Iron phase transformations

Claussen W. E. (1960) Detection of the a-y iron phase transformation by differential thermal analysis. Rev. Sci. Instr. 31, 878-881. 

Phase transformations in the solid state (like those in iron), too, have latent heats. They may be small, but with sensitive equipment for measuring cooling curves or heating curves, they are easily detected. 

The shock-compression induced structural phase transformation in iron from the low pressure bcc phase to the high pressure hep phase is one of the most visible problems studied in shock-compression science, and its discovery was responsible for widespread recognition of the capabilities of the high pressure shock-compression experiment. The properties of many shock-induced phase transitions are summarized in Duvall and Graham [77D01]. 

Martensitic phase transformations are discussed for the last hundred years without loss of actuality. A concise definition of these structural phase transformations has been given by G.B. Olson stating that martensite is a diffusionless, lattice distortive, shear dominant transformation by nucleation and growth . In this work we present ab initio zero temperature calculations for two model systems, FeaNi and CuZn close in concentration to the martensitic region. Iron-nickel is a typical representative of the ferrous alloys with fee bet transition whereas the copper-zink alloy undergoes a transformation from the open to close packed structure. ... 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

Schwertmannite and the chemical modeling of iron in acid sulfate waters. Geochimica et Cosmochimica Acta, 60, 2111-2121. Jonsson, J. Persson, P., Sjoberg, S., Lovgren, L. 2005. Schwertmannite precipitated from acid mine drainage phase transformation, sulphate release and surface properties. Applied Geochemistry, 20, 179-191. 

Appelo CAJ, VanDerWeiden MJJ, Toumassat C, Charlet L (2002) Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic. Environ Sci Techno 36 3096-3103 Ardizzone S, Formaro L (1983) Temperature induced phase transformation of metastable Fe(OH), in the presence of ferrous ions. Mat Chem Phys 8 125-133... 

Contradictory opinions have been referred to in the literature particularly on the nature of the iron-tarmate and its interaction with the rusted steel due to the diversity of the material used in different studies. Studies have included the use of tannic acid [7-10], gallic acid [11], oak tannin [12, 13], pine tannin [14] and mimosa tannin [15]. In order to establish the correlation between the ferric-taimate formation and the low inhibition efficiency observed at high pH from the electrochemical studies, phase transformations of pre-rusted steels in the presence of tannins were evaluated. In this work the quantum chemical calculations are conducted to analyse the relationship between the molecular stracture and properties of ferric-taimate complex and its inhibitory mechanism. 

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

The disappearance of the magnetization above the Curie (or Neel) temperature (see Chap. 6) can be used to identify iron oxides. Thermal reactions - dehydration/dehy-droxylation, oxidation/reduction - with accompanying phase transformations may give additional information about phases present. Therefore, not only the heating cycle, but also the cooling cycle is usually recorded to give the so-called Js(T) curve. 

Ideally, a phase transformation should be investigated using a combination of techniques which enable changes in composition, structure, surface area, morphology and porosity of the solid phases and in the composition of the solution to be monitored, together with the reaction kinetics. This type of comprehensive investigation is rare for iron oxide interconversions in most cases only one or two of the above aspects of the transformation have been considered. 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

Martinez, C.E. McBride, M.B. (1998) Coprecipitates of Cd, Cu, Pb and Zn in iron oxides solid phase transformation and metal solubility after aging and thermal treatment. Clays Clay Min. 46 537-545... 

As you can see, the process by which the iron-carbon alloy is processed and solidified is jnst as important as the overall stoichiometry. Although a discussion regarding phase transformations is more the realm of kinetic processes, it is nonetheless pertinent to snmmarize here the types of important ferrous alloys, particularly those in the cast iron categories. This is done in Fignre 2.13. 

The phase transformations in the catalyst play an important role in determining the activity, attrition resistance, and deactivation of this catalyst. Activation of this precipitated catalyst transforms single crystals of hematite to smaller crystallites of carbide. While the transformation from hematite to magnetite is extremely rapid, the magnetite to carbide transition is much slower under the conditions of temperature and pressure employed in this study. As carbon deposits on the carbide particles, it serves to further prise the carbide particles apart. In a commercial slurry phase reactor the carbide particles break away leading to catalyst attrition. The implication of this work for the attrition resistance of iron FT catalysts is explored in detail elsewhere.18... 

Certain alloys of iron, nickel, and cobalt (Kovar, Fernico, etc.) have thermal expansion curves which nearly match those of borosilicate glasses, and a good bond may be formed between the two. Kovar is similar to carbon steel in its chemical properties. For example, it oxidizes when heated in air and is not wet by mercury. It may be machined, welded, copper brazed, and soft soldered. Silver solders should not be used with Kovar since they may cause embrittlement. At low temperatures Kovar undergoes a phase transformation, and the change in expansion coefficient below this temperature may be sufficient to cause failure of a glass-to-Kovar seal. The transformation temperature usually is below... 

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