Editing Phase transition (section)

===Structural===
{{See also|Polymorphism (materials science)}}
[[File:Pure iron phase diagram (EN).svg|thumb|250px|A phase diagram showing the [[allotropes of iron]], distinguishing between several different [[crystal structure]]s including [[Allotropes of iron|ferrite]] (α-iron) and [[austenite]] (γ-iron).]]

Phase transitions can also occur when a solid changes to a different structure without changing its chemical makeup. In elements, this is known as [[allotropy]], whereas in compounds it is known as [[polymorphism (materials science)|polymorphism]]. The change from one [[crystal structure]] to another, from a crystalline solid to an [[amorphous solid]], or from one amorphous structure to another ([[polyamorphism|{{not a typo|polyamorphs}}]]) are all examples of solid to solid phase transitions.

The [[martensitic transformation]] occurs as one of the many phase transformations in carbon steel and stands as a model for [[displacive phase transformations]]. Order-disorder transitions such as in alpha-[[titanium aluminide]]s. As with states of matter, there is also a [[metastable]] to equilibrium phase transformation for structural phase transitions. A metastable polymorph which forms rapidly due to lower surface energy will transform to an equilibrium phase given sufficient thermal input to overcome an energetic barrier.