Ferrite phases

Miscibility of C2F with hypothetic C2A is total, imtil the composition CgA2F. 

Tetrahedra have only common vertex, the same calcium octahcdra 

Aluminum is divided uniformly between tetrahedra and octahedra until the composition C2FQ Aq 33. However, in the case of higher aliuninum content it substitute iron principally in tetrahedral layers, which causes decrease of tetrahedra dimensions in relation to octahedral layer and their such twisting that they occupy more symmetric position. Simultaneously calcium ions ate a httle shifted which causes higher symmetry and transformation of stmcture to the special group Imma [176], In brownmillerite about 75 % A1 is substituting Fe in tetrahedral sites, and about 25 % in octahedral [101]. C AF has the highest stabihty in this series of solid solutions [100], 

Solid solutions C2Fj A studied, among others, Woermatm et al. [177]. They found two discontinuities in the changes of unit cell parameters for / =0.30 and 0.50 (Fig. 2.61). However, the change of inclination is very low. First discontinuity is hnked with the polymorphic transformation of pirre C2F at 430 °C, which is caused by discontinuity in increase of the imit cell volume [100]. This polymorphic transformation does not appear in the case of soUd solution for/r=0.33, for stmcture change from Pnma to Imma, which has shown Smith [176]. 

Magnesiirm oxide can form the soUd solutions with C2F, A phase. Two HnHc of these soUd solutions were fotmd [177]. First, in which Mg2+ substitute Fe, is 

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The enhanced strength and corrosion properties of duplex stainless steels depend on maintaining equal amounts of the austenite and ferrite phases. The welding thermal cycle can dismpt this balance therefore, proper weld-parameter and filler metal selection is essential. Precipitation-hardened stainless steels derive their additional strength from alloy precipitates in an austenitic or martensitic stainless steel matrix. To obtain weld properties neat those of the base metal, these steels are heat treated after welding. 

Pordand cement clinker is formed by the reactions of calcium oxide and acidic components to give C S, C2S, C A, and a ferrite phase approximating... 

The hydration of the ferrite phase (C AF) is of greatest interest in mixtures containing lime and other cement compounds because of the strong tendency to form soHd solutions. When the sulfate in solution is very low, soHd solutions are formed between the cubic C AH and analogous iron hydrate C FHg. In the presence of water and siUca, soHd solutions such as C3 ASH4-C3FSH4 may be formed (33). Table 7 Hsts some of the important phases formed in the hydration of mixtures of pure compounds. 

Additional high temperature changes cause decarburization, wherein carbon in the ferrite phase of carbon steel can be oxidized to carbon dioxide. 

Duplex, and super-duplex stainless steels, contain high percentages of chromium. They are called duplex because their structure is a mixture of the austenitic and ferritic phases. They have a better corrosion resistance than the austenitic stainless steels and are less susceptible to stress corrosion cracking. The chromium content of duplex stainless steels is around 20 per cent, and around 25 per cent in the super-duplex grades. The super-duplex steels where developed for use in aggressive off-shore environments. 

Dibutylstibinic acid, 3 73 Dibutyltin diacetate, 24 823 Dibutyltin dilaurate, 24 823 Dicadmium triantimonide, 4 510 Dicalcium alumino monosilicate, phase in Portland cement clinker, 5 472t Dicalcium ferrite, phase in Portland cement clinker, 5 472t Dicalcium hexaborate pentahydrate,... 

There may be a chelating effect whereby TEA reacts with the ferrite phase of Portland cement [10], as illustrated in Fig. 5.2. 

Step cooling will not simulate embrittlement of 1 WCM Mb, though it occurs (e.g., a 100°F [38°C] increase in transition temperature was reported after 8 y at 930° F [500° C]). This is because the embrittlement in 11/iCr-1 Mo is caused by precipitation of carbides in the ferrite phase rather than segregation of impurities to the grain boundaries. Temper embrittlement can be reversed by heating at 1,150°F (620°C) for 2 h per inch of thickness. 

The ferrite phase makes up 5 15% of normal Portland cement clinkers. It is tetracalcium aluminoferrile (CajAIFeOj) substantially modified in composition by variation in Al/Fe ratio and incorporation of foreign ions. The rale at which it reacts with water appears to be somewhat variable, perhaps due to differences in composition or other characteristics, but in general is high initially and intermediate between those of alite and belite at later ages. 

