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 transformationstransformation classifications]]

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A '''diffusionless transformation''', commonly known as '''displacive transformation''', denotes [[solid-state chemistry|solid-state]] alterations in [[Crystal structure|crystal structures]] 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 example of such a phenomenon is the martensitic transformation, a notable occurrence observed in the context of steel materials.

A '''diffusionless transformation''' is a [[Phase transition|phase change]] that occurs without the long-range [[diffusion]] of [[atoms]] but rather by some form of cooperative, homogeneous movement of many atoms that results in a change in the crystal structure. These movements are small, usually less than the interatomic distances, and the atoms maintain their relative relationships. The systematic movement of large numbers of atoms leads some to refer to these as ''military'' transformations in contrast to ''civilian'' diffusion-based phase changes.<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>

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 being 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, but 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, while beingis 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,. butHowever, an increasing number of alternatives, such as [[shape memory alloy]]s, are becoming more important as well.

== Classification and definitions ==

WhenThe a structural change occursphenomenon byin the coordinated movement ofwhich atoms (or groups of atoms) relativecoordinate to displace their neighborsneighboring thencounterparts theresulting in structural changemodification is termedknown as ''a displacive'' transformation. ThisThe coversscope of displacive transformations is extensive, encompassing a broaddiverse rangearray of transformationsstructural andchanges. As a soresult, furtheradditional classifications have been developeddevised [Cohento 1979]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.

Homogeneous lattice-distortive strains, also known as Bain strains, are strains that transform one [[Bravais lattice]] into a different one. This can be represented by a strain [[Matrix (mathematics)|matrix]] '''S''' which transforms one vector, '''y''', into a new vector, '''x''':

:<math>y=Sx</math>

This is homogeneous, as straight lines are transformed into new straight lines. Examples of such transformations include a [[Cubic crystal system|cubic lattice]] increasing in size on all three axes (dilation) or [[Shearing (physics)|shearing]] into a [[Monoclinic crystal system|monoclinic]] structure.

[[File:diffusionless shuffles distortions.svg|350px|thumbnail|right]]

Shuffles, asaptly thenamed, name suggests,refer involveto the smallminute movementdisplacement of atoms within the unit cell. As a resultNotably, pure shuffles typically do not normallyinduce resulta modification in athe shape change of the unit cell; -instead, onlythey predominantly impact its symmetry and structureoverall structural configuration.

[[Phase transition|Phase transformations]] normallytypically resultgive inrise to the creationformation of an interface betweendelineating the transformed and parent materialmaterials. The energy requiredrequisite tofor generateestablishing this new interface willis dependcontingent onupon its nature -characteristics, essentiallyspecifically how well the two structures fit togetherinterlock. An additional energy termconsideration occursarises ifwhen the transformation includesinvolves a change in shape. changeIn such sinceinstances, if the new phase is constrained by the surrounding material, this may give rise to [[Elasticity (physics)|elastic]] or [[plastic]] deformation andmay henceoccur, introducing a [[Strain (materials sciencemechanics)|strain]] energy term. The ratiointerplay ofbetween these [[Surface science|interfacial]] and strain energy terms has a notable effectsignificantly oninfluences the kinetics of the transformation and the morphology of the newresulting phase. ThusNotably, in shuffle transformations, wherecharacterized distortionsby areminimal smalldistortions, are dominated by interfacial energies andtend canto bepredominate, usefullydistinguishing separatedthem from lattice-distortive transformations where the impact of strain energy tends to have ais greatermore effectpronounced.

A subclassification of lattice-distortive displacements can be made by considering the dilationaldilutional and shear components of the distortion. In transformations dominated by the shear component, it is possible to find a line in the new phase that is undistorted from the [[parent phase]] while all lines are distorted when the dilation is predominant. Shear-dominated transformations can be further classified according to the magnitude of the strain energies involved compared to the innate [[Atom vibrations|vibrations]] of the atoms in the lattice and hence whether the strain energies have a notable influence on the kinetics of the transformation and the morphology of the resulting phase. If the strain energy is a significant factor, then the transformations are dubbed ''martensitic'' and, if it is not the transformation is referred to as ''quasi-martensitic''.

==Iron-Carboncarbon Martensiticmartensitic 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 |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 (BCT). 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 differences in the symmetry of the crystal structures, the chemical bonding between them remains similar.

