Materials Science Glossary - A

Ab-initio. [lat. “from the beginning”] parameter free methods based on quantum mechanics and electrodynamics, solely universal fundamental physical constants are needed. The term ab initio may be used in different contexts. From a natural science and technical point of view these are the following: A calculation is said to be “ab initio” or "from first principles" if it relies on basic and established laws of nature without additional assumptions or special models in contrast to the practices in according measurements. The difference between an ab initio calculation and experimental results may show mistakes in the experimental set up, it may deliver a more accurate determination of the influencing parameters or display a so far unknown effect. The use of measured values is about so called semi-empirical calculations.  In chemistry there are two fields in which to use the term. The ab initio synthesis means the production of a chemical compound based on elementary chemicals. Or the concept may refer to ab initio quantum chemistry methods. In combination with quantum mechanics this means the solvation of the Schrödinger equation solely with nature constants. For example, an ab initio calculation of the properties of liquid water might start with the properties of the constituent hydrogen and oxygen atoms and the laws of electrodynamics. From these basics, the properties of isolated individual water molecules would be derived, followed by computations of the interactions of larger and larger groups of water molecules, until the bulk properties of water had been determined.

abnormal grain growth. Grain growth wherein the mean grain size increases slowly at first, then, after a certain incubation period, some grains increase abruptly in size, almost linearly, with time. Only a minority of the grains grow in the course of abnormal grain growth. These grains can reach the size of several mm, whereas the matrix grains retain its nearly initial size of several µm until being 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 (referred to as groove drag). The matrix can also be stabilized by low mobility of the majority of grain boundaries, characteristic of materials with a strong single-component texture. The grains growing in the course of abnormal grain growth differ from the matrix grains by an increased outbound directed capillary driving force owing to their increased initial size. Sometimes, their growth can be supported by a surface-energy driving force or by a driving force owing to decreased dislocation density (as in 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 feature, abnormal grain growth is often referred to as secondary recrystallization. In some cases, abnormal grain growth is quite helpful, as, e.g., in FeSi electrical steels, where it leads to the Goss texture formation and to a significant improvement in magnetic properties. In other cases, it is detrimental, as, e.g., in crystalline ceramics (see solid-state sintering). Abnormal grain growth is also termed discontinuous or exaggerated grain growth.
 

Abrasion / abrasive. A hard and wear-resistant material (commonly a ceramic) that is used to wear, grind, or cut away other material. Abrasion refers to the displacement and/or detachment of metallic particles from a surface as a consequence of being exposed to flowing solids, fluids or gases.

absorption. The optical phenomenon whereby the energy of a photon of light is assimilated within a substance, normally by electronic polarization or by an electron excitation event.

adsorbate. A component which belongs to an adsorption layer. Term is often used when the species which are in this adsorption layer come from the vapor phase.

adsorption. Positive or negative excess (atoms or moles) of a component, per unit area of interface, in a system with an interface in comparison with a system consisting just of the abutting bulk phases

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.

acceptor state (level). For a semiconductor or insulator, an energy level lying within yet near the bottom of the energy band gap that may accept electrons from the valence band, leaving behind holes. The level is normally introduced by an impurity atom.

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.

activation energy (Q). The energy required to initiate a reaction, such as diffusion.

activation polarization. The condition wherein the rate of an electrochemical reaction is controlled by the one slowest step in a sequence of steps that occur in series.

acicular. Needle-shaped. The name has its origin in the fact that plate-like crystallites, such as Widmannstätten ferrite or steel martensite, typcially appear like needles on plane sections studied by optical microscopy 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 Ms temperature of a lens-or needle-like shape when imaged in 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 10 15}, and its lattice is typically oriented with respect to the austenite lattice according to the Nishiyama-Wassermann or Greninger–Troiano orientation relationships, respectively. Acicular martensite is also called lenticular or plate martensite.

addition (or chain reaction) polymerization. The process by which monomer units are attached one at a time, in chainlike fashion, to form a linear polymer macromolecule.

adhesive. A substance that bonds together the surfaces of two other materials (termed adherends).

