Dictionary of Materials science

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A α-Al O Pure alumina. Polycrystalline Al O is known as corundum and single 2 3 2 3 crystals as sapphire. Its crystal structure can be described as consisting 2– 3+ of two sublattices: an FCC sublattice of O ions and a sublattice of Al ions occupying two thirds of the octahedral sites in the first one. α-Fe Allotropic form of iron having BCC crystal structure and existing at tem- peratures below 910°C at atmospheric pressure. α isomorphous Ti system Ti–X alloy system in which the alloying element X is the α-stabilizer, i.e., it raises the temperature of the β ↔ α polymorphic transformation. α-phase in Ti alloys A solid solution of alloying elements in α-Ti. α′-martensite See titanium martensite. α″-martensite See titanium martensite. α-stabilizer In physical metallurgy of Ti alloys, an alloying element increasing the thermodynamic stability of α-phase and thereby raising the β/(α + β) transus in the corresponding phase diagram. In physical metallurgy of steels, it is referred to as ferrite-stabilizer. α-Ti Allotropic form of titanium having a hexagonal crystal structure and exist- ing at temperatures below 882°C at atmospheric pressure. The axial ratio of its lattice c/a = 1.58, i.e., a little smaller than in an ideal HCP structure. α Ti alloy Titanium alloy in which α-phase is the only phase constituent after air-cooling from the β-field in the phase diagram concerned. Alloys with a small fraction of β-phase (∼5 vol%) are usually related to the same group and are called near-α alloys. All the α alloys contain α-stabilizers. (α + β) brass Brass with two phase constituents: a copper-based substitutional solid solution (α-phase) and an electron compound (β-phase). (α + β) Ti alloy Alloy whose phase constituents are α- and β-phases after air- cooling from the (α + β)-field in the phase diagram concerned. Slow cooling of these alloys from the β-field results in a microstructure com- prising grain-boundary allotriomorphs of the α-phase (known as “pri- mary” α) and packets of similarly oriented α-platelets with the β-phase layers between the platelets. A /Ae mperature In the Fe–Fe C diagram, the temperature of an eutectoid 1 1 te 3 reaction corresponding to the PSK line in the diagram. Since the reaction, on cooling, starts at a certain undercooling (see nucleation), the temper- © 2003 by CRC Press LLC ature of its commencement, Ar , is lower than A . The start temperature 1 1 of the same reaction on heating, Ac , is greater than A e to superheat- 1 1 du ing. The difference between Ac d Ar s named thermal, or transfor- 1 an 1 i mation, hysteresis. A /Ae mperature Temperature of magnetic transformation in ferrite 2 2 te (∼770°C). See Curie temperature. A /Ae mperature In Fe–Fe C phase diagram, a temperature of the polymor- 3 3 te 3 phic transformation γ ↔ α corresponding to the GS line in the diagram. Critical points on cooling and heating are known as Ar ) and 3 (Ar3 A3 Ac A ), respectively. For details, see A mperature. 3 (Ac3 3 1 te A /Ae mperature In Fe–Fe C phase diagram, a temperature of the polymor- 4 4 te 3 phic transformation δ-ferrite ↔ austenite. A /Ae mperature In Fe–Fe C phase diagram, a temperature corresponding cm cm te 3 to the equilibrium austenite ↔ cementite; it is shown by the ES line in the diagram. aberration Defect observed in optical and electron microscopes. It reveals itself in a colored (in optical microscopy) or slightly eroded or distorted image. The main types of aberration are: chromatic, spherical, distortion, astig- matism, and coma. abnormal grain growth (AG) Grain growth wherein the mean grain size changes slowly at first, then, after a certain incubation period, increases –5 abruptly, almost linearly, with time. Only a minority of the grains (10 ) grow in the course of abnormal grain growth. These grains can reach the size of several mm, whereas the matrix grains retain its initial size of several µ m until it is consumed. The reason why the small grains cannot grow or grow slowly is retardation of their boundary migration by various drag forces as, e.g., by grain-boundary solute segregation (also known as impurity drag), by small precipitates (see particle drag), or by thermal grooves in thin films and strips (see groove drag). The matrix can also be stabilized by low mobility of the majority of grain boundaries, character- istic of materials with a strong single-component texture. The grains growing in the course of AG differ from the matrix grains by an increased capillary driving force owing to their increased initial size (see normal grain growth). Sometimes, their growth can be supported by a surface- energy driving force or by a driving force owing to decreased dislocation density (see strain-induced grain boundary migration). Time dependence of the volume fraction of abnormally large grains is similar to that of primary recrystallized grains; owing to this, AG is often referred to as secondary recrystallization. In some cases, AG is quite helpful, as, e.g., in electrical steels, where it leads to the Goss texture formation and to a significant improvement in magnetic properties. In other cases, it is det- rimental, as, e.g., in crystalline ceramics (see solid-state sintering). AG is also termed discontinuous or exaggerated grain growth. abnormal pearlite In hypereutectoid steels, a microstructure formed by pearlite colonies separated by extended ferrite fields from the network of pro- eutectoid cementite. © 2003 by CRC Press LLC absorption Phenomenon of taking up atoms or energy from the environment into a body. A reduction in the intensity of certain radiation passing through a substance is described by an absorption coefficient. absorption coefficient Quantity describing a reduction of the integrated inten- sity of some radiation passed through a homogeneous substance. See linear absorption coefficient and mass absorption coefficient. absorption contrast Image contrast associated with different x-ray (electron) absorption in the sample areas having different thicknesses or densities. It is also known as amplitude contrast. absorption edge See x-ray absorption spectrum. absorption