Materials Science Glossary - I

impact energy (notch toughness). A measure of the energy absorbed during the fracture of a specimen of standard  dimensions and geometry when subjected to very rapid (impact) loading. Charpy and Izod impact tests are used to measure this parameter, which is important in assessing the ductile-to-brittle transition behavior of a material. The Charpy impact test method for metallic materials is specified by the European EN 10045 standard. This specification defines terms, dimension and tolerances of test pieces, type of the notch (U or V), test force, verification of impact testing machines etc.

imperfection. A deviation from perfection; normally applied to crystalline materials wherein there is a deviation from atomic/molecular order and/or continuity.

Impurity. Incidentally present substance or chemical element, unlike alloying element. In semiconductors, impurity frequently means the same as dopant. 

index of refraction (n). The ratio of the velocity of light in a vacuum to the velocity in some medium.

impurity drag. Inhibition of grain boundary migration by grain-boundary segregations. Since the segregations decrease the grain-bound­ary energy, they reduce the capillary driving force, thus causing a drag force. At the same time, impurity drag is most often used in the sense that the segregated impurity reduces the effective mobility of grain bound­aries because the diffusivity of impurity atoms differs from that of the host atoms. Impurity drag is also called solute drag. 

incoherent interface. Phase boundary in which there is no coincidence of the lattice points of neighboring lattices, in contrast to coherent or partially coherent interfaces.

incoherent precipitate/particle. Second phase precipitate whose interface with the matrix phase is incoherent. Incoherent precipitates have little to no orientation relationship with the matrix.

incoherent twin boundary. Twin boundary whose plane does not coincide with the twinning plane (see twin). A boundary of this kind is always joined to either a coherent twin boundary or the boundaries of the twinned grain. The energy and mobility of an incoherent twin boundary are rather close to those of general high-angle grain boundaries, in contrast to a coherent twin boundary. 

incubation period. In materials science, the time duration (at a constant temper­ature) necessary for the first stable nuclei of a new phase to occur. The incubation period found experimentally is often greater than the true incubation period, due to an insufficient sensitivity of investigation tech­niques used.

index of refraction (n). The ratio of the velocity of light in a vacuum to the velocity in some medium.

inelastic scattering. Interaction of x-rays (electrons) with a crystalline material accompanied by changes in the wavelength (energy) of scattered radiation. For instance, secondary electrons, including Auger-electrons, arise because of an inelastic electron scattering. X-ray fluorescence and Comp­ton scattering result from an inelastic x-ray scattering.

ingot. In metallurgy, a product obtained through the solidification of liquid metal in a mold. The ingot macrostructure consists of three zones: a chill zone of equiaxed, relatively small grains close to the mold wall, a columnar zone, and a central equiaxed zone. A shrinkage cavity develops in the upper-central part of a metallic ingot due to a decrease of the specific volume during solidification. In ingots obtained by directional solidifica­tion, there is only a columnar zone. Single crystals can be also obtained by crystallization using seed crystals. 

inhibitor. A chemical substance that, when added in relatively low concentrations, retards a chemical reaction or metallurgical process. In electrical FeSi steels the term inhibitor refers specifically to small precipitates, usually MnS or MnCuS compounds as well as AlN that retard conventional grain growth to enable secondary recrystallization of Goss grains.

insulator (electrical). A nonmetallic material that has a filled valence band at 0 K and a relatively wide energy band gap. Consequently, the room-temperature electrical conductivity is very low.

integrated circuit. Millions of electronic circuit elements (transistors, diodes, resistors, capacitors, etc.) incorporated on a very small silicon chip.

intercritical heat treatment. Heat treatment comprising a heating stage at tem­peratures inside an intercritical range.

intercritical range. Temperature range between the boundaries of a two-phase field in the corresponding phase diagram.

intercrystalline. Evolving over grain boundaries or phase boundaries. Intergranular is a synonym for intercrystalline. 

interdiffusion. Diffusion of atoms of one metal into another metal.

intergranular corrosion. Preferential corrosion along grain-boundary regions of polycrystalline materials.

intergranular fracture. Fracture of polycrystalline materials by crack propagation along grain boundaries.

intermediate solid solution. A solid solution or phase having a composition range that does not extend to either of the pure components of the system.