aging / age Hardening. Hardening by aging, usually after rapid cooling or cold working as a decomposition process originating from a supersaturated solid solution. The process of aging refers to a change in properties of metals and alloys which occurs slowly at room or low temperatures and proceeds more rapidly at higher temperatures. The change in properties is often, but not always, due to a phase change (precipitation), but never involves a global change in chemical composition of the metal or alloy. The size and number of the precipitates depends on the aging temperature and time and on the supersaturation, as well as on the solution substructure (depending on heterogeneous or homogeneous nucleation, or respectively, the spinodal process). Their arrangement is affected by the microstructure of the supersaturated solution and the aging conditions. For instance, if precipitates nucleate and grow inside the parent grains, Widmannstätten structure can appear. If they nucleate and grow predominately at the subgrain boundaries and grain boundaries of the parent phase, the precipitates can form a network corresponding to the boundary network of the parent phase. In addition, narrow precipitation-free zones near the grain boundaries can occur. In some alloys such as maraging steels or many Al alloys very fine, i.e.  nanosized precipitates are formed whose eraly stage is often governed by spinodal processes.

AHSS: Advanced High Strength Steels. Group of steels that is characterized by a mulitphase microstructure and an excellent strength-ducitility ratio. There is a general trend in sheet steel development towards an improved balance of formability and strength. AHSS show outstanding yield strength without the compromise of ductility loss. The physical background of this development is mainly that their microstructure is designed to contain constituents with strong distinction in mechanical properties. The final material behavior is affected by the coexistence of the different microstructure components, their different mechanical behavior and their mutual interactions. Multiphase high strength steels with a ferrite matrix are addressed as AHSS 1. Generation. The term AHSS 2. Generation or 2.G is used for single phase austenitic high Mn steels which microstructure characteristically develops throughout plastic forming either by transformation induced plasticity due to strain induced martensite formation (see TRIP), by continuous microstructure refinement due to twinning induced plasticity (→TWIP) or by pronounced microband formation  (see MBIP). Recently, AHSS 3.G is used as a term for medium Mn steels with a multiphase microstructure containing a high austenite fraction. It is possible to vary the mechanical properties by adjusting type, morphology and orientation and above all volume fraction, size and distribution of the different phases. These transformations improve the formability by significant strain hardening and prolonging the uniform straining behavior. This effect can be seen in the ECO-Index as the Product of UTS x TEL which delivers a rough indicator for the overall performance.

All electron method. Quantum-mechanical methods which explicitly include all the electrons of the participating atoms in the calculation of the electronic wave function (or density) in contrast to those which only include the outer (valence) electrons.  The definition sounds trivial and relates to the most desirable case in the quantum theory of matter. In order to effectively deal with (very) large systems, however, a large number of theoretical methods nowadays sacrifices the detailed treatment of the core electrons in favor of speed. Indeed, the core electrons are not heavily engaged in the chemical bonding or chemical reactions, as indicated by the sheer existence of the periodic table. Hence, it is often sufficient and accurate enough to group the atomic nucleus and the atom’s inner electrons into one effective potential. In fact, there exist different ways to deal with these core potentials (> basis set), for example pseudo or PAW potentials. In some cases, e.g., for the calculation of NMR chemical shifts, it is advantageous to explicitly consider all the electrons, no matter what. A popular example of such an all-electron method is LAPW and the corresponding WIEN2k code [1]. Another route is given by LMTO theory and the TB-LMTO-ASA (>LMTO) code [2], likewise an all-electron approach. Here, additional speed is gained by using rapidly decaying orbitals which enforces a local description of the electronic structure. This makes the program a very fast but also reliable tool.

allotropy. The possibility of the existence of two or more different crystal structures for a substance in different temperature or pressure ranges (generally an elemental solid). Allotropic transformation relates to first-order transitions.

alloy. A metallic material consisting of two or more elements where a predominatly metallic bonding character prevails. These alloying elements are usually added to improve mechanical and corrosion-resistance properties. The elements may form a random solution of one metal in another or separate components or phases of different composition. Alloys may be interstitial (in which smaller alloy atoms occupy interstices) or substitutional, in which one alloy element replaces an atom of a similar sized element in a crystal structure.

alloyed steel. A ferrous (or iron-based) alloy that contains appreciable concentrations of alloying elements (other than C and residual amounts of Mn, Si, S, and P).

alternating copolymer. A copolymer in which two different repeat units alternate positions along the molecular chain. 

