Materials Science Glossary - P

paramagnetism. A relatively weak form of magnetism that results from the independent alignment of atomic dipoles (magnetic) with an applied magnetic field.

particle-reinforced composite. A composite for which the dispersed phase is equiaxed.

passivity. The loss of chemical reactivity, under particular environmental conditions, by some active metals and alloys, often due to the formation of a protective film.

Pauli exclusion principle. The postulate that for an individual atom, at most two electrons, which necessarily have opposite spins, can occupy the same state.

pearlite. A two-phase microstructure found in some steels and cast irons; it results from the transformation of austenite of eutectoid composition and consists of alternating layers (or lamellae) of -ferrite and cementite.

periodic table. The arrangement of the chemical elements with increasing atomic number according to the periodic variation in electron structure. Nonmetallic elements are positioned at the far right-hand side of the table.

peritectic reaction. A reaction wherein, upon cooling, a solid and a liquid phase transform isothermally and reversibly to a solid phase having a different composition.

permeability (magnetic, _). The proportionality constant between B and H fields. The value of the permeability of a vacuum (_0) is 1.257 _ 10_6 H/m.

permittivity (_). The proportionality constant between the dielectric displacement D and the electric field e. The value of the permittivity for a vacuum is 8.85 _ 10_12 F/m.

phase. A homogeneous portion of a system that has uniform physical and chemical characteristics.

phase diagram. A graphical representation of the relationships among environmental constraints (e.g., temperature and sometimes pressure), composition, and regions of phase stability, ordinarily under conditions of equilibrium.

phase equilibrium. See equilibrium (phase).

phase transformation. A change in the number and/or character of the phases that constitute the microstructure of an alloy.

phonon. A single quantum of vibrational or elastic energy.

phosphorescence. Luminescence that occurs at times greater than on the order of a second after an electron excitation event.

photoconductivity. Electrical conductivity that results from photoninduced electron excitations in which light is absorbed.

photomicrograph. A photograph made with a microscope that records a microstructural image.

photon. A quantum unit of electromagnetic energy.

piezoelectric. A dielectric material in which polarization is induced by the application of external forces.

Pilling–Bedworth ratio (P–B ratio). The ratio of metal oxide volume to metal volume; used to predict whether a scale that forms will protect a metal from further oxidation.

pitting. A form of very localized corrosion wherein small pits or holes form, usually in a vertical direction.

plain carbon steel. A ferrous alloy in which carbon is the prime alloying element.

Planck’s constant (h). A universal constant that has a value of 6.63 x 10^34 J s. The energy of a photon of electromagnetic radiation is the product of h and the radiation frequency.

plane strain. The condition, important in fracture mechanical analyses, wherein, for tensile loading, there is zero strain in a direction perpendicular to both the stress axis and the direction of crack propagation; this condition is found in thick plates, and the zero-strain direction is perpendicular to the plate surface.

plane strain fracture toughness (KIc). For the condition of plane strain, the measure of a material’s resistance to fracture when a crack is present.

plastic. A solid organic polymer of high molecular weight that has some structural rigidity under load and is used in general-purpose applications. It may also contain additives such as fillers, plasticizers, and flame retardants.

plastic deformation. Deformation that is permanent or non-recoverable (also referred to as inelastic) after release of the applied load. It is accompanied by permanent atomic displacements.

plasticizer. A low-molecular-weight polymer additive that enhances flexibility and workability and reduces stiffness and brittleness, resulting in a decrease in the glass transition temperature Tg.

point defect. A crystalline defect associated with one or, at most, several atomic sites.

Poisson’s ratio (_). For elastic deformation, the negative ratio of lateral and axial strains that result from an applied axial stress.

polarization (P). The total electric dipole moment per unit volume of dielectric material. Also, a measure of the contribution to the total dielectric displacement by a dielectric material.

polarization (corrosion). The displacement of an electrode potential from its equilibrium value as a result of current flow.

polarization (electronic). For an atom, the displacement of the center of the negatively charged electron cloud relative to the positive nucleus that is induced by an electric field.

polarization (ionic). Polarization as a result of the displacement of anions and cations in opposite directions.

polarization (orientation). Polarization resulting from the alignment (by rotation) of permanent electric dipole moments with an applied electric field.

polar molecule. A molecule in which there exists a permanent electric dipole moment by virtue of the asymmetrical distribution of positively and negatively charged regions.

polycrystalline. Refers to crystalline materials that are composed of more than one crystal or grain.

polymer. A compound of high molecular weight (normally organic) the structure of which is composed of chains of small repeat units.

polymer-matrix composite (PMC). A composite material for which the matrix is a polymer resin, and having fibers (normally glass, carbon, or aramid) as the dispersed phase.

polymorphism. The ability of a solid material to exist in more than one form or crystal structure.

powder metallurgy (P/M). The fabrication of metal pieces having intricate and precise shapes by the compaction of metal powders, followed by a densification heat treatment. 

precipitation hardening. Hardening and strengthening of a metal alloy by extremely small and uniformly dispersed particles that precipitate from a supersaturated solid solution; sometimes also called age hardening.

precipitation heat treatment. A heat treatment used to precipitate a new phase from a supersaturated solid solution. For precipitation hardening, it is termed artificial aging.

prepreg. Continuous fiber reinforcement preimpregnated with a polymer resin that is then partially cured.

prestressed concrete. Concrete into which compressive stresses have been introduced using steel wires or rods.

primary bonds. Interatomic bonds that are relatively strong and for which bonding energies are relatively large. Primary bonding types are ionic, covalent, and metallic.

primary phase. A phase that exists in addition to the eutectic structure.

principle of combined action. The supposition, often valid, that new properties, better properties, better property combinations, and/or a higher level of properties can be fashioned by the judicious combination of two or more distinct

materials.

process annealing. Annealing of previously cold-worked products (commonly steel alloys in sheet or wire form) below the lower critical (eutectoid) temperature.

proeutectoid cementite. Primary cementite that exists in addition to pearlite for hypereutectoid steels.

proeutectoid ferrite. Primary ferrite that exists in addition to pearlite for hypoeutectoid steels.

property. A material trait expressed in terms of the measured response to a specific imposed stimulus.

proportional limit. The point on a stress–strain curve at which the straight line proportionality between stress and strain ceases.

p-type semiconductor. A semiconductor for which the predominant charge carriers responsible for electrical conduction are holes. Normally, acceptor impurity atoms give rise to the excess holes.