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. Metallurgy is a domain of materials science and of materials engineering that studies the physical and chemical behavior of metallic elements and their mixtures, which are called alloys. ... This article is about minerals in the geologic sense; for nutrient minerals see dietary mineral; for the band see Mineral (band). ... Quartz crystal A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... Rose des Sables (Sand Rose), formed of gypsum crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... Austenite is a solid solution of carbon and iron that exists in steel above the critical temperature of 1333°F (about 723°C). ...

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. The 1890s were sometimes referred to as the Mauve Decade, because William Henry Perkins aniline dye allowed the widespread use of that colour in fashion, and also as the Gay Nineties, under the then-current usage of the word gay which referred simply to merriment and frivolity, with no... // Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. ... 1852 microscope Compound microscope made by John Cuff in 1750 A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ... A shape memory alloy (SMA) (also known as memory metal or smart wire) is a metal that remembers its geometry. ... Zirconia (ZrO2) is a white crystalline oxide of zirconium. ...

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. Austenite is a solid solution of carbon and iron that exists in steel above the critical temperature of 1333°F (about 723°C). ... Cryogenics is the study of very low temperatures or the production of the same, and is often confused with cryobiology, the study of the effect of low temperatures on organisms, or the study of cryopreservation. ... Katana of the 16th of 17th Century, with its saya. ... Quenching is a general term for non-radiative de-excitation. ...

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. Iron-carbon phase diagram, showing the conditions under which ferrite (α) is stable. ... Cementite or iron carbide is a chemical compound with the formula Fe3C, and an orthorhombic crystal structure. ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... Diffusion is the spontaneous spreading of something such as particles, heat, or momentum. ... Bubbles in a soft drink each nucleate independently, responing to a decrease in pressure. ... Rose des Sables (Sand Rose), formed of gypsum crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Metastability is the ability of a non-equilibrium state to persist for a long period of time. ...

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. The activation energy in chemistry is the energy needed by a system to initiate a particular process. ... General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Atomic mass 183. ... Tempering is a heat treatment technique for metals and alloys, most often the toughening of martensitic steel. ...

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. Cementite or iron carbide is a chemical compound with the formula Fe3C, and an orthorhombic crystal structure. ...

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. Work hardening is an increase in mechanical strength due to plastic deformation. ...

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. A shape memory alloy (SMA) (also known as memory metal or smart wire) is a metal that remembers its geometry. ... This article is about the building; for another meaning, see stability. ...

External Links

• Extensive resources from Cambridge University Press
• The cubic-to-tetragonal transition