==Iron-carbon martensitic transformation==<!-- [[Martensitic transformation]] links here -->
The difference between [[austenite]] and [[martensite]] is minor.<ref>{{Citation |last=Duhamel |first=C. |title=Diffusionless transformations |date=May 2008-05 |url=https://www.worldscientific.com/doi/abs/10.1142/9789812790590_0006 |work=Basics of Thermodynamics and Phase Transitions in Complex Intermetallics |volume=Volume 1 |pages=119–145 |access-date=2023-08-11 |series=Book Series on Complex Metallic Alloys |publisher=WORLD SCIENTIFIC |doi=10.1142/9789812790590_0006 |isbn=978-981-279-058-3 |last2=Venkataraman |first2=S. |last3=Scudino |first3=S. |last4=Eckert |first4=J.}}</ref> While the unit cell of austenite is a perfect cube, the transformation to martensite involves a distortion of this cube into a body-centered tetragonal shape, as interstitial carbon atoms do not have time to diffuse out during the displacive transformation.<ref>{{cite book |last=Shewmon |first=Paul G. |title=Transformations in Metals |publisher=McGraw-Hill |year=1969 |isbn=978-0-07-056694-1 |___location=New York |page=333 |language=en}}</ref> The unit cell becomes slightly longer in one dimension and shorter in the other two. The mathematical description of the two crystal structures is quite different, for reasons of symmetry, but the chemical bonding remains very similar. Unlike [[cementite]], which has bonding similar to ceramic materials, the hardness of martensite is difficult to explain chemically.
The explanation hinges on the crystal's subtle change in dimension. Even a microscopic crystallite is millions of unit cells long. Since all of these units face the same direction, distortions of even a fraction of a percent get magnified into a major mismatch between neighboring materials. The mismatch is sorted out by the creation of myriad [[crystal defect]]s, in [[work hardening]]. Similar to the process in work-hardened steel, these defects prevent atoms from sliding past one another in an organized fashion, causing the material to become harder.
