Diffusionless transformation: Difference between revisions

Revision as of 21:12, 27 June 2024

Revision as of 01:53, 12 July 2024

Line 30:

==Iron-carbon martensitic transformation==<!-- [[Martensitic transformation]] links here -->

The distinction between [[austenite|austenitic]] and [[martensite|martensitic]] steels is subtle in nature.<ref>{{Citation |last1=Duhamel |first1=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 (BCCBCT). 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 descriptionssymmetry of thesethe crystal structures owing to symmetry considerations, the chemical bonding between them remains highly similar.

The iron-carbon martensitic transformation generates an increase in hardness. The martensitic phase of the steel is supersaturated in carbon and thus undergoes [[solid solution strengthening]].<ref>{{Cite book |last=Banerjee |first=S. |url=https://www.worldcat.org/title/156890507 |title=Phase transformations: examples from titanium and zirconium alloys |last2=Mukhopadhyay |first2=P. |date=2007 |publisher=Elsevier/Pergamon |isbn=978-0-08-042145-2 |series=Pergamon materials series |___location=Amsterdam ; Oxford |oclc=156890507}}</ref> Similar to [[Work hardening|work-hardened]] steels, defects prevent atoms from sliding past one another in an organized fashion, causing the material to become harder.

In contrast to [[cementite]], which features bonding akin to ceramic materials, the chemical basis for the hardness of martensite is challenging to elucidate.

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 [[crystal defect]]s in [[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 have mechanical properties, which were eventually explained by analogy to martensite. Unlike the iron-carbon system, alloys in the [[nickel]]-titanium system can be treated to make the "martensitic" phase [[thermodynamics|thermodynamically]] stable.

==Pseudo martensitic transformation==