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.

δ-Fe cannot exist above 5.2 GPa, with austenite instead transitioning directly to a molten phase at these high pressures.

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.

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.