==Iron-carbon martensitic transformation==<!-- [[Martensitic transformation]] links here -->
The distinction between [[austenite]] and [[martensite]] is subtle in nature.<ref>{{Citation |lastlast1=Duhamel |firstfirst1=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.|bibcode=2008btpt.book..119D }}</ref> Austenite exhibits a face-centered cubic (FCC) unit cell, whereas the transformation to martensite entails a distortion of this cube into a body-centered tetragonal shape (BCC). This transformation occurs due to a displacive process, where interstitial carbon atoms lack the time to diffuse out.<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> Consequently, the unit cell undergoes a slight elongation in one dimension and contraction in the other two. Despite marked differences in the mathematical descriptions of these crystal structures owing to symmetry considerations, the chemical bonding between them remains highly similar.
In contrast to [[cementite]], which features bonding akin to ceramic materials, the chemical basis for the hardness of martensite is challenging to elucidate.