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 diffusion may be observed in sintering and diffusional creep of ionic crystals. Relates also to the field of electromigration in metals.

amorphous. Substance without crystalline structure.

anelastic deformation. Time-dependent elastic (nonpermanent) deformation.

anion. A negatively charged, nonmetallic ion.

anisotropic. Having different physical and mechanical properties in various directions. Anisotropy of single crystals is a result of crystalline anisotropy, whereas that of a polycrystal is dependent on crystallographic texture (and so on the crystalline anisotropy) as well as on the microstructural anisotropy as, e.g., the banded structure and alignment or carbide stringers in steels or an elongated grain structure in heat-resistant alloys.

annealing. A generic term used to denote a heat treatment wherein the microstructure and, consequently, the properties of a material are altered.“ Annealing” frequently refers

to a heat treatment whereby a previously cold-worked metal is softened by allowing it to recrystallize.

annealing point (glass). The temperature at which residual stresses in a glass are eliminated within about 15 min; this corresponds to a glass viscosity of about 10^12 (10^12 P).

annealing twin. Twin occurring during any type of heat treatment, typically during  solidifcation from the retained heat (self heating) or during primary recrystallization or grain growth. Annealing twins are usually observed in materials with low stacking-fault energy such as in TWIP steels, austenitic stainless steels or brass, especially on annealing after preceding 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 incoherent twin boundaries. Annealing twins with two coherent boundaries looks like a straight band.

anode. The electrode in an electrochemical cell or galvanic couple that experiences oxidation, or gives up electrons.

antiferromagnetism. A phenomenon observed in some materials (e.g.,MnO):complete magnetic moment cancellation occurs as a result of antiparallel coupling of adjacent atoms or ions. The macroscopic solid possesses no net magnetic moment.

antiphase boundary. Boundary between adjacent antiphase domains within a grain of an ordered crystalline material. The antiphase boundary is characterized by an increased energy because the arrangement of atoms of different components at the boundary is distorted and energetically unfavorable in comparison to their arrangement inside the fully ordered domains.

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.

Arrhenius equation. Description of the expoential temperature dependence of some kinetic parameter, A, of any thermally activated process according to A =A_0 exp (–Q/k T) where A_0 is a pre-exponential factor, Q is the activation energy (or energy barrier of that process), T is the absolute temperature, and k 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–Fe3C alloys, a critical point observed on cooling.

artificial aging. For precipitation hardening, aging above room temperature.

astigmatism. Optical aberration revealing itself in a distortion of the cylindrical symmetry of an image.

atactic. A type of polymer chain configuration (stereoisomer) wherein side groups are randomly positioned on one side of the chain or the other.

athermal transformation. A phase transformation process that is not or very weakly thermally activated, and usually diffusionless, such as with the martensitic transformation. Normally, the transformation takes place with great speed (i.e., is independent of time), and the extent of reaction depends on temperature. Often, even athermal transformations are not entirely diffusionless but are as a rule accompanied by atomic scale relaxation processes. This applies particularly for Fe-C martensite.

atomic force microscope. (AFM) Device for studying the surface atomic structure of solids. AFM is similar in design to STM, but measures the force between the sharp microscope tip and surface atoms.

atomic mass unit (amu). A measure of atomic mass; one-twelfth of the mass of an atom of C12.

atomic number (Z). For a chemical element, the number of protons within the atomic nucleus.

atomic packing factor (APF). The fraction of the volume of a unit cell that is occupied by rigid sphere atoms or ions. The largest atomic packing factor is 0.74 in FCC and (ideal) HCP lattices; it is a little smaller (0.68) in BCC lattice, and very low (0.34) in the diamond lattice. 

atomic vibration. The vibration of an atom about its normal position in a substance.

atomic weight (A). The weighted average of the atomic masses of an atom’s naturally occurring isotopes. It may be expressed in terms of atomic mass units (on an atomic basis), or the mass per mole of atoms.

atom percent (at%). A concentration specification on the basis of the number of moles (or atoms) of a particular element relative to the total number of moles (or atoms) of all elements within an alloy.

