Martensite

    Martensite, named after the German metallurgist Adolf Martens (1850–1914), is any crystal structure that is formed by displacive transformation, as opposed to much slower diffusive transformations. It includes a class of hard minerals occurring as lath- or plate-shaped crystal grains. When viewed in cross-section, the lenticular (lens-shaped) crystal grains appear acicular (needle-shaped), which is how they are sometimes incorrectly described. "Martensite" most commonly refers to a very hard constituent of steel (the alloy of iron and carbon) important in some tool steels. The martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure.

In the 1890s, Martens studied samples of different steels under a microscope, and found that the hardest steels had a regular crystalline structure. He was the first to explain the cause of the widely differing mechanical properties of steels. Martensitic structures have since been found in many other practical materials, including shape memory alloys and transformation-toughened ceramics.

Martensite has a different crystalline structure (tetragonal) than the face-centered-cubic austenite from which it is formed, but identical chemical or alloy composition. The transition between these two structures requires very little thermal activation energy because it occurs displacively or martensiticly by the subtle but rapid rearrangement of atomic positions, and has been known to occur even at cryogenic temperatures. Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume:^[1] this can be seen vividly in the Japanese katana, which is straight before quenching. Differential quenching causes martensite to form predominantly in the edge of the blade rather than the back; as the edge expands, the blade takes on a gently curved shape.

Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is a metastable phase, the kinetic product of rapid cooling of steel containing sufficient carbon. Since chemical processes (the attainment of equilibrium) accelerate at higher temperature, martensite is easily destroyed by the application of heat. This process is called tempering. In some alloys, the effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead. Since quenching can be difficult to control, many steels are quenched to produce an overabundance of martensite, then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. Too much martensite leaves steel brittle, too little leaves it soft.

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Martensitic transformation: mysterious properties explained

The difference between austenite and martensite is, in some ways, quite small: while the unit cell of austenite is, on average, a perfect little cube, the transformation to martensite sees this cube distorted by interstitial carbon atoms that do not have time to diffuse out during displacive transformation, so that it is a tiny bit longer than before in one dimension and a little bit shorter in the other two. The mathematical description of the two structures is quite different, for reasons of symmetry (see external links), but the chemical bonding remains very similar. Unlike cementite, which has bonding reminiscent of ceramic materials, the hardness of martensite is difficult to explain in chemical terms.

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 become magnified into a major mismatch between neighboring materials. The mismatch is sorted out by the creation of a myriad of crystal defects, in a process reminiscent of work hardening. As in work-hardened steel, these defects prevent atoms from sliding past one another in an organized fashion, causing the material to become harder.

Shape memory alloy also has surprising mechanical properties, that were eventually explained by an analogy to martensite. Unlike the iron-carbon system, alloys in the nickel-titanium system can be chosen that make the "martensitic" phase thermodynamically stable.

Pseudomartensitic transformation

In addition to displacive transformation and diffusive transformation, a new phase transformation that involves displasive sublattice transition and atomic diffusion was discovered by Chen et al.^[2] using modern diffraction technique. The new transformation mechanism has been christened by the scientists Pseudomartensitic transformation.^[3].

See also

• Spring steel
• Maraging steel
• Cementite
• Eutectoid
• Ferrite (iron)
• Martensite

References

1. ^ Ashby, Michael F.; & David R. H. Jones [1986] (1992). Engineering Materials 2, with corrections (in English), Oxford: Pergamon Press. ISBN 0-08-032532-7. 
2. ^ Jiuhua Chen, Donald J. Weidner, John B. Parise, Michael T. Vaughan, and Paul Raterron, (2001)Observation of Cation Reordering during the Olivine-Spinel Transition in Fayalite by In Situ Synchrotron X-Ray Diffraction at High Pressure and Temperature Phys. Rev. Lett, 86, pp. 4072–4075.
3. ^ Kristin Leutwyler New phase transition Scientific American, May 2, 2001.