Allotropes of iron

Some controversial experimental evidence suggests the existence of a fifth high-pressure form that is stable at very high pressures and temperatures.

The inner core of the Earth is generally assumed to consist essentially of a crystalline iron-nickel alloy with ε structure.

The A2 line forms the boundary between the beta iron and alpha fields in the phase diagram in Figure 1.

The beta designation maintains continuity of the Greek-letter progression of phases in iron and steel: α-Fe, β-Fe, austenite (γ-Fe), high-temperature δ-Fe, and high-pressure hexaferrum (ε-Fe).

The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic α-Fe.

Above the A2 boundary, the hysteresis mechanism disappears and the required amount of energy per degree of temperature increase is thus substantially larger than below A2.

Load-matching circuits may be needed to vary the impedance in the induction power source to compensate for the change.

The triple point of hexaferrum, ferrite, and austenite is 10.5 GPa at 750 K.[15] Antiferromagnetism in alloys of epsilon-Fe with Mn, Os and Ru has been observed.

[17] An alternate stable form, if it exists, may appear at pressures of at least 50 GPa and temperatures of at least 1,500 K; it has been thought to have an orthorhombic or a double hcp structure.

[1] As of December 2011[update], recent and ongoing experiments are being conducted on high-pressure and superdense carbon allotropes.

Adding Gamma loop additives keeps the iron in a body-centered cubic structure and prevents the steel from suffering phase transition to other solid states.