factor Quantity characterizing an angular dependence of the inten- sity of diffracted x-ray radiation, the dependence being a result of the x- ray absorption. The absorption factor can increase with the Bragg angle, as e.g., in the Debye-Scherrer method, or remain independent of it, as e.g., in the diffractometric method. Absorption factor is taken into account in x-ray structure analysis. absorption spectrum Wavelength spectrum of an absorbed radiation. acceptor Dopant in semiconductors increasing the concentration of charge car- riers. The energy level of the acceptor valence electrons lies within the band gap close to its bottom. Owing to this, valence electrons from the filled valence band can be activated to the acceptor level, which, in turn, produces empty levels (known as holes) in the valence band, and thus promotes the electron conductivity. For instance, in elemental semicon- ductors (Si, Ge), acceptors can be substitutional solutes with a smaller valence than that of host atoms. accommodation strain See coherency strain. achromatic lens/objective In optical microscopes, a lens corrected for chro- matic aberrations in two colors (usually red and green), as well as for spherical aberrations. acicular Needle-shaped. The name has its origin in the fact that plate-like crys- tallites, as e.g., Widmannstätten ferrite or steel martensite, look like nee- dles on plane sections studied by optical microscopy, PEEM, and SEM. acicular ferrite Ferrite crystallite growing, apparently, as in the course of bai- nitic transformation. It has a lath-like shape and an increased dislocation density. The lathes form packets in which they are parallel to each other, and the boundaries between them inside a packet are low-angle. Several packets can occur within an austenite grain. Acicular ferrite is also termed Widmannstätten ferrite. acicular martensite Crystallite of martensite in steels with a low M temperature s of a lens- or needle-like shape in the cross-section. Martensite plates have a clearly visible longitudinal center line called midrib (i.e., middle ribbon). An increased density of transformation twins and dislocations is observed close to the midrib. The adjacent martensite plates of acicular martensite are non-parallel. The habit planes of acicular martensite are 259 or 3 A 10 15 , and its lattice is oriented with respect to the austenite lattice A according to the Nishiyama and Greninger–Troiano orientation relation- © 2003 by CRC Press LLC ships, respectively. Acicular martensite is also called lenticular or plate martensite. Ac temperature In Fe–Fe C alloys, a critical point observed on heating and 3 denoted by Ac , Ac , or Ac , for A , A , or A , respectively. See super- 1 3 cm 1 3 cm heating. activation analysis Technique for chemical analysis wherein a sample is pre- liminary irradiated, and a secondary radiation of some component is used for determining its amount. activation energy Additional free energy necessary for the commencement of some thermally activated reactions (e.g., diffusion, recrystallization, phase transformations, etc.). If activation energy is denoted by H, the Gibbs free energy is implied (in this case, activation energy can be referred to as activation enthalpy). If not, either the Gibbs or Helmholtz free energy may be meant. Units of activation energy are J/mol or eV/at. activation enthalpy See activation energy. active slip system Slip system over which the dislocation glide motion takes place. adatom Atom from the environment adsorbed at the surface of an adsorbent. adiabatic approximation The assumption that all processes in a system proceed without heat exchange with the environment. adsorbate See adsorption. adsorbent See adsorption. adsorption Spontaneous attachment of atoms (or molecules) of some substance from the environment to the surface of some body, the substance being called adsorbate and the body adsorbent. Adsorption is accompanied by a decrease of surface energy. Adsorption results in the formation of an adsorption layer in which the adsorbate concentration is greater than in the environment. A layer of this kind can also form at some lattice defects, such as grain boundaries and interfaces, the environment and adsorbate being the bulk of the grains and solute atoms, respectively. In this case, adsorption is referred to as equilibrium segregation. See also physical adsorption and chemisorption. after-effect Any alteration evolving after the completion of an external action. age hardening An increase in hardness and strength caused by precipitation treatment resulting in precipitation of dispersed phase(s) from a supersat- urated solid solution. It is frequently referred to as precipitation strength- ening. aging Decomposition of a supersaturated solid solution. The size and number of precipitates depends on the aging temperature and time and on the supersaturation, as well as on the solution substructure (see heteroge- neous nucleation). Their arrangement is affected by the microstructure of the supersaturated solution and the previously mentioned aging condi- tions. For instance, if precipitates nucleate and grow inside the parent grains, Widmannstätten structure can appear. If they nucleate and grow predominately at the subboundaries and grain boundaries of the parent phase, the precipitates can form a network corresponding to the boundary © 2003 by CRC Press LLC network of the parent phase. In addition, narrow precipitation-free zones near the grain boundaries can occur. aging in Ti alloys Phase changes accompanying the decomposition of retained β-phase or metastable β-phase (β ) that occurred on tempering. m These changes are commonly referred to as aging, although both β and β e unsaturated with respect to the equilibrium β-phase at the aging m ar temperature. In the course of aging, phases with a decreased solute con- centration precipitate from the metastable β-phases. As a result, the latter become solute-rich and their composition tends to the equilibrium β- phase. Possible sequences of phase changes during aging in (α + β) alloys can be described as follows: β → β ω → β + α or β → β β → m m + m 1 + 2 β + α. Here, β nd β e metastable BCC phases