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{{Short description|Shift of atomic positions in a crystal structure}}
[[File:diffusionless classification.svg|350px|thumbnail
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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, homogenous 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 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 | title=マルテンサイトの格子欠陥 | journal=日本金属学会会報 | publisher=日本金属学会 | volume=6 | issue=7 | date=1967 | issn=1884-5835 | doi=10.2320/materia1962.6.497 | pages=497-506| url=https://www.jstage.jst.go.jp/article/materia1962/6/7/6_7_497/_article/-char/ja}}</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, but an increasing number of alternatives, such as [[shape memory alloy]]s, are becoming more important as well.▼
▲
▲The most commonly encountered transformation of this type is the [[Adolf Martens|martensitic]] transformation, which
== Classification and definitions ==
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,
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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,
Phase transformations normally result in the creation of an interface between the transformed and parent material. The energy required to generate this new interface will depend on its nature - essentially how well the two structures fit together. An additional energy term occurs if the transformation includes a shape change since, if the new phase is constrained by the surrounding material, this may give rise to [[Elasticity (physics)|elastic]] or [[plastic]] deformation and hence a [[Strain (materials science)|strain]] energy term. The ratio of these interfacial and strain energy terms has a notable effect on the kinetics of the transformation and the morphology of the new phase. Thus, shuffle transformations, where distortions are small, are dominated by interfacial energies and can be usefully separated from lattice-distortive transformations where the strain energy tends to have a greater effect.▼
▲[[Phase transition|Phase transformations]]
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'' and if it is not the transformation is referred to as ''quasi-martensitic''.▼
▲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''
==Iron-Carbon Martensitic transformation==<!-- [[Martensitic transformation]] links here -->▼
▲==Iron-
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 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.
==Pseudo martensitic transformation==
In addition to displacive transformation and diffusive transformation, a new type of phase transformation that involves a displacive sublattice transition and atomic diffusion was discovered using a high-pressure
==References==
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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.;
==External links==
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