Martensite in AISI 4140 steel

0. QtubIronPillarJPG|thumb|right| Iron pillar at Delhi India containing 98% wrought iron]] Wrought iron is commercially pure Iron. Ductile iron, also called ductile cast iron, spheroidal graphite iron, or nodular cast iron, is a type of Cast iron invented in 1943 by 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. Metallurgy is a domain of Materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their A single-displacement reaction, also called single-replacement reaction, is when an element or ion moves out of one compound and into another Atomic Diffusion is a process whereby the random thermally-activated hopping of Atoms in a Solid results in the net transport of atoms It includes a class of hard minerals occurring as lath- or plate-shaped crystal grains. A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating 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. Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 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. A quench refers to a rapid Cooling. In Polymer chemistry and Materials science, quenching is used to prevent low-temperature processes such as phase Austenite (or gamma phase iron is a metallic non-magnetic solid solution of Iron and an Alloying element

In the 1890s, Martens studied samples of different steels under a microscope, and found that the hardest steels had a regular crystalline structure. The 1890s were sometimes referred to as the " Mauve Decade" because William Henry Perkin 's aniline dye allowed the widespread use of that A microscope ( Greek: ( micron) = small + ( skopein) = to look or see is an instrument for viewing objects that are 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. A shape memory alloy (SMA also known as a smart alloy, memory metal, or muscle wire) is an Alloy that "remembers" its shape Zirconia redirects here For the Sailor Moon character see Dead Moon Circus.

Martensite has a different crystalline structure (body-centered-tetragonal) than the face-centered-cubic austenite from which it is formed, but identical chemical or alloy composition. Austenite (or gamma phase iron is a metallic non-magnetic solid solution of Iron and an Alloying element 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. In Chemistry, activation energy, also called midnight energy, is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined Cryogenics is often used incorrectly to refer to Cryonics, cryopreserving humans or animals 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. A is a type of Japanese sword ( nihontō) and often is called a "samurai sword 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. 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 Metastability is a general scientific concept which describes states of delicate equilibrium Since chemical processes (the attainment of equilibrium) accelerate at higher temperature, martensite is easily destroyed by the application of heat. In Chemistry, activation energy, also called midnight energy, is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined This process is called tempering. Tempering is a Heat treatment technique for metals and Alloys In Steels tempering is done to "toughen" the metal by transforming brittle 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. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 Cementite or iron carbide is a Chemical compound with the formula Fe3C (or Fe2CFe and an Orthorhombic crystal structure 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. Hardness refers to various properties of Matter in the Solid phase that give it high resistance to various kinds of shape change when Force

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. 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, strain hardening, or cold work is the strengthening of a material by macroscopically speaking plastic deformation (which has the 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. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning "

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

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

References

1. ^ Ashby, Michael F. Cementite or iron carbide is a Chemical compound with the formula Fe3C (or Fe2CFe and an Orthorhombic crystal structure Ferrite or alpha iron ( α-Fe) is a Materials science term for Iron, or a Solid solution with iron as the main constituent with a Maraging steels (a Portmanteau of Martensitic and aging are iron Alloys which are known for possessing superior strength and toughness without losing Spring steel is a low alloy Medium carbon steel with a very high Yield strength. ; & 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. Events 1194 - King Richard I of England gives Portsmouth its first Royal Charter. Year 2001 ( MMI) was a Common year starting on Monday according to the Gregorian calendar.

External links

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