differing in composition 1 a 2 ar from β nd from one another; they supposedly occur by spinodal decom- m a position of β . The microstructure after aging consists of two microcon- m stituents: β-matrix and relatively uniformly distributed, dispersed α-phase particles. See ω-phase and aging treatment. aging treatment Heat treatment aimed at age hardening; it comprises solution and precipitation treatments. aging treatment of Ti alloys Heat treatment that comprises heating of quenched alloys with metastable α′-, α″-, and ω-phases and retained β- or metastable β-phase. As for α′-martensite, α″-martensite, and ω-phase, the treatment should be named tempering, whereas the term “aging treat- ment” should relate solely to the previously mentioned β-phases. See aging in Ti alloys and tempering of titanium martensite. air-cooling Cooling in still air. aliovalent solute/impurity Solute in ionic crystals whose valence differs from that of a host ion. Aliovalent solutes disturb the electrical neutrality and must be associated with other defects (either lattice defects or electrons) compensating their charge. allotropic change Transformation of one allotropic form into another, the trans- formation evolving as a first-order transition. See also polymorphic trans- formation. allotropic form/modification In a single-component solid, one of several stable phases differing from the others by crystal structure, and transforming one into another spontaneously at the corresponding temperature and pressure. There can be more than two allotropic forms. They are usually denoted by Greek letters in alphabetic order, starting with alpha for the lowest temperature form. See allotropy. allotropy In a single-component solid, the existence of stable phases with dif- ferent crystal structures in different temperature or pressure ranges. Al l o - tropic transformation relates to first-order transitions. See also polymor- phism. alloy Metallic material consisting of a base metal and one or more alloying elements partially or completely dissolving in the base metal. Alloys are frequently denoted by symbols of their components, the symbol of the base metal being usually underlined, as, e.g., Cu–Zn alloy for brasses. © 2003 by CRC Press LLC alloy carbide Intermediate phase in alloy steels consisting of carbon and alloy- ing element(s). It is also termed special carbide. alloying composition Auxiliary alloy used in the alloy production instead of pure alloying elements. It is also known as master alloy. alloying element Component added deliberately with the aim of improving the properties of an alloy. Alloying elements can affect the existence range of equilibrium phases present in an unalloyed material, or lead to the occurrence of new phases, or both. In addition, alloying elements strongly affect the kinetics of phase transformations and thus the microstructure formation in alloyed materials. See also dopant. alloy steel Steel comprising one or several alloying elements, along with carbon. alloy system See system. alpha brass Brass with only one phase constituent, that is, a Cu-based solid solution. ambipolar diffusion Coupled migration of oppositely charged ions and lattice defects under the influence of an electric field, either external or internal. In the latter case, the oppositely charged species migrate together because their separate migration disturbs the electrical neutrality. Ambipolar dif- fusion may be observed in sintering and diffusional creep of ionic crystals. Compare with electromigration in metals. amorphous solid Phase characterized only by a short-range order and by a missing long-range order in atomic structure. Amorphous phase can be obtained by quenching the melt below a glass transition temperature (see glassy phase), by ion bombardment, by heavy plastic deformation (e.g., by mechanical alloying), by rapid film deposition, etc. amplitude contrast See absorption contrast. analytical electron microscope (AEM) TEM used for chemical analysis of small areas (∼10 nm in diameter), e.g., by means of EELA. Andrade creep Transient creep described by the empirical time dependence of the creep strain, ε, in tension tests: 1/3 ε ∝ at (a is a constant and t is time). Andrade creep is observed at higher temperatures than logarithmic creep. anelasticity Deviation from the behavior according to Hooke’s law that reveals itself in two constituents of elastic strain: an instantaneous one, occurring simultaneously with the application of an external force and corresponding to the Hooke law; and a time-dependent constituent, ε(t), changing with time after the force application, t, at a constant temperature as follows: ε(t) = (σ/E)1–exp(–t/τ ) R where E is Young’s modulus, σ is the tensile stress, and τ is the relaxation R time. The relaxation time is constant at a fixed σ and a constant temper- ature and is dependent on the nature of anelasticity. As seen in the equa - © 2003 by CRC Press LLC tion, at various t, there may be different values of the elasticity modulus, its extremities being the Young’s modulus E corresponding to the Hooke’s law at t τ nd what is known as the relaxation modulus E E at t R a R τ . See also internal friction. R anisotropic Having different physical and mechanical properties in various directions. Anisotropy of single crystals is a result of crystalline aniso- tropy, whereas that of a polycrystal is dependent on crystallographic texture (and so on the crystalline anisotropy) as well as on the microstruc- tural anisotropy as, e.g., banded structure or carbide stringers in steels or an elongated grain structure in heat-resistant alloys (see Nabarro− Herring or Coble creep). Anisotropy can be observed not only in crystal- line solids but also in some liquids (see liquid crystals). annealing/anneal Heat treatment resulting in the occurrence of equilibrium phases (see, e.g., graphitization anneal, solution annealing), in removing of deformation or amorphization effects or in attaining a required grain size or texture (see, e.g., recrystallization annealing), or in relieving chem- ical inhomogeneity and macroscopic residual stresses (see homogenizing, stress-relief annealing). In metallic alloys, annealing