Tentative composition for ferrite phase in sulphate-resisting clinkers. 

Fig. 1.7 Portions of XRD powder patterns of clinkers containing (A) cubic, (B) orthorhombic and (C) pseudotetragonal modifications of the aluminate phase. Peaks marked A and F are due to aluminate and ferrite phases, respectively, and arc rc-indexed, where necessary, to correspond to axes in the text and Table 1.7, and to calculated intensities. After Regourd and Guinier (Rl).

Table 1.7 Crystal data for ferrite phases in the series Ca,(Al,.Fe,...

In many clinkers, the ferrite phase is closely mixed with aluminate due to a similarity in cell parameters, oriented intergrowth can occur (MIS). The close admixture often renders X-ray microanalysis difficult or unreliable. For ordinary Portland cement clinkers, the compositions found in dilferent laboratories are nevertheless remarkably consistent. Table 1.2 includes an average value based on the results of investigations using X-ray microanalysis (H8,K1,B2,U1,H3,B4) or chemical analysis of separated material (Yl). Table 1.3 includes suggested site occupancies corresponding to these data. 

Sulphate-resisting Portland cements have relatively high ratios of iron to aluminium, and the ferrite phase cannot have the composition given above if it contains most of the iron. Tables 1.2 and 1.3 include a tentative composition and atomic ratios corresponding to it, based on scanty data for the interstitial material as a whole (G3,G4) and the requirement of reasonable site occupancies. 

Crystal data and X-ray powder patterns for ferrite phase containing foreign ions... 

Regourd and Guinier (Rl) reported unit cell parameters for the ferrite phase in five clinkers. The ranges observed were a = 0.5517-0.5555 nm, h = 1.455-1.462 nm, c = 0.5335-0.5350 nm. Boikova (B4) reported XRD powder spacings for clinker ferrites which indicate similar values. The similarity of these cell parameters to those of the laboratory preparations... 

Fig. 2.5 shows part of the ternary system CaQ-Al203-Fe203. C3A, C12A7 and CA can all accommodate some Fe for C3A under equilibrium conditions at 1325°C, the limit is about 4.5%, expressed as Fe203 (M20). The ferrite phase in equilibrium with iron-substituted C3A can have compositions with. V between 0.48 and 0.7 in the formula Ca2(Al,jFej jj)205 if CaO is also present,. v is fixed at 0.48, i.e. the composition is close to C4AF. Some reduction of Fe to Fe occurs when the ferrite phase is prepared from mixes with compositions in the Ca2(AljjFej - j)205 series in air it leads to the formation of minor amounts of other phases, which are not observed when similar experiments are carried out in oxygen (M20). 

Fig. 2.5 Part of the system CaO-AljOj-FejOj. Hatched lines indicate solid solutions. Ferrite phase compositions are indicated by the quantity x, which relates to the formula CajfAl Fe, After Newkirk and Thwaite (N8) and Majumdar...

Swayze (S8) noted that equilibrium was often difficult to achieve in this system. One effect was the tendency for crystals of the ferrite phase to be zoned. For bulk compositions in the Ca2(Al Fei J2O5 series, the liquid is of higher Al/Fe ratio than the ferrite phase with which it is in equilibrium the crystals that are initially deposited on cooling such a liquid therefore have a lower Al/Fe ratio than the bulk composition of the mix. On further cooling, the Al/Fe ratio of the material deposited progressively increases. Equilibrium within the ferrite crystals is difficult to attain, causing them to remain zoned, with cores richer in Fe and outer regions richer in AF" than the mean composition. 

A second effect was the tendency for protected phases to be formed. If a liquid having a composition somewhat on the CaO-rich side of the boundary between the CaO and CjA primary phase fields (Fig. 2.5) is cooled calcium oxide is initially deposited and the liquid composition moves away from CaO and towards that boundary. When the latter is reached, and assuming that equilibrium were to be maintained, calcium oxide would redissolve, C3A would be deposited, and the liquid composition would move along the boundary. In reality, the C3A quickly surrounds the particles of calcium oxide, which thus form a protected phase, effectively removed from the system. This can markedly affect the composition of the ferrite phase which is subsequently formed on further cooling. 

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