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[[File:diffusionless classification.svg|350px|thumbnail|right|Diffusionless transformationstransformation classifications.]]
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Diffusionless transformations, also referred to as displacive transformations, are solid-state changes in the crystal structure that do not rely on the diffusion of atoms over long distances. Instead, they occur due to coordinated shifts in atomic positions, where atoms move by a distance less than the span between neighboring atoms while maintaining their relative arrangement. An illustrative instance of this is the martensitic transformation observed in steel. The term "martensite" was initially used to designate the hard and finely dispersed constituent that forms in rapidly cooled steels. Subsequently, it was discovered that other materials, including non-ferrous alloys and ceramics, can undergo diffusionless transformations as well. As a result, the term "martensite" has taken on a more inclusive meaning to encompass the resulting product of such transformations. With diffusionless transformations, there is some form of cooperative, homogeneous movement that results in a change to the [[crystal structure]] during a [[Phase transition|phase change]]. These movements are small, usually less than their interatomic distances, and the neighbors of an atom remain close. The systematic movement of large numbers of atoms led to some to refer to these as ''military'' transformations in contrast to ''civilian'' diffusion-based phase changes, initially by [[Frederick Charles Frank]] and [[John Wyrill Christian]].<ref>D.A. Porter and K.E. Easterling, Phase transformations in metals and alloys, ''Chapman & Hall'', 1992, p.172 {{ISBN|0-412-45030-5}}</ref><ref>{{cite journal |author=西山 善次 |date=1967 |title=マルテンサイトの格子欠陥 |script-title=ja:... |url=https://www.jstage.jst.go.jp/article/materia1962/6/7/6_7_497/_article/-char/ja |url-status=live |journal=日本金属学会会報 |language=Japanese |publisher=日本金属学会 |volume=6 |issue=7 |pages=497–506 |doi=10.2320/materia1962.6.497 |issn=1884-5835 |archive-url=https://web.archive.org/web/20230617075122/https://www.jstage.jst.go.jp/article/materia1962/6/7/6_7_497/_article/-char/ja |archive-date=2023-06-17 |via=J-STAGE |doi-access=free}}</ref>
The most commonly encountered transformation of this type is the [[Adolf Martens|martensitic]] transformation which, while probably the most studied, is only one subset of non-diffusional transformations. The martensitic transformation in [[steel]] represents the most economically significant example of this category of phase transformations. However, an increasing number of alternatives, such as [[shape memory alloy]]s, are becoming more important as well.
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.
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/>nickel-[[titanium]] systemstem can be chosen that make the "martensitic" phase [[thermodynamics|thermodynamically]] stable.
==Pseudo martensitic transformation==
In addition to displacive transformation and diffusive transformation, a new type of phase transformation that involves a displacive sublattice transition and atomic diffusion was discovered using a high-pressure xX-ray diffraction system.<ref>{{cite journal |last1=Chen |first1=Jiuhua |last2=Weidner |first2=Donald J. |last3=Parise |first3=John B. |last4=Vaughan |first4=Michael T. |last5=Raterron |first5=Paul |date=2001-04-30 |title=Observation of Cation Reordering during the Olivine-Spinel Transition in Fayalite by In Situ Synchrotron X-Ray Diffraction at High Pressure and Temperature |url=https://link.aps.org/doi/10.1103/PhysRevLett.86.4072 |url-status=live |journal=Physical Review Letters |publisher=American Physical Society (APS) |volume=86 |issue=18 |pages=4072–4075 |bibcode=2001PhRvL..86.4072C |doi=10.1103/physrevlett.86.4072 |issn=0031-9007 |pmid=11328098 |url-access=subscription |archive-url=https://web.archive.org/web/20230617080425/https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.86.4072 |archive-date=2023-06-17}}</ref> The new transformation mechanism has been christened pseudo martensitic transformation.<ref>{{Cite web |last=Leutwyler |first=Kristin |date=May 2, 2001 |title=New Phase Transition May Explain Deep Earthquakes |url=https://www.scientificamerican.com/article/new-phase-transition-may/ |url-status=live |archive-url=https://web.archive.org/web/20141117205256/http://www.scientificamerican.com/article/new-phase-transition-may/ |archive-date=2014-11-17 |access-date=2023-06-17 |website=Scientific American}}</ref>
==References==
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