Martensite in AISI 4140 steel

0.35%C Steel, water-quenched from 870°C

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 lathe- 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) used in such products as springs and piano wire. 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. Image File history File links Metadata Size of this preview: 750 × 600 pixelsFull resolution (3840 × 3072 pixel, file size: 941 KB, MIME type: image/jpeg) Martensite in optical microscope. ... Image File history File links Metadata Size of this preview: 750 × 600 pixelsFull resolution (3840 × 3072 pixel, file size: 941 KB, MIME type: image/jpeg) Martensite in optical microscope. ... Image File history File links Size of this preview: 800 × 598 pixelsFull resolution (2248 × 1680 pixel, file size: 2. ... Image File history File links Size of this preview: 800 × 598 pixelsFull resolution (2248 × 1680 pixel, file size: 2. ... A Blacksmith, the father of the modern metallurgist. ... It has been suggested that this article or section be merged with Single replacement scheme. ... Atomic diffusion is a process whereby the random thermally-activated hopping of atoms in a solid results in the net transport of atoms. ... A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties. ... Quartz crystal Synthetic bismuth hopper crystal Insulin crystals Gallium, a metal that easily forms large single crystals A huge monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA. In chemistry and mineralogy, a crystal is a solid in which the constituent atoms... The steel cable of a colliery winding tower. ... Helical or coil springs designed for tension A spring is a flexible elastic object used to store mechanical energy. ... Piano wire is a specialized type of wire made for use in piano and other musical instrument strings, as well as many other purposes. ... A quench refers to a rapid cooling. ... Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. ...

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... The steel cable of a colliery winding tower. ... Robert Hookes microscope (1665) - an engineered device used to study living systems. ... A shape memory alloy (SMA, also known as a smart alloy or memory metal) is an alloy that remembers its geometry. ... Zirconia (ZrO2) is a white crystalline oxide of zirconium. ...

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. Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. ... The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. ... 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. ... This article or section does not cite any references or sources. ...

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. 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. ... The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. ... General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Standard atomic weight 183. ... Cementite or iron carbide is a chemical compound with the formula Fe3C, and an orthorhombic crystal structure. ... 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 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. 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 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. Crystalline solids have a very regular atomic structure: that is, the local positions of atoms with respect to each other are repeated at the atomic scale. ... Work hardening, or strain 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 a smart alloy or memory metal) is an alloy that remembers its geometry. ... Leland Stanfords horse stable, still in use Horse kept in stable A stable is a building in which livestock, usually horses, are kept. ...

Pseudomartensitic Transformation

In addition to displacive transformation and diffusive transformation, a new phase transformation that involves displasive sublattice transition and atomic diffusion is discovered by Chen et al.^[2] using modern diffraction technique. The new transformation mechanism is christened by the scientists Pseudomartensitic transformation.^[3]. It has been suggested that this article or section be merged with Single replacement scheme. ... Atomic diffusion is a process whereby the random thermally-activated hopping of atoms in a solid results in the net transport of atoms. ...

See also

• Spring steel
• Maraging steel

Martensite, named after the German metallurgist Adolf Martens, is a class of hard minerals occurring as lathe- or plate-shaped crystals. ... Maraging steel is an iron-based steel alloy which is known for possessing superior strength without losing malleability. ...

External links

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

The headquarters of the Cambridge University Press, in Trumpington Street, Cambridge. ...

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 02, 2001