is a preliminary treatment preparing the microstructure or phase composition to a final treatment (see, e.g., austenitization and solution treatment). Annealing after amorphization of single-crystalline semiconductors can restore sin- gle-crystalline structure. annealing texture Preferred orientation evolved in the course of primary recrys- tallization or grain growth. Recrystallization texture occurs because recrystallization nuclei are of nonrandom orientations and grow into the deformed matrix at different rates. It can be similar to deformation texture or quite different from it. Texture changes during grain growth are con- nected with different driving forces for growth of variously oriented grains and different mobility of their boundaries (see compromise texture). Grain growth commonly (but not always) results in weakening of the primary recrystallization texture. Annealing texture is usually characterized by an increased scatter and a decreased intensity in comparison to the initial deformation texture, except for a cube texture in some cold-rolled FCC alloys and the Goss texture in ferritic steels. annealing twin Twin occurring during primary recrystallization or grain growth. Annealing twins are usually observed in materials with low stacking-fault energy, especially on annealing after heavy plastic deformation. An annealing twin, depending on its position inside a grain, can have one or two coherent twin boundaries joining up with grain boundaries or inco- herent twin boundaries. The twin with two coherent boundaries looks like a straight band. anomalous x-ray transmission Abnormally low x-ray absorption observed in thick perfect crystals adjusted at the exact Bragg angle. It is also known as the Borrmann effect. antiferromagnetic Material characterized (below Néel point) by a negative energy of exchange interaction and equal but oppositely directed magnetic © 2003 by CRC Press LLC moments of different magnetic sublattices. The intrinsic magnetization in antiferromagnetics is lacking due to equality of the magnetic moments of the sublattices. antiferromagnetic Curie point See Néel point. antiphase boundary Boundary of antiphase domains within a grain of an ordered solid solution. Antiphase boundary is characterized by an increased energy because the arrangement of atoms of different compo- nents at the boundary is distorted in comparison to their arrangement inside domains (see Figure A.1). Antiphase boundary FIGURE A.1 Antiphase domains and an antiphase boundary inside a grain of an ordered solid solution. Open and solid circles represent atoms of different components. antiphase domain Grain part having a crystal structure of an ordered solid solution. Identical sublattices in the adjacent antiphase domains inside one grain are shifted relative to each other (see Figure A.1), the shift being unequal to the translation vector of the corresponding superlattice. If the superlattice is of a noncubic system, identical sublattices of the adjacent antiphase domains inside a grain can have different spatial orientations. antisite defect Lattice defect in ionic crystals produced by an ion of some sign occupying a site in the sublattice formed by ions of the opposite sign. Antisite defect is analogous to an antistructural atom in metallic crystals. See structural disorder. antistructural atom See structural disorder. aperture diaphragm In optical microscopes, a diaphragm that restricts the inci- dent beam and affects the illumination intensity, image contrast, resolving power, and depth of focus. apochromatic lens/objective In optical microscopes, a lens corrected for chro- matic aberration in three color regions (violet, green, and red) and for spherical aberration in two color regions (violet and green). Apochro- matic objective has a better color correction than achromatic objective. arrest point See critical point and thermal analysis. © 2003 by CRC Press LLC Arrhenius equation Description of the temperature dependence of some kinetic parameter, A, of any thermally activated process: A = A exp (–Q/cT) 0 Here, A s a pre-exponential factor, Q is the activation energy, T is the 0 i absolute temperature, and c is either the gas constant (if the activation of one molecule is considered) or the Boltzmann constant (if the activation of one atom, or molecule, is concerned). Ar temperature In Fe–Fe C alloys, a critical point observed on cooling and 3 denoted by Ar , Ar Ar , for A , A , or A , respectively. See under- 1 3 or cm 1 3 cm cooling. artifact Feature caused by preparation or manipulation of a sample or, some- times, by investigation conditions. artificial aging Aging treatment at temperatures higher than ambient. asterism Radial elongation of reflection spots in Laue diffraction patterns owing to residual stresses or substructures in a single crystal. astigmatism Optical aberration revealing itself in a distortion of the cylindrical symmetry of an image. asymmetric boundary Tilt grain boundary whose plane divides the angle between identical planes in the lattices of the adjacent grains into two unequal parts. athermal transformation Phase transition developing without any thermal acti- vation (thus, the transformation is diffusionless). The volume fraction of the transformation products depends mostly on temperature (or, more precisely, on supercooling). At a fixed temperature in the transformation range, after some period of a rapid increase, the volume fraction changes little, if at all. See shear-type transformation and martensitic transformation. atomic force microscope (AFM) Device for studying the surface atomic struc- ture of solids. AFM is similar in design to STM, but measures the force between the sharp microscope tip and surface atoms. 