Phase Transitions and Allotropism of Iron

The wide variations in the properties of iron and iron alloys must be related to the existence of pure soHd iron in more than one phase, i.e., several crystallographic structures. This characteristic of many chemical elements including iron is called allotropism. Allotropism must not be confused with the term applied to a pure compound (e.g., a molecule or an alloy) that exhibits several crystal lattices and is calledpofy/wo/p/i/s i. The temperature at which a change in 

Alpha-iron (a-Fe). Between room temperature and a transition temperature of 769°C, pure iron exhibits a body-centered cubic (bcc) crystal lattice (a = 286.645 pm at 25°C). Alpha-iron is a soft, ductile metal with a density of 7875 kg.m l Alpha-iron is also ferromagnetic, with a saturation magnetization at room temperature of 220 A-m -kg , and the cubic anisotropy constants are = 4.7 x 10 J-m and = 1.5 - 3.0 x 10 J m Carbon exhibits a poor solubility in alpha-iron with a maximum content of 0.025 wt.% C at 723°C. It is important to note that the word ferrite describes a solid solution of carbon into alpha-iron, though it is sometimes improperly used to describe alpha-iron (Section 2.1.9)  

Beta-iron (P-Fe). When heated above its Curie temperature of 769 C, alpha-iron loses its ferromagnetic properties but retains its body-centered cubic structure (i.e., second-order transition). This particular form of iron is called beta-iron, which is considered a different allotropic form owing to its paramagnetic properties. However, because no changes in the crystal lattice structure occurs, it is customary to consider it nonmagnetic alpha-iron  

Gamma-iron (y-Fe). At OlO C, the crystallographic structure of iron changes into a face-centered cubic (fee) structure (a = 364.680 pm at 916°C). At this transition temperature, a considerable absorption of latent heat occurs due to the endothermic reaction, and the volume of the iron unit cell expands to 25 vol.%. Gamma-iron is nonmagnetic and has 

Space group (Hermann-Mauguin) Im3m Im3m Fm3m Im3m P6,/mmc 

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