Martensite, named after the German metallurgist Adolf Martens, is a class of hard minerals occurring as lathe- or plate-shaped crystals. When viewed in cross-section, the crystals appear acicular (needle-shaped), which is how they are sometimes incorrectly described. The crystals are a body-centred tetragonal (BCT) form of iron and carbon, and result from the rapid cooling of austenite during quenching.

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 very similar crystalline structure to austenite, and identical chemical composition. As such, a transition between these two allotropes requires very little thermal activation energy, and has been known to occur even at cryogenic temperatures. Martensite has a lower density than ferrite, so that the transformation between phases often results in a relative change of volume: 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.

Because phases such as ferrite, cementite, and austenite are more chemically stable at any composition and temperature, martensite is not in thermodynamic equilibrium; for this reason, martensite is not shown in the equilibrium phase diagram of the iron-carbon system. It only forms because transitions between the stable phases rely on such processes as diffusion and the nucleation of new crystallites with mismatching crystal structures, both of which can be very slow. Martensite can be seen as an interim structure that the material takes on until a stable state can be reached; this phenomenon is known as metastability.

Since chemical processes accelerate at higher temperature, martensite is easily destroyed by the application of heat. In some alloys, this 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, most 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.

Martensitic Transformation: Mysterious Properties Explained

The difference between austenite and martensite is, in some ways, quite small: while the average unit cell of austenite is, on average, a perfect little cube, the transformation to martensite sees this cube distorted, so that it's 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 myriad 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.

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