ADiffusionless '''diffusionlesstransformations, transformation'''also iscalled adisplacive transformationtransformations, are solid state transformations that do not require diffusion, i.e. long range movements of atoms, for a change in the positioncrystal ofstructure manyto atomsoccur. byWith diffusionless transformations, there is some form of cooperative, homogenous 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 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.
