Content deleted Content added
Johan Brahm (talk | contribs) Minor rewording, added a source on the martensitic transformation in steel. |
Jacy Ensley (talk | contribs) Updated references to increase accessibility and prevent linkrot. |
||
Line 5:
}}
A '''diffusionless transformation''' is a [[Phase transition|phase change]] 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 |
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.
Line 29:
==Iron-carbon martensitic transformation==<!-- [[Martensitic transformation]] links here -->
The difference between [[austenite]] and [[martensite]] is minor. While the unit cell of austenite is a perfect cube, the transformation to martensite involves a distortion of this cube into a body-centered tetragonal shape, as interstitial carbon atoms do not have time to diffuse out during the displacive transformation.<ref>{{cite book |
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 myriad [[crystal defect]]s, in [[work hardening]]. 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.
Line 36:
==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 x-ray diffraction system.<ref>{{cite journal |
==References==
| |||