12 atomic mass Atomic mass, in units, equal to 1/12 mass of C atom. atomic packing factor Volume fraction of a unit cell occupied by atoms pre- sented by rigid spheres of equal radii. The largest atomic packing factor is 0.74 in FCC and HCP lattices; it is a little smaller (0.68) in BCC lattice, and very low (0.34) in the diamond lattice. Atomic packing factor is also known as packing factor. at% Atomic percentage; it is used in cases in which the components are chemical elements. A weight percentage of a component A, W , in a binary system A A–B can be calculated from its atomic percentage, A , by the formula: A W = 100/1 + (100 – A )M /(A M ) A A B A A where M and M are the atomic masses of A and B, respectively. In cases A B in which the components are compounds, mol% is used instead of at%. © 2003 by CRC Press LLC atomic radius Conventional value not connected with an atomic size, but relat- ing to a crystal lattice, i.e., the interatomic spacing is assumed equal to the sum of atomic radii. This is the reason why atomic radius depends on the bond type (i.e., metallic, ionic, or covalent), as well as on the coor- dination number in the crystal lattice considered. See metallic, ionic, and covalent radii. atomic scattering factor Coefficient characterizing the intensity of the elasti- cally scattered radiation. It increases with the atomic number and decreases with (sin θ)/λ, where θ is the glancing angle and λ is the 5 ti wavelength. The atomic scattering factor for electrons is ∼10 mes greater than for x-rays, which enables the application of electron diffrac- tion for studying relatively thin objects, commonly of thickness smaller than 01 . µ m. Atomic scattering factor is taken into account in x-ray struc- ture analysis. atomic size See atomic radius. atomic structure In materials science, a description of an atomic arrangement in phases, e.g., amorphous, or in lattice defects. atomic volume Vol ume of unit cell per atom. atomizing Procedure for obtaining small solid droplets from melt, the droplets being ultra-fine grained because the cooling rate during their solidification 3 K is ∼10 /s. They are used for producing massive articles by consolidating and sintering. atom probe field ion microscopy (APFIM) Technique for mass-spectrometric identification of single atoms removed from the sample tip in FIM by means of pulse field evaporation. Besides the studies of the surface atomic structure, APFIM is used for analyzing the nucleation and growth of precipitates, ordering phenomena (see order−disorder transformation and short-range ordering), and segregation at crystal defects. Auger electron Secondary electron emitted by an atom whose electron vacancy at an inner shell has been created by a high-energy primary electron. An electron from a higher energy shell subsequently fills the electron vacancy, whereas another electron, referred to as the Auger electron, is emitted from the other shell. The energy spectrum of Auger electrons is a char- acteristic of the atom and can be used for chemical analysis (see Auger- electron spectroscopy). Auger-electron spectroscopy (AES) Technique for chemical analysis utilizing the energy spectrum of Auger-electrons. Since Auger-electrons are of low- energy, AES can analyze very thin surface layers only (∼1 nm in depth), with the lateral resolution 20 to 50 nm. AES can also yield a depth profile of chemical composition using ion etching for the layer-by-layer removal of the material studied. ausforming Thermo-mechanical treatment comprising two main stages: warm deformation of a steel article at temperatures of bainitic range for the time period smaller than the incubation period of bainitic transformation; and quenching of the article, which results in the martensite or bainite formation from the deformed austenite. An increased dislocation density © 2003 by CRC Press LLC in the austenite (after the first stage) is inherited by the martensite or bainitic ferrite (after the second stage), which increases the article’s hard- ness. Ausforming is also referred to as low-temperature thermo-mechan- ical treatment. austempering Heat treatment comprising austenitization of a steel article, cool- ing it to a bainitic range at a rate higher than the critical cooling rate and holding at a fixed temperature until the completion of bainitic transfor- mation. austenite Solid solution of alloying elements and/or carbon in γ-Fe. It is named after British metallurgist W. C. Roberts-Austen. austenite finish temperature (A ) Temperature at which the transformation of f martensite into austenite completes upon heating. The same designation is also applied to nonferrous alloys in which martensite transforms into some parent phase. austenite stabilization Decrease, in comparison to a continuous cooling, in the amount of martensite occurring from austenite when cooling is interrupted at a temperature between M d M . This can be explained by the relax- s an f ation of stresses induced in the austenite by martensite crystals occurring before the interruption. The relaxation, in turn, leads to the dislocation rearrangement and their interaction with martensite/austenite interfaces, which makes the interfaces immobile. austenite-stabilizer Alloying element expanding the γ-phase field in the corre- sponding phase diagram, which manifests itself in a decrease of the A 3 temperature and an increase of the A mperature in binary alloys Fe–M 4 te as well as in a decrease of A emperature in ternary alloys Fe–C–M (M 1 t is an alloying element). The solubility of austenite-stabilizers in ferrite is much lower than in austenite. Under the influence of austenite-stabilizers, austenite can become thermodynamically stable down to room tempera- ture. See, e.g., austenitic steels. austenite start temperature (A ) Temperature at which the transformation of s martensite into austenite starts upon heating. The same designation is also applied to nonferrous alloys in which martensite transforms into some parent phase. austenitic-ferritic steel Alloy steel whose structure after normalizing consists of austenite and ferrite. austenitic-martensitic steel Alloy steel whose structure after normalizing con- sists of austenite and martensite. austenitic range Temperature range wherein a purely austenitic structure can be obtained in steels upon heating. austenitic steel Alloy steel whose structure after normalizing consists predom- inately of austenite. This is a result of an increase in the thermodynamic stability of austenite by alloying elements. If austenite is thermodynami- cally unstable, it can transform into martensite (see, e.g., maraging steel and