atom Probe Tomography. Atom probe tomography (APT) is a destructive characterization method that enables measuring the 3D distribution of elements at near-atomic spatial resolution with a chemical sensitivity, equal for all elements, in the range of a few ppm. Typical measurement volumes are 50 nm x 50 nm x 200 nm. The technique is ideally suited for measuring nano precipitates, the chemistry of an interface or segregation at the nm scale. APT samples are needle-shaped and commonly prepared electrochemically or by focused ion beam milling. The measurement is performed between 20 and 100 K under ultra-high vacuum conditions. A high base voltage is applied between sample and a hollow-cone-shaped local electrode placed closely above the apex of the tip. At frequencies of 100 – 250 kHz either the voltage is raised cyclically (voltage mode, more accurate but less sensitive to the samples) or the apex is locally heated by a laser (laser mode, for materials that tend to fracture in voltage mode) just above the threshold of field-evaporation of ions from the apex of the tip where the curvature is the highest and thus the electric field is the strongest. In the ideal case one individual atom is ionized each 100-1000 pulses and accelerated towards a detector system where position and time of flight are measured. By converting the time-of-flight to a mass-to-charge ratio the ions can be identified. The additional positional information combined with the information of the sequence of the detector hit events enables to reconstruct the 3D distribution of elements in the sample.  In the reconstructed 3D atom map different atomic species can be depicted in different colors and concentration gradients can be visualized using iso-concentration surfaces. The atom maps enable the analysis of clusters or precipitate size distributions, chemical gradients can be plotted even across curved interfaces (using so called “proximity histograms”), and the segregation of  elements such as carbon, nitrogen or boron can be accurately quantified, also if they occur only in very small concentrations. The spatial resolution of APT is instrument-, measurement condition- and material dependent. Only for a few material systems it is high enough to preserve the atomic lattice in the reconstructed data. For all other materials crystallographic information on grain orientations, grain boundaries, dislocations and stacking faults inside the atom probe samples must be characterized by additionally performing transmission electron microscopy directly on the atom probe tips before the destructive atom probe experiment (correlative TEM/APT), which is an experimental challenge.

atomic radius. Conventional value not connected with an atomic size, but relating 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 coordination number in the crystal lattice considered.

atomic scattering factor. Coefficient characterizing the intensity of the elastically scattered radiation. It increases with the atomic number and decreases with (sin θ)/λ, where θ is the glancing angle and λ is the wavelength. The atomic scattering factor for electrons is ∼10^5 times greater than for x-rays, which enables the application of electron diffraction for studying relatively thin objects.

atomizing / atomization. Procedure for obtaining small solid droplets from melt, the droplets being ultra-fine grained because the cooling rate during their solidification is ∼10^3 K/s. They are used for producing more massive products and samples by consolidating and sintering. Relates to the field of powder metallurgy.

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 characteristic of the atom and can be used for chemical analysis, a technique referred to as 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 (about 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 sample at temperatures within the bainitic range for a time period smaller than the incubation period of bainitic transformation; and then quenching of the specimen, which results in the martensite or bainite formation from the deformed austenite. An increased dislocation density in the austenite (after the first stage) is inherited by the martensite or bainitic ferrite (after the second stage), which increases the sample’s hardness. Ausforming is also referred to as low-temperature thermo-mechanical treatment.

austempering. Heat treatment comprising austenitization of a steel, cooling 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 transformation.

austenite. Face-centered cubic iron; also iron and steel alloys that have the FCC crystal structure.

austenite finish temperature (Af). Temperature at which the transformation of 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 ferrite or martensite occurring from austenite when cooling is interrupted at a temperature between Ms and Mf. This can be explained by the relaxation 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 stabilization can also be reached by compositional adjustment.

austenite-stabilizer. Alloying element expanding the gamma (FCC)-phase field in the corresponding phase diagram, which manifests itself in a decrease of the A3 temperature and an increase of the A4 temperature in binary alloys Fe–M as well as in a decrease of A1 temperature in ternary alloys Fe–C–M (M 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 temperature.

austenite start temperature (As). Temperature at which the transformation of martensite into austenite starts upon heating. The same designation is also applied to nonferrous alloys in which martensite transforms into some parent phase.

austenitic steel. Alloy steel whose structure after normalizing consists predominately of austenite. This is a result of an increase in the thermodynamic stability of austenite by alloying elements such as C, Mn, and Ni. If austenite is thermodynamically unstable, it can transform into martensite (see, e.g., maraging steel and transformation-induced plasticity).

austenitizing. Forming austenite (FCC iron phase) by heating a ferrous alloy above its upper critical temperature—to within the austenite phase region from the phase diagram.