Diffusionless transformation: Difference between revisions

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{{Short description|Shift of atomic positions in a crystal structure}}

[[File:diffusionless classification.svg|350px|thumbnail|right|Diffusionless transformation classifications.]]

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{{Refimprove|date=July 2008}}

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DiffusionlessA transformations'''diffusionless transformation''', commonly known as '''displacive transformationstransformation''', denotedenotes [[solid-state chemistry|solid-state]] alterations in the[[Crystal structure|crystal structurestructures]] that do not hinge on the diffusion of atoms across extensive distances. Rather, these transformations manifest as a result of synchronized shifts in atomic positions, wherein atoms undergo displacements of distances smaller than the spacing between adjacent atoms, all while preserving their relative arrangement. An exemplarexample of such a phenomenon is the martensitic transformation, a notable occurrence observed in the context of steel materials. The term "[[martensite]]" was originally coined to describe the rigid and finely dispersed constituent that emerges in steels subjected to rapid cooling. Subsequent investigations revealed that materials beyond ferrous alloys, such as non-ferrous alloys and ceramics, can also undergo diffusionless transformations. Consequently, the term "martensite" has evolved to encompass the resultant product arising from such transformations in a more inclusive manner. In the context of diffusionless transformations, a cooperative and homogeneous movement occurs, leading to a modification in 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 term "[[martensite]]" was originally coined to describe the rigid and finely dispersed constituent that emerges in steels subjected to rapid cooling. Subsequent investigations revealed that materials beyond ferrous alloys, such as non-ferrous alloys and ceramics, can also undergo diffusionless transformations. Consequently, the term "martensite" has evolved to encompass the resultant product arising from such transformations in a more inclusive manner. In the context of diffusionless transformations, a cooperative and homogeneous movement occurs, leading to a modification in 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 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 systematic movement of large numbers of atoms led some to refer to them as ''military'' transformations, in contrast to ''civilian'' diffusion-based phase changes, initially by [[Charles Frank (physicist)|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, whileis probably the most studied, but 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.

== Classification and definitions ==

WhenThe aphenomenon structuralin modification takes place through the coordinated displacement ofwhich atoms or groups of atoms in relationcoordinate to displace their neighboring counterparts, thisresulting phenomenonin structural modification is identifiedknown as a displacive transformation. The scope of displacive transformations is extensive, encompassing a diverse array of structural changes. ConsequentlyAs a result, additional classifications have been devised to provide a more nuanced understanding of these transformations.<ref name="Cohen" />

The first distinction can be drawn between transformations dominated by ''lattice-distortive strains'' and those where ''shuffles'' are of greater importance.

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

The distinction between [[austenite|austenitic]] and [[martensite|martensitic]] steels 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 |url-access=subscription }}</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.

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|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.

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.

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==

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==External links==

*[httphttps://www.msm.cam.ac.uk/phase-trans/2002/martensite.html Extensive resources from Cambridge University]

*[httphttps://www.aem.umn.edu/people/faculty/shield/hane/tet.html The cubic-to-tetragonal transition]

*[httphttps://www.esomat.org/ European Symposium on Martensitic Transformations (ESOMAT)]

*[httphttps://sourceforge.net/projects/tclab/ PTC Lab for martensite crystallography]

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