transformation-induced plasticity). austenitization Holding stage of a heat treatment resulting in the formation of a completely austenitic structure. © 2003 by CRC Press LLC autoelectronic emission See field emission. autoradiography Technique for studying chemical microinhomogeneity by re g - istering the radiation of radioactive elements (tracers) contained in the specimen on a high-resolution screen (film), displaying the disposition of the tracers in the surface layer. Avogadro number Amount of atoms, ions, or molecules in a mole of any sub- 23 −1 stance; N ≅ 6.022·10 mol . A Avrami equation Description of transformation kinetics, assuming that new phase nuclei occur at predetermined sites only. As a result of this assump- tion, the nucleation rate decreases with time. In this case, the kinetic equation is: n V/V 1– exp(–kt ) 0 = where V and V e the transformed and the initial volume fractions, 0 ar respectively, k is a kinetic constant, t is the transformation time, and 3 ð n ð 4 (in three-dimensional cases) or 2 ð n ð 3 (in two-dimensional cases). If the nucleation rate is constant, the Avrami equation is identical to the Johnson–Mehl–Kolmogorov equation. In cases in which all the nucleation sites are exhausted at an early stage: 3 V/V 1 – exp–(4πN /3)G 3t 0 = 0 where N the initial number of the nucleation sites and G is the linear 0 is growth rate. axial angle In a unit cell, an angle between a pair of its axes. See lattice param- eters and unit cell (Figure U.2). axial ratio In hexagonal crystal systems, the ratio of lattice constants c and a. © 2003 by CRC Press LLC 0970_frame_B Page 13 Friday, August 1, 2003 7:01 PM B β-Al O Impure alumina whose main impurity is Na O. 2 3 2 β eutectoid Ti system Name of a Ti−X alloy system in which the β-stabilizer X has a limited solubility in β-Ti, and a eutectoid reaction β ↔ α + γ takes place (γ is an intermediate phase or a terminal solid solution). β-Fe Obsolete designation of the paramagnetic α-Fe existing at temperatures between 768 and 910°C at atmospheric pressure (i.e., between A nd A ). 2 a 3 Correspondingly, a solid solution in β-Fe was named β-ferrite. β isomorphous Ti system Name of a Ti–X alloy system in which the alloying element X is the β-stabilizer and there is no eutectoid reaction in the corresponding phase diagram. β se in Ti alloys See metastable β-phase. m pha β-phase in Ti alloys Solid solution of alloying elements in β-Ti. β-stabilizer Alloying element expanding the β-phase field in phase diagrams of Ti alloys and thereby lowering β/(α + β) transus. β-Ti High-temperature allotropic form of titanium having BCC crystal structure and existing above 882°C up to the melting point at atmospheric pressure. β Ti alloy Alloy with β-stabilizers wherein β-phase is the only phase constituent after air-cooling from temperatures above the β/(α + β) transus. Alloys with a small (∼5 vol%) amount of α-phase are related to the same group and termed near-β alloys. If the β → α transition does not evolve on air- cooling, these alloys are named metastable β alloys. background In x-ray structure analysis and texture analysis, an intensity of scat- tered x-ray radiation between diffraction lines caused mainly by: x-ray flu- orescent radiation emitted by the specimen, diffraction of the white radiation on the polycrystalline specimen, Compton scattering, and diffuse scattering. back-reflection Laue method Technique wherein an x-ray source and a flat film (screen) registering an x-ray diffraction pattern are placed on the same side of the sample. backscattered electron Electron elastically scattered in the direction that is opposite to the direction of the primary beam. The yield of backscattered electrons increases with the atomic number of the substance studied. Backscattered electrons are used in SEM for gaining data on the topog- raphy, microstructure, and chemistry of the specimen surface, as well as for crystallographic studies (see electron channeling). © 2003 by CRC Press LLC 0970_frame_B Page 14 Friday, August 1, 2003 7:01 PM bainite Microconstituent in steels occurring on transformation of undercooled austenite in a bainitic range. Bainite consists of ferrite and cementite (or ε-carbide). It is named after American scientist E. C. Bain. See bainitic transformation, upper bainite, and lower bainite. bainite start temperature (B ) In alloy steels, temperature of the start of bai- s nitic transformation on cooling from an austenitic range. bainitic range Temperature range wherein bainite can be obtained upon cooling from an austenitic range. The upper limit of bainitic range is the B s temperature in alloy steels and the lower limit of pearlitic range in plain carbon steels. The lower limit of the bainitic range is the M emperature s t (see Figure B.1). Ae 1 Pearlite B s Bainite M s Log time FIGURE B.1 TTT diagram for eutectoid alloy steel (scheme). Temperature range Ae –B is 1 s referred to as pearlitic, B –M as bainitic, and M –M as martensitic (temperature M is not shown). s s s f f bainitic transformation In steels, phase transformation of undercooled austen- ite at temperatures of bainitic range. In this range, the atoms of both iron and substitutional alloying elements cannot migrate by diffusion, whereas the carbon atoms can. Bainitic transformation (BT) evolves as follows. Carbon diffusion inside austenite leads to its chemical inhomogeneity, i.e., in some areas, the carbon content becomes reduced and in the others, increased. Since the M ncreases with a reduction of the carbon concentra- s i tion in austenite, martensitic transformation evolves in the low-carbon areas. An occurring metastable low-carbon martensite decomposes into ferrite because of the elimination of its carbon content through the carbide pre- cipitation. If this proceeds into the upper part of bainitic range, diffusion paths of carbon atoms can be long enough, and the carbides occur only at the boundaries of the ferrite crystallites (see upper bainite). If the temper- ature is low, the diffusion paths are short, and the carbides form inside the ferrite grains (see lower bainite). In the high-carbon areas, the austenite transforms into a ferrite-cementite mixture in the upper part