A '''diffusion lessdiffusionless 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 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.