The difference between [[austenite]] and [[martensite]] is small. While the unit cell of austenite is, a perfect cube, the transformation to martensite distorts this cube by interstitial carbon atoms that do not have time to diffuse out during displacive transformation. The unit cell becomes slightly longer in one dimension and shorter in the other two. The mathematical description of the two 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 explanationiron-carbon hingesmartensitic on thetransformation crystal'sgenerates subtlean changeincrease in dimensionhardness. EvenThe amartensitic microscopic crystallite is millionsphase of unitthe cellssteel long.is Sincesupersaturated allin ofcarbon theseand unitsthus faceundergoes the[[solid samesolution direction,strengthening]].<ref>{{Cite distortionsbook of|last=Banerjee even|first=S. a|url=https://www.worldcat.org/title/156890507 fraction|title=Phase oftransformations: aexamples percentfrom gettitanium magnifiedand intozirconium aalloys major|last2=Mukhopadhyay mismatch between neighboring materials|first2=P. The|date=2007 mismatch|publisher=Elsevier/Pergamon is|isbn=978-0-08-042145-2 sorted|series=Pergamon outmaterials byseries the|___location=Amsterdam creation; ofOxford myriad|oclc=156890507}}</ref> [[crystalSimilar defect]]s, into [[workWork hardening]]. Similar to the process in |work-hardened]] steelsteels, these defects prevent atoms from sliding past one another in an organized fashion, causing the material to become harder.

==PseudomartensiticPseudo martensitic transformation==

Shape memory alloys also have mechanical properties, which were eventually explained by analogy to martensite. Unlike the iron-carbon system, alloys in the nickel-titanium system can be chosen that make the "martensitic" phase [[thermodynamics|thermodynamically]] stable.

In addition to displacive transformation and diffusive transformation, a new type of phase transformation that involves a displasivedisplacive 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 | datepages=2001-04-304072–4075 | issnbibcode=0031-90072001PhRvL..86.4072C | doi=10.1103/physrevlett.86.4072 | pagesissn=4072–4075|0031-9007 |pmid=11328098 |url-access=subscription bibcode|archive-url=2001PhRvLhttps://web.archive.org/web/20230617080425/https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.86.4072C4072 |archive-date=2023-06-17}}</ref> The new transformation mechanism has been christened aspseudo a pseudomartensiticmartensitic transformation.<ref>Kristin{{Cite web |last=Leutwyler [http|first=Kristin |date=May 2, 2001 |title=New Phase Transition May Explain Deep Earthquakes |url=https://www.sciamscientificamerican.com/article.cfm?articleID=000E8826/new-A6AFphase-1C5Etransition-B882809EC588ED9Fmay/ New|url-status=live |archive-url=https://web.archive.org/web/20141117205256/http://www.scientificamerican.com/article/new-phase -transition]-may/ ''Scientific|archive-date=2014-11-17 American'',|access-date=2023-06-17 May|website=Scientific 2, 2001.American}}</ref>

==Pseudomartensitic transformation==

In addition to displacive transformation and diffusive transformation, a new type of phase transformation that involves a displasive sublattice transition and atomic diffusion was discovered using a high-pressure x-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 | title=Observation of Cation Reordering during the Olivine-Spinel Transition in Fayalite by In Situ Synchrotron X-Ray Diffraction at High Pressure and Temperature | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=86 | issue=18 | date=2001-04-30 | issn=0031-9007 | doi=10.1103/physrevlett.86.4072 | pages=4072–4075| pmid=11328098 | bibcode=2001PhRvL..86.4072C }}</ref> The new transformation mechanism has been christened as a pseudomartensitic transformation.<ref>Kristin Leutwyler [http://www.sciam.com/article.cfm?articleID=000E8826-A6AF-1C5E-B882809EC588ED9F New phase transition] ''Scientific American'', May 2, 2001.</ref>

==References==

===Notes===

{{Reflist}}|refs=

<ref name="Cohen">{{cite conference |url=https://www.phase-trans.msm.cam.ac.uk/2009/ICOMAT79/1.pdf |title=On the Classification of Displacive Phase Transformations |last1=Cohen |first1=Morris |last2=Olson |first2=G. B. |last3=Clapp |first3=P. C. |date=1979 |pages=1{{ndash}}11 |___location= |conference=International Conference on Martensitic Transformations}}</ref>

}}

===Bibliography===

* Christian, J.W., ''Theory of Transformations in Metals and Alloys'', Pergamon Press (1975)

* Khachaturyan, A.G., ''Theory of Structural Transformations in Solids'', Dover Publications, NY (1983)

* Green, D.J.; HanninkHannick, R.; Swain, M.V. (1989). ''Transformation Toughening of Ceramics''. Boca Raton: CRC Press. {{ISBN|0-8493-6594-5}}.

==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/ PTCLabPTC Lab for martensite crystallography]

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