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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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.

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 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.

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 thesethem as ''military'' transformations, in contrast to ''civilian'' diffusion-based phase changes, initially by [[FrederickCharles 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 a structural change occursphenomenon byin the coordinated movement ofwhich atoms (or groups of atoms) relativecoordinate to displace their neighbors,neighboring thecounterparts changeresulting in structural modification is termedknown as a ''displacive'' transformation. ThisThe coversscope of displacive transformations is extensive, encompassing a broaddiverse rangearray of transformationsstructural sochanges. furtherAs a result, 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.

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[[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 [[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 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|austenitic]] and [[martensite|martensitic]] steels is minorsubtle 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> 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 (BCCBCT). tetragonalThis transformation occurs shapedue 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 slightlya slight longerelongation in one dimension and shortercontraction in the other two. TheDespite mathematicaldifferences descriptionin the symmetry of the two crystal structures is quite different for reasons of symmetry, but the chemical bonding remainsbetween verythem similar. Unlike [[cementite|cementite,]] which has bondingremains 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 [[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|n]]<nowiki/>ickel-titanium system can be treated to make the "martensitic" phase [[thermodynamics|thermodynamically]] stable.

==Pseudo martensitic transformation==

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* 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/ PTC Lab for martensite crystallography]

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