==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 |url=https://www.worldscientific.com/doi/abs/10.1142/9789812790590_0006 |work=Basics of Thermodynamics and Phase Transitions in Complex Intermetallics |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 aface perfectcentred cubecubic (FCC), the transformation to martensite involves a distortion of this cube into a body-centered (BCC) 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|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|work hardening,]]. ,which results from dislocations within the crystal lattices at the atomic level generated from atomic displacements which serve to prevent the motion of crystal planes under an applied strain.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.
Shape memory alloys also have mechanical properties, which were eventually explained by analogy to martensite. Unlike the iron-carbon system, alloys in the [[nickel|n]]<nowiki/>nickelickel-titanium stemsystem can be chosentreated thatto make the "martensitic" phase [[thermodynamics|thermodynamically]] stable.
