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'''Diffusionless transformations''', alsocommonly referred toknown as displacive transformations, aredenote solid-state changesalterations in the [[crystal structure]] that do not relyhinge on the [[diffusion]] of atoms overacross longextensive distances. InsteadRather, theythese occurtransformations duemanifest toas a result of coordinatedsynchronized shifts in atomic positions, wherewherein atoms moveundergo bydisplacements aof distancedistances lesssmaller than the spanspacing between neighboringadjacent atoms, all while maintainingpreserving their relative arrangement. An illustrative instanceexemplar of thissuch a phenomenon is the martensitic transformation, a notable occurrence observed in the context of steel materials. The term "[[martensite]]" was initiallyoriginally usedcoined to designatedescribe the hardrigid and finely dispersed constituent that formsemerges in rapidlysteels cooledsubjected steels.to Subsequently,rapid itcooling. wasSubsequent discoveredinvestigations revealed that other materials beyond ferrous alloys, includingsuch as non-ferrous alloys and ceramics, can also undergo diffusionless transformations as well. As a resultConsequently, the term "martensite" has taken on a more inclusive meaningevolved to encompass the resultingresultant product ofarising from such transformations. Within diffusionlessa transformations,more thereinclusive ismanner. someIn formthe context of cooperativediffusionless transformations, a cooperative and homogeneous movement thatoccurs, resultsleading into a changemodification toin 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.
== Classification and definitions ==
When a structural changemodification occurstakes byplace through the coordinated movementdisplacement of atoms (or groups of atoms) relativein relation to their neighborsneighboring counterparts, thethis changephenomenon is termedidentified as a ''displacive'' transformation. ThisThe coversscope of displacive transformations is extensive, encompassing a broaddiverse rangearray of transformationsstructural sochanges. furtherConsequently, additional classifications have been developeddevised 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.
[[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 naturecharacteristics, - 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 [[InterfacialSurface energyscience|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 dilutional 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'', if not the transformation is referred to as ''quasi-martensitic''.
==Iron-carbon martensitic transformation==<!-- [[Martensitic transformation]] links here -->
The differencedistinction between [[austenite]] and [[martensite]] is minorsubtle in nature.<ref>{{Citation |last=Duhamel |first=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.}}</ref> WhileAustenite theexhibits unit cell of austenite isa face centred-centered cubic (FCC) unit cell, whereas the transformation to martensite involvesentails a distortion of this cube into a body-centered tetragonal shape (BCC). tetragonalThis shapetransformation occurs due to a displacive process, aswhere interstitial carbon atoms dolack not havethe 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> TheConsequently, the unit cell becomesundergoes a slightlyslight longerelongation in one dimension and shortercontraction in the other two. TheDespite mathematicalmarked descriptiondifferences ofin the twomathematical crystaldescriptions structuresof isthese quitecrystal differentstructures forowing reasonsto ofsymmetry symmetryconsiderations, but the chemical bonding remainsbetween verythem similar.remains Unlike [[cementite|cementite,]] which has bondinghighly similar to ceramic materials the hardness of martensite is difficult to explain chemically.
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 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.
Shape memory alloys have mechanical properties, which were eventually explained by analogy to martensite. Unlike the iron-carbon system, alloys in the [[nickel|n]]<nowiki/>ickel-titanium system can be treated to make the "martensitic" phase [[thermodynamics|thermodynamically]] stable. ▼
▲Shape memory alloys have mechanical properties, which were eventually explained by analogy to martensite. Unlike the iron-carbon system, alloys in the [[nickel |n]] <nowiki/>ickel-titanium system can be treated to make the "martensitic" phase [[thermodynamics|thermodynamically]] stable.
==Pseudo martensitic transformation==
* 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}}.
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