Body-centred cubic iron

Al, cubic close-packed, copper Cu, Ag, Au, Al A2, body-centred cubic, iron Fe, Mo, W, Na... 

Crangle J, Hallam GC (1963) The magnetization of face-centred cubic and body-centred cubic iron - - nickel alloys. Proc R Soc A Math, Phys Eng Sci 272(1348) 119-132... 

Ferrite the body-centred cubic form of iron (a-iron) and the solid solutions of one or more elements in b.c.c. iron. 

Fig. 6. Face-centred cubic unit cell of copper (left), and body-centred cubic unit cell of a iron (both shown by broken linos). In each case a unit containing one pattern-unit (one atom) is heavily outlined.

T(hxJrky.Jr z) for this atom is 180° when h- -k- -l is odd.) Therefore, reflections from planes having A+ +1 odd are not fotlnd in the diffraction pattern of a-iron. On the other hand, all planes such as 110, 200, 310, and 211 which have h+k+I even give strong reflections, because all the atoms lie on these planes. (In other words, the phase angle for the centre atom is 0° when h -rk- l is even.) It is important to note that this is true not only for body-centred cubic crystals but for a l other body-centred crystals, whatever their symmetry. 

Iron has a body-centred cubic lattice (see Figure 5.16) with a unit cell side of 286 pm. Calculate the number of iron atoms per cm2 of surface for each of the Fe(100), Fe(110) and Fe(lll) crystal faces. Nitrogen adsorbs dissociatively on the Fe(100) surface and the LEED pattern is that of a C(2 x 2) adsorbed layer. Assuming saturation of this layer, calculate the number of adsorbed nitrogen atoms per cm2 of surface. 

Iron behaves similarly there is a sudden increase in the uptake of hydrogen at about 900°, just the transition temperature of the a-y phase change, followed by an equally sudden release of hydrogen at about 1400°, the y-d transition temperature. Both the a and d phases are body-centred cubic. The y phase is face-centred so presumably there is greater interstitial space in that phase. 

Q Steel is one of the most important everyday materials which depend on the interstitial solid solution of iron with carbon. Although iron exists in three polymorphs, two body-centred cubic and one face-centred cubic, only one of these forms readily absorbs... 

The distance from the nearest iron atom to the interstitial site is 1.4 A and 1.8 A respectively for body-centred and face-centred polymorphs. While the site in the body-centred cubic lattice is too small to accommodate the carbon without significant distortion, the site in the face-centred cubic lattice is suitable, and the carbon can be accommodated. Approximately one-third of the empty octahedral sites can be occupied, giving the formula FejC. 

Another example of polymorphic change involving a change in co-ordination is that between the cubic body-centred and cubic close-packed structures of a- and y-iron. Although a change in co-ordination takes place, it should nevertheless be noted that the transformation can be achieved by a purely displacive mechanism, and accordingly takes place readily. In fig. 9.02 four unit cells of the cubic body-centred a-iron... 

Figure 4.5 Mechanism of martensitic transformation of a- to y-iron. (a) Shearing during which the body-centred cubic (b.c.c.) stacking order ABABA... is transformed to the face-centred cubic (f.c.c.) stacking order BACBA..., (b) shearing within the matrix lattice, (c) inner plastic deformation...

This equation may be applied to body-centred cubic a-iron ... 

X-ray diffraction patterns of face-centred cubic and body-centred cubic metals is shown in Pig. 18. It might be mentioned at this stage that X-ray examination has shown why it is that there is a definite contraction in volume and increase in density when a-iron changes to y-iron at the Ac3 point (900° C.), at which there is a change of arrangement of the iron atoms from that of the rather loosely packed body-centred cubic lattice of the a-iron to that of the cubic close-packed lattice of the y-iron. It should also be noted that the face-centred cubic... 

Order and Disorder in Iron-Aluminium Alloys. A mosl valuable routribution lo Hie X-ray examination of superlaHice structures was made in their study of tlie iron-aluminium alloys by Bradley and Jay in 198 2. The atoms of these metals have diameters which are such that they fall only just within the favourable range for the formation of wide solid solution of aluminium in iron. The lattice of the solvent metal, iron, is in this case body-centred cubic, and eight adjacent lattice units... 

Above 130 kbar there is a phase change to a non-magnetic hexagonal phase which has a chemical isomer shift 0-17 mm s more negative than for the body-centred cubic a-iron [13, 15]. The relative line intensities are also affected by pressure due to a pressure-induced polarisation of the iron foil... 

The structure ofa-iron is body-centred cubic, a-iron is ferromagnetic. The Atomium, the symbol of the Brussels World Fair of 1958, is a 102 m high construction in the form of a 165-billion-times enlarged unit cell of an a-lron crystal. [43]... 

The Gaussian Approximation Potential scheme is not limited to simple semiconductors. We demonstrate this by applying the scheme to a metallic system, namely the body-centred cubic (bcc) phase of iron. We included configurations in the training set where the lattice vectors of the one-atom primitive cell were randomised and where the positions of the atoms in 8 and 16-atom supercells were also... 

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