of bainitic range. In the lower part of the range, carbide precipitation in these areas leads to a further local reduction of the carbon content in the austenite and © 2003 by CRC Press LLC Temperature0970_frame_B Page 15 Friday, August 1, 2003 7:01 PM to the transformation chain described. Sometimes, a certain part of the austenite remains untransformed (see retained austenite). Thus, BT includes a diffusion-controlled carbon partitioning inside austenite, a nondiffusional phase transition of austenite into martensite, and a diffusion-controlled precipitation of carbides from the martensite and austenite. The BT kinetics are governed by the slowest process, i.e., by the carbon diffusion, and are the same as in the other diffusional transformations. At the same time, BT is similar to martensitic transformation in the sense that it ceases continu- ously at a constant temperature below B , and a certain amount of austenite s remains untransformed. Alloying elements affect BT by reducing the carbon diffusivity and changing the elastic modulus of austenite, which retards the transformation and lowers the B temperature. s bainitic steel Alloy steel whose microstructure after normalizing consists pre- dominately of bainite. bamboo structure Microstructure of thin wires formed by a row of grains whose diameter is equal to the wire diameter. banded structure Microstructure of an article fabricated from hypoeutectoid car- bon steel, wherein pearlite and proeutectoid ferrite form alternating bands parallel to the direction of the preceding hot deformation. Banded structure has its origin in the coring in a steel ingot. For instance, in silicon steels, proeutectoid ferrite occurs in the areas of the hot-deformed and dynamically recrystallized austenite where there is an increased silicon concentration, i.e., on the periphery of the prior dendrite arms. Banded structure leads to high anisotropy of the mechanical properties of steel articles. band gap See band structure. band structure Spectrum of available energy states for electrons in crystals. The spectrum is composed of almost-continuous bands of permitted energy states separated by the gaps of forbidden energy (these are called band gaps). The bond type and crystal structure determine the spectrum. The electrons of the upper-atom shell fill a valence band. The band of higher permitted energies, next to the valence band, is known as a conduction band; it can be completely or partially empty. Electron conductivity is only possible if valence electrons can be activated to the energy level corre- sponding to the conduction band. In metals, the valence and conduction bands lie close to each other or superimpose. This explains the high con- ductivity of metals in which there are always available energy states in the conduction band. In intrinsic semiconductors and insulators, the valence band is filled completely and the conduction band is empty, but the latter is separated from the former by a band gap. Thus, the electron conductivity in these materials is only possible if valence electrons of the highest energy can acquire the activation energy necessary to overcome the gap. In intrin- sic semiconductors, this takes place under the influence of thermal, elec- trical, magnetic, or light excitation, because the band gap in these materials is relatively small. In intrinsic insulators, the band gap is large, so there is no electron conductivity in these materials. Certain impurities, known as donors and acceptors, introduce permitted energy levels into the band gap © 2003 by CRC Press LLC 0970_frame_B Page 16 Friday, August 1, 2003 7:01 PM close to its borders, which reduces the activation energy necessary for electrons to reach the conduction band and significantly increases the number of charge carriers. Under the influence of some impurities, both covalent and ionic crystals can become semiconductors. basal plane In crystallography, 0001 plane in hexagonal structure. See Miller–Bravais indices. basal slip Slip over a basal plane along 〈1120 〉 direction; it is commonly observed in HCP alloys with an axial ratio c/a ≥ 1.633. base In materials science, a component used as a basis for alloying. base-centered lattice Orthorhombic or monoclinic Bravais lattices in which, along with the lattice points at the vertices of the corresponding unit cell, there are additional points at the centers of two opposite faces. It is also referred to as based lattice. based lattice See base-centered lattice. Bauschinger effect In the specimen strained initially in one direction and then in the reverse direction, a decrease of the yield stress observed on the second loading. Microscopic residual stresses induced upon the first load- ing cause this effect due to inhomogeneity of plastic flow. bend contour See extinction contour. bicrystal Solid body consisting of only two crystallites of the same or different phases. The latter case is usual in semiconductor heterojunctions wherein bicrystal is formed by a heteroepitaxial single-crystalline film on a single- crystalline substrate or by two single-crystalline films. If bicrystal consists of crystallites of the same phase, they are disoriented. bimetallic Consisting of two brazed or welded metallic strips of different com- position and properties. For instance, thermobimetals are produced from strips with different coefficients of thermal expansion. bimodal Description of a curve with two distinct maxima. binodal Dome-shaped surface or a curve in a ternary phase diagram and a binary diagram, respectively, bordering a miscibility gap (see Figures B.2 and B.3). binary Consisting of two components. black-heart malleable cast iron Malleable iron with a pearlitic matrix. Bloch wall Domain wall characteristic of massive ferromagnetics or ferrimag- netics. Inside the wall, the magnetization vector rotates around an axis perpendicular to the wall plane, going from one domain to the other. In thin films, such a wall structure is thermodynamically unfavorable (see 1/2 Néel wall). The thickness of a 180° Bloch wall is proportional to (A/K) , where A is the energy of exchange interaction and K is the constant of magnetic crystalline anisotropy; e.g., Bloch-wall