Materials Science Glossary - S

sacrificial anode. An active metal or alloy that preferentially corrodes and protects another metal or alloy to which it is electrically coupled.

safe stress (_w). A stress used for design purposes; for ductile metals, it is the yield strength divided by a factor of safety.

sandwich panel. A type of structural composite consisting of two stiff and strong outer faces that are separated by a  lightweight core material.

saturated. A term describing a carbon atom that participates in only single covalent bonds with four other atoms.

saturation magnetization, flux density (Ms, Bs). The maximum magnetization (or flux density) for a ferromagnetic or ferromagnetic material.

scanning electron microscope (SEM). A microscope that produces an image by using an electron beam that scans the surface of a specimen; an image is produced by reflected electron beams. Examination of surface and/or microstructural

features at high magnifications is possible.

scanning probe microscope (SPM). A microscope that does not produce an image using light radiation. Rather, a very small and sharp probe raster scans across the specimen surface; out-of-surface plane deflections in response to electronic

or other interactions with the probe are monitored, from which a topographical map of the specimen surface (on a nanometer scale) is produced.

Scheil model / segregation model. The Scheil model considers kinetic effects for predicting chemical gradients that form during solidification of metallurgical alloys. It is based on assuming three boundary conditions that are characteristic for solidification of alloys: (a) diffusion inside the liquid is assumed to be infinite; (b) diffusion in the solid  is assumed to be zero; (c) equilibrium at the liquid-solid interface is maintained.

Schottky defect. In an ionic solid, a defect consisting of a cation– vacancy and anion–vacancy pair.

scission. A polymer degradation process whereby molecular chain bonds are ruptured by chemical reactions or by exposure to radiation or heat.

screw dislocation. A linear crystalline defect associated with the lattice distortion created when normally parallel planes are joined together to form a helical ramp. The Burgers vector is parallel to the dislocation line.

secondary bonds. Interatomic and intermolecular bonds that are relatively weak and for which bonding energies are relatively small. Normally atomic or molecular dipoles are involved. Examples of secondary bonding types are van der Waals

forces and hydrogen bonding.

selective leaching. A form of corrosion wherein one element or constituent of an alloy is preferentially dissolved.

self-diffusion. Atomic migration in pure metals.

self-interstitial. A host atom or ion that is positioned on an interstitial lattice site.

semiconductor. A nonmetallic material that has a filled valence band at 0 K and a relatively narrow energy band gap.

shear. A force applied so as to cause or tend to cause two adjacent parts of the same body to slide relative to each other, in a direction parallel to their plane of contact.

shear strain ( ). The tangent of the shear angle that results from an applied shear load.

shear stress ( ). The instantaneous applied shear load divided by the original cross-sectional area across which it is applied.

single crystal. A crystalline solid for which the periodic and repeated atomic pattern extends throughout its entirety without interruption.

sintering. Particle coalescence of a powdered aggregate by diffusion that is accomplished by firing at an elevated temperature.

slip. Plastic deformation as the result of dislocation motion; also, the shear displacement of two adjacent planes of atoms.

slip casting. A forming technique used for some ceramic materials.A slip, or suspension of solid particles in water, is poured into a porous mold. A solid layer forms on the inside wall as water is absorbed by the mold, leaving a shell (or ultimately

a solid piece) having the shape of the mold.

slip system. The combination of a crystallographic plane and, within that plane, a crystallographic direction along which slip (i.e., dislocation motion) occurs.

softening point (glass). The maximum temperature at which a glass piece may be handled without permanent deformation; this corresponds to a viscosity of approximately 4 x 10^6 Pa s.

soft magnetic material. A ferromagnetic or ferrimagnetic material having a small B versus H hysteresis loop, which may be magnetized and demagnetized with relative ease.

soldering. A technique for joining metals using a filler metal alloy that has a melting temperature less than about 425°C.

solid solution. A homogeneous crystalline phase that contains two or more chemical species. Both substitutional and interstitial solid solutions are possible.

solid-solution strengthening. Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility.

solidus line. On a phase diagram, the locus of points at which solidification is complete upon equilibrium cooling, or at which melting begins upon equilibrium heating.

solubility limit. The maximum concentration of solute that may be added without forming a new phase. 

solute. One component or element of a solution present in a minor concentration. It is dissolved in the solvent.

solution heat treatment. The process used to form a solid solution by dissolving precipitate particles. Often, the solid solution is supersaturated and metastable at ambient conditions as a result of rapid cooling from an elevated temperature.

solvent. The component of a solution present in the greatest amount. It is the component that dissolves a solute.

solvus line. The locus of points on a phase diagram representing the limit of solid solubility as a function of temperature.

specific heat (cp, cv). The heat capacity per unit mass of material.

specific modulus (specific stiffness). The ratio of elastic modulus to specific gravity for a material.

specific strength. The ratio of tensile strength to specific gravity for a material.

spheroidite. Microstructure found in steel alloys consisting of spherelike cementite particles within a ferrite matrix. It is  roduced by an appropriate elevated-temperature heat treatment of pearlite, bainite, or martensite and is relatively soft.

spheroidizing. For steels, a heat treatment normally carried out at a temperature just below the eutectoid in which the spheroidite microstructure is produced.

spherulite. An aggregate of ribbonlike polymer crystallites (lamellae) radiating from a common central nucleation site; the crystallites are separated by amorphous regions.

spinning. The process by which fibers are formed. A multitude of fibers are spun as molten or dissolved material is forced through many small orifices.

stabilizer. A polymer additive that counteracts deteriorative processes.

stainless steel. A steel alloy that is highly resistant to corrosion in a variety of environments. The predominant

alloying element is chromium, which must be present in a concentration of at least 11 wt%; other alloy additions, to include

nickel and molybdenum, are also possible. Stainless steels are commonly divided into five groups: Martensitic stainless steels; Ferritic stainless steels; Austenitic stainless steels; Duplex (ferritic-austenitic) stainless steels; Precipitation-hardening stainless steels.

