressure iron-hydrogen alloys melt at a significantly lower temperature than pure iron:
183:
of iron with the alloying elements that gives iron-hydrogen alloy its range of unique properties.
1301:
1296:
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370:
365:(BCC) crystalline structure; in the absence of reactive chemicals, at ambient temperature and 13
51:
774:
during the formation of the Earth. Above 5 GPa, iron will split water yielding the hydride and
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430:
330:
326:
322:
207:
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142:
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The speed of compressional sound waves in FeH rises as pressure rises, at 10 GPa it is at 6.3
918:, American Association for the Advancement of Science, volume 253, issue 5018, pages 421-424
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378:
338:
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437:
stable structures should be DHCP below 37 GPa, HCP between 37–83 GPa, and FCC above 83 GPa.
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deficit” has later been revised downwards: 2 to 5% by some estimates or 1 to 2% by others.
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San-Martin, A.; Manchester, F. D. (1 April 1990). "The Fe-H (Iron-Hydrogen) System".
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1052:(American Association for the Advancement of Science), volume 278, pages 1781-1784.
1017:"Neutron diffraction investigation of the dhcp and hcp iron hydrides and deuterides"
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The best-known high-pressure phase in the iron-hydrogen system (characterized by
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25:
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The density deficit is thought to be due to mixture of lighter elements such as
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1234:
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956:
863:
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346:
746:. The only parameters that are known with confidence are the speed of the
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377:), with only small amounts of hydrogen diffusing into it forming a solid
306:
824:) would explain a 2–5% density deficit. and match the observed speed of
734:
The slope of the melting point curve with pressure (dT/dP) is 13 K/GPa.
1131:
A. S. Mikhaylushkin, N. V. Skorodumova, R. Ahuja, B. Johansson (2006),
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by a number of chemical processes. One such process, known as hydrogen
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333:. Non-saturated iron hydride does not benefit from heat treatment.
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Iron is able to take on two crystalline forms (allotropic forms),
804:
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1019:. Journal of Alloys and Compounds, volume 264, pages 214–222
771:
235:
161:
17:
165:
1150:, AIP Conference Proceedings, volume 837, pages 161–167
243:
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into metallic iron (with diffusion length of about 500
177:
body centered cubic (BCC) and face centered cubic (FCC)
1113:"Fe-C-H system at pressures of the Earth's inner core"
742:
Very little is known about the composition of Earth's
934:
274:, but can be used to produce iron-hydrogen alloys.
65:. Unsourced material may be challenged and removed.
639:, at 40 GPa 8.3 km/s and 70 GPa 9 km/s.
737:
1288:
609:This material has metallic appearance and is an
384:Starting at about 3.5 GPa of pressure, hydrogen
369:of pressure it converts to the “ε” form, with
361:The common form of iron is the “α” form, with
928:
266:, is more commonly applied to metals such as
145:, or 'barretter', containing an iron hydride
1238:Physics of the Earth and Planetary Interiors
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1115:. Physics-Uspekhi, volume 55, pages 489-497
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281:, the most stable form of pure iron is the
1133:"Structural and magnetic properties of FeH
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417:, as having an approximate composition FeH
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16:This article is about the iron–hydrogen
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534:are thermodynamically stable, whereas
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63:adding citations to reliable sources
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1195:Earth and Planetary Science Letters
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413:and others analysed a sample using
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246:, usually an iron oxide, such as
937:Bulletin of Alloy Phase Diagrams
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39:
943:(2). Springer-Verlag: 173–184.
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50:needs additional citations for
738:Occurrence in the Earth’s core
234:Iron is commonly found in the
1:
1058:10.1126/science.278.5344.1781
1025:10.1016/S0925-8388(97)00298-3
889:
803:and iron hydride by reacting
568:double hexagonal close packed
423:double hexagonal close packed
309:out of solution as elemental
986:10.1080/08957959.2010.522447
924:10.1126/science.253.5018.421
440:Other hydrogenated forms FeH
7:
874:Non-stoichiometric compound
847:
522:and an unexpected compound
10:
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1280:10.1016/j.pepi.2003.07.030
1258:10.1016/j.pepi.2008.05.017
1211:10.1016/j.epsl.2011.11.002
1092:10.1016/j.epsl.2011.05.015
15:
859:Transition metal hydride
799:Indeed, Okuchi obtained
578:. Hydrogen atoms occupy
566:and others, 1989) has a
357:High pressure properties
1203:2012E&PSL.313...79S
371:hexagonal close packing
1111:, Omar Gianola (2012)
1107:Zulfiya G. Bazhanova,
652:hexagonal close packed
603:at 84 GPa and 1300 K.
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431:hexagonal close packed
409:experiments. In 1991
331:hydrogen embrittlement
150:
143:iron-hydrogen resistor
869:Interstitial compound
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379:interstitial solution
140:
74:"Iron–hydrogen alloy"
1137:(x=0.25; 0.50;0.75)"
611:electrical conductor
202:behaviour, need for
59:improve this article
1250:2009PEPI..174..192S
1086:3–4, pages 409–414
580:octahedral cavities
230:Material properties
154:Iron–hydrogen alloy
1197:. 313–314: 79–85.
949:10.1007/BF02841704
884:Allotropes of iron
805:magnesium silicate
710:Melting point (C)
626:elasticity modulus
560:
425:(DHCP) structure.
407:diamond anvil cell
363:body centred cubic
287:face-centred cubic
283:body-centred cubic
242:in the form of an
151:
22:chemical compounds
1156:10.1063/1.2213072
879:Metallic hydrogen
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415:X-ray diffraction
343:recrystallisation
325:, quenching, and
224:planetary geology
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48:This article
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795:→ 2FeH + FeO
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57:Please help
52:verification
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822:0.13 ± 0.03
615:resistivity
375:atmospheres
307:precipitate
210:behaviour,
1291:Categories
890:References
756:outer core
744:inner core
507:metastable
272:molybdenum
181:allotropes
85:newspapers
20:. For the
957:0197-0216
801:magnetite
624:The bulk
572:unit cell
327:tempering
323:annealing
248:magnetite
208:tempering
204:annealing
200:quenching
1140:Archived
848:See also
826:pressure
748:pressure
574:is 0.87
546:is not.
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398:diffuses
396:rapidly
339:recovery
311:hydrogen
268:tungsten
264:roasting
260:iron ore
252:hematite
196:hardness
170:hydrogen
160:, is an
147:filament
1246:Bibcode
1199:Bibcode
1146:. In:
1050:Science
916:Science
776:ferrous
764:silicon
480:), and
256:smelted
188:ductile
99:scholar
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813:silica
782:3Fe +
778:ions:
768:carbon
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772:magma
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236:Earth
162:alloy
106:JSTOR
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953:ISSN
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690:7.5
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421:and
419:0.94
270:and
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78:news
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1276:doi
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920:doi
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820:FeH
766:or
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524:FeH
512:FeH
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