Diffusionless transformation: Difference between revisions

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Diffusionless transformations, also calledreferred to as displacive transformations, are solid -state transformationschanges in the crystal structure that do not requirerely on the diffusion, i.e.of atoms over long rangedistances. movementsInstead, ofthey atomsoccur due to coordinated shifts in atomic positions, forwhere atoms move by a changedistance 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 crystalhard structureand 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 occurencompass the resulting product of such transformations. With diffusionless transformations, there is some form of cooperative, homogenous 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.

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==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=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.