intermetallic compound. A compound of two or more metals that has a distinct chemical formula. On a phase diagram it appears as an intermediate phase that often exists only over a very narrow range of compositions. Solid phase whose field in the phase diagram does not include pure components of the system concerned. Intermediate phases can be of a constant or a varying composition (the latter are usually observed in metallic systems). See Hume–Rothery phase, intermetallic compound, interstitial phase, Laves phase, carbide. Examples are NiAl, TiAl or FeAl.

internal friction. Technique for studying anelasticity by imposing free strain oscillations on a thin specimen.

internal oxidation. Nucleation and growth of oxide particles inside a solid solu­tion containing readily oxidizable solutes, as, e.g., aluminum or silicon in copper. The oxides occur during annealing due to the diffusion of oxygen into the specimen from the environment. Internal oxidation is one of the methods for producing ODS alloys. 

interphase precipitation. In low-alloy steels, straight parallel rows of small carbonitride particles, usually with Ti, V and Nb, inside ferrite grains. The rows occur in the course of the growth of platelet-like ferrite grains that proceed by the intermittent ledge motion over their flat interfaces with austenite. 

interstitial diffusion. A diffusion mechanism whereby atomic motion is from interstitial site to interstitial site.

interstitial phase. Intermediate phase of a varying chemical composition close to the stoichiometry MeX or Me2X, where Me is a transition metal, and X is a nonmetal (usually hydrogen, carbon, or nitrogen) whose atomic radius is between 0.41 and 0.59 of the radius of the Me atoms. The crystal structure of interstitial phases can be described as consisting of two sublattices, one formed by the Me atoms, and the other by the X atoms. The former can be FCC, BCC, or HCP, whereas the sites of the latter lie in certain octahedral voids of the former. Interstitial phase is also known as interstitial compound. 

interstitial solid solution. Solid solution wherein solute atoms are arranged in lattice voids of the host lattice. This is only possible when the atomic size of the solute atoms is smaller than that of the host atoms, as, e.g., for carbon and nitrogen in iron, or oxygen in titanium. Interstitial solutes usually bring about severe static lattice distortions and, because of this, an increased solid-solution strengthening.

intrinsic semiconductor. A semiconductor material for which the electrical behavior is characteristic of the pure material; that is, in which electrical conductivity depends only on temperature and the band gap energy.

intrinsic stacking fault. Disturbance in the stacking sequence of the close-packed planes caused by the lack of a part of one plane, e.g., due to the collapse of a vacancy disc lying in the plane. For instance, in FCC structure with a perfect stacking sequence …ABCABCABC…, removing a plane B results in a sequence …ABCACABCA… with the layers AC arranged erroneously (their stacking sequence is typical of HCP struc­ture). Because of this distorted stacking sequence, intrinsic stacking fault is characterized by a certain excess energy. Since it is geometrically identical to a twin one layer in thickness (the layer sequence in a twin is …ABCABACBA...), its energy is two times greater than the energy of the coherent twin boundary. 

invariant point. A point on a binary phase diagram at which three phases are in equilibrium.

ionic bond. A coulombic interatomic bond that exists between two adjacent and oppositely charged ions.

isomerism. The phenomenon whereby two or more polymer molecules or repeat units have the same composition but different structural arrangements and properties.

isomorphous. Having the same structure. In the phase diagram sense, isomorphicity means having the same crystal structure or complete solid solubility for all compositions.

isotactic. A type of polymer chain configuration (stereoisomer) wherein all side groups are positioned on the same side of the chain molecule.

isothermal. At a constant temperature.

isothermal transformation (T–T–T) diagram. A plot of temperature versus the logarithm of time for a steel or Ti alloy alloy of definite composition. Used to determine when phase transformations begin and end for an isothermal (constant-temperature) heat treatment of a previously austenitized alloy (in case of steels).

isotopes. Atoms of the same element that have different atomic masses. The atoms have the same changes but different masses. Examples are hydrogen, deuterium and tritium.

isotropic. Having identical values of a property in all crystallographic directions. In the field of elasticity only one element in the periodic system has near isotropic elasticity, namely, tungsten. Alloys can be tuned to have near isotropic elasticity, e.g. Ti30wt%Nb.

Izod test. One of two tests (see also Charpy test) that may be used to measure the impact energy of a standard notched specimen. An impact blow is imparted to the specimen by a weighted pendulum.