Martensitic stainless steels are essentially alloys of chromium and carbon that possess a martensitic crystal structure in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are usually less resistant to corrosion than some other grades of stainless steel.  Chromium content usually does not exceed 18wt.%, while carbon content may exceed 1.0 wt.%.  The chromium and carbon contents are adjusted to ensure a martensitic structure after hardening. Excess carbides may be present to enhance wear resistance or as in the case of knife blades, to maintain cutting edges.

Ferritic stainless steels are chromium containing alloys with Ferritic, body centered cubic (bcc) crystal structures. Chromium content is typically less than 30%.  The ferritic stainless steels are ferromagnetic.  They may have good ductility and formability, but high-temperature mechanical properties are relatively inferior to the austenitic stainless steels.  Toughness is limited at low temperatures and in heavy sections.

Austenitic stainless steels have a austenitic, face centered cubic (fcc) crystal structure. Austenite is formed through the generous use of austenitizing elements such as nickel, manganese, and nitrogen.  Austenitic stainless steels are effectively nonmagnetic in the annealed condition and can be hardened only by cold working.  Some ferromagnetism may be noticed due to cold working or welding.  They typically have reasonable cryogenic and high temperature strength properties. Chromium content typically is in the range of 16 to 26wt.%; nickel content is commonly less than 35wt.%.

Duplex stainless steels are a mixture of bcc ferrite and fcc austenite crystal structures. The percentage each phase is a dependent on the composition and heat treatment. Most Duplex stainless steels are intended to contain around equal amounts of ferrite and austenite phases in the annealed condition. The primary alloying elements are chromium and nickel.  Duplex stainless steels generally have similar corrosion resistance to austenitic alloys except they typically have better stress corrosion cracking resistance.  Duplex stainless steels also generally have greater tensile and yield strengths, but poorer toughness than austenitic stainless steels.

Precipitation hardening stainless steels are chromium-nickel alloys. Precipitation-hardening stainless steels may be either austenitic or martensitic in the annealed condition.  In most cases, precipitation hardening stainless steels attain high strength by precipitation hardening of the martensitic structure.

standard half-cell. An electrochemical cell consisting of a pure metal immersed in a 1 M aqueous solution of its ions, which is electrically coupled to the standard hydrogen electrode.

steady-state diffusion. The diffusion condition for which there is no net accumulation or depletion of diffusing species. The diffusion flux is independent of time.

stereoisomerism. Polymer isomerism in which side groups within repeat units are bonded along the molecular chain in the same order but in different spatial arrangements.

stoichiometry. For ionic compounds, the state of having exactly the ratio of cations to anions specified by the chemical formula.

strain, engineering (_). The change in gauge length of a specimen (in the direction of an applied stress) divided by its original gauge length. In mechanics it is the symmetric portion of the displacement gradient tensor.

strain hardening. The increase in hardness and strength of a ductile metal as it is plastically deformed below

its recrystallization temperature.

strain point (glass). The maximum temperature at which glass fractures without plastic deformation;

strain, true. See true strain.

stress concentration. The concentration or amplification of an applied stress at the tip of a notch or small crack.

stress corrosion (cracking). A form of failure that results from the combined action of a tensile stress and a corrosion environment; it occurs at lower stress levels than are required when the corrosion environment is absent.

stress, engineering ( ). The instantaneous load applied to a specimen divided by its cross-sectional area before any deformation.

stress raiser. A small flaw (internal or surface) or a structural discontinuity at which an applied tensile stress will be amplified and from which cracks may propagate.

stress relief. A heat treatment for the removal of residual stresses.

structural clay products. Ceramic products made principally of clay and used in applications where structural integrity is important (e.g., bricks, tiles, pipes).

structural composite. A composite, whose properties depend on the geometrical design of the structural elements. Laminar composites and sandwich panels are two subclasses.

structure. The arrangement of the internal components of matter: electron structure (on a subatomic level), crystal structure (on an atomic level), and microstructure (on a microscopic level).

substitutional solid solution. A solid solution wherein the solute atoms replace or substitute for the host atoms.

superconductivity. A phenomenon observed in some materials: the disappearance of the electrical resistivity at temperatures approaching 0 K.

supercooling. Cooling to below a phase transition temperature without the occurrence of the transformation.

superheating. Heating to above a phase transition temperature without the occurrence of the transformation.

syndiotactic. A type of polymer chain configuration (stereoisomer) in which side groups regularly alternate positions on opposite sides of the chain.

system. Two meanings are possible: (1) a specific body of material that is being considered, and (2) a series of possible alloys consisting of the same components.