thickness equals ∼50 nm in α-Fe and ∼3 nm in an intermediate phase Fe Nd B. 14 2 blocky martensite See lath martensite. body-centered cubic (BCC) structure Crystal structure whose coordination number equals 8; atomic packing factor is 0.68; the close-packed planes and the close-packed directions are 110 and 〈111〉, respectively (see Figure B.4); the radius of a tetrahedral void in the structure is 0.290R; and that of an octahedral void is 0.153R, where R is the atomic radius. © 2003 by CRC Press LLC 0970_frame_B Page 17 Friday, August 1, 2003 7:01 PM L α α + α 1 2 A B FIGURE B.2 Binary diagram with a miscibility gap in the solid state: α and α denote α 1 2 solid solutions of different compositions. Solid line shows a binodal. L yx z α A %B FIGURE B.3 Part of a binary phase diagram with a monotectic reaction. The dome-shaped curve is a binodal bordering (L + L ) field in the case of monotectic reaction or (β + β ) field. 1 2 1 2 FIGURE B.4 Unit cell of BCC crystal structure. Open circle shows an atom inside the cell body. © 2003 by CRC Press LLC Temperature Temperature0970_frame_B Page 18 Friday, August 1, 2003 7:01 PM body-centered lattice Cubic, tetragonal, or orthorhombic Bravais lattices in which, along with the lattice points at the vertices of the corresponding unit cell, there is one additional point at the cell’s center. Boltzmann constant Quantity k = R/N , where R is the gas constant and N A A is –23 –5 the Avogadro number: k = 1.381⋅10 J/K or 8.62⋅10 eV/K. bond energy Energy necessary to break interatomic bonds and separate the atoms. Bond energy increases in the order: van der Waals → metallic → ionic or covalent bond. Bordoni peak/relaxation Internal friction peak observed in cold-worked FCC metals due to the generation and lateral motion of double kinks. The measurements of Bordoni peak are used for determining the kink energy. Borrmann effect See anomalous x-ray transmission. Bragg angle Glancing angle appearing in the Bragg law. Bragg diffraction condition See Bragg’s law. Bragg reflection X-ray reflection corresponding to Bragg’s law. Bragg’s law Condition for x-ray (or electron) diffraction on parallel lattice planes spaced at a distance d , where h, k, and l are the Miller indices hkl of the planes: nλ = 2 d n θ hkl si where λ is the wavelength, θ is the angle between the primary beam and the corresponding planes (the glancing angle or Bragg angle), and n is an order of reflection, i.e., an integral value consistent with the condition nλ/2d 1. It is assumed that the primary beam is strictly monochromatic hkl and parallel, and that the crystal studied has a perfect lattice. The angle θ does not correspond to the angle of incidence, ϑ, considered in optics: θ = π/2 – ϑ This explains why θ is termed glancing angle. brass Cu alloy where zinc is the main alloying element. Bravais lattice One of 14 possible crystal lattices: cubic (primitive, body-cen- tered, and face-centered); tetragonal (primitive and body-centered); orthorhombic (primitive, body-centered, base-centered, and face-cen- tered); primitive rhombohedral or trigonal; primitive hexagonal; mono- clinic (primitive and base-centered); and primitive triclinic. The names of crystal systems are italicized. bremsstrahlung See white radiation. bright-field illumination In optical microscopy, such illumination that flat hor- izontal features of an opaque sample appear bright, whereas all the inclined features appear dark; e.g., in single-phase materials, grains are bright and grain boundaries dark. This is due to the fact that the horizontal features reflect the incident light into an objective, whereas the inclined features do not. © 2003 by CRC Press LLC 0970_frame_B Page 19 Friday, August 1, 2003 7:01 PM bright-field image TEM image produced by a directly transmitted electron beam. Bright features in the image correspond to areas with an undistorted lattice, provided the image results from diffraction contrast. bronze Cu-based alloy in which zinc is a minor alloying element. Bronzes are denoted by the name of the main alloying element as, e.g., aluminum bronze, silicon bronze, lead bronze, etc. Bs/Def orientation One of the main texture components, 011〈211〉, observed in cold-rolled FCC metallic materials of low stacking-fault energy as well as in the cold-rolled copper. Bs/Rex orientation Recrystallization texture component, 236〈385〉, observed in cold-rolled FCC metallic materials of low stacking-fault energy. bulk diffusion Mass transport through the grain interiors in a polycrystalline material. It is also termed lattice diffusion or volume diffusion. bulk modulus Elastic modulus at hydrostatic pressure. In non-textured polycrys- tals, it is isotropic and is usually denoted by K. Its magnitude relates to Young’s modulus, E, as follows: K = E/3(1 – 2ν) where ν is Poisson’s ratio. See Hooke’s law. Burger orientation relationship Orientation relationship between an HCP phase, α, and a BCC phase, β: 0001 110 , 〈1120 〉 〈111〉 . α β α β Burgers circuit Closed circuit in a perfect crystal lattice; it helps determine the type of a linear defect in an imperfect crystal with the same lattice. If a circuit, identical to that in the perfect crystal, is drawn around a linear defect and turns to be opened, the defect is a dislocation; if it is closed, it is a disclination. The circuit is drawn counterclockwise around the defect. Thus, the defect sense should be chosen first. Burgers vector Vector, b, invariant for a given dislocation line and characteriz- ing the magnitude of lattice distortions associated with it (see dislocation energy and dislocation stress field). The sense of the Burgers vector is defined as follows: the end of the vector should be taken at the end of the Burgers circuit, and its head at the start point of the circuit. Burgers vector of a perfect dislocation is the translation vector in the crystal structure concerned. For example, the Burgers vector of perfect dislocations in BCC structure is 1/2 〈111〉, i.e., it lies along 〈111〉 direction and its length equals half the body diagonal, i.e., b = 1/2 a 3 (a is the lattice constant). Burgers vector can be determined experimentally using TEM. © 2003 by CRC Press LLC