Test 2

grain growth: The increase in average grain size of a polycrystalline material; for most materials, an elevated-temperature heat treatment is necessary.

• ductile fracture: A mode of fracture attended by extensive gross plastic deformation.

• brittle fracture: Fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation.

• transgranular fracture: Fracture of polycrystalline materials by crack propagation through the grains.

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

• fracture mechanics: A technique of fracture analysis used to determine the stress level at which

preexisting cracks of known size will propagate, leading to fracture.

• fracture toughness: The measure of a material's resistance to fracture when a crack is present.

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

• 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.

• 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.

• ductile-to-brittle transition: The transition from ductile to brittle behavior with a decrease in

temperature exhibited by some low-strength steel (BCC) alloys; the temperature range over which the transition occurs is determined by Charpy and Izod impact tests.

• fatigue: Failure, at relatively low stress levels, of structures that are subjected to fluctuating and cyclic stresses.

• fatigue limit: For fatigue, the maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail.

• fatigue strength: The maximum stress level that a material can sustain without failing, for some specified number of cycles.

• fatigue life: The total number of stress cycles that cause a fatigue failure at some specified stress amplitude.

• case hardening: Hardening of the outer surface (or case) of a steel component by a carburizing or nitriding process; used to improve wear and fatigue resistance.

• thermal fatigue: A type of fatigue failure in which the cyclic stresses are introduced by fluctuating thermal stresses.

• corrosion fatigue: A type of failure that results from the simultaneous action of a cyclic stress and chemical attack.

• creep: The time-dependent permanent deformation that occurs under stress; for most materials it is important only at elevated temperatures.

• component: A chemical constituent (element or compound) of an alloy that may be used to specify its composition.

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

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

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

• equilibrium (phase): The state of a system in which the phase characteristics remain constant over indefinite time periods. At equilibrium the free energy is a minimum.

• free energy: A thermodynamic quantity that is a function of both the internal energy and entropy (or randomness) of a system. At equilibrium, the free energy is at a minimum.

• phase equilibrium: See equilibrium (phase).

• metastable: A nonequilibrium state that may persist for a very long time.

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

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

• tie line: A horizontal line constructed across a two-phase region of a binary phase diagram; its

intersections with the phase boundaries on either end represent the equilibrium compositions of the respective phases at the temperature in question.

• lever rule: A mathematical expression by which the relative phase amounts in a two-phase alloy at equilibrium may be computed.

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

• 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.

• liquidus line: On a binary phase diagram, the line or boundary separating liquid- and liquid + solid-phase regions. For an alloy, the liquidus temperature is the temperature at which a solid phase first forms under conditions of equilibrium cooling.

• eutectic reaction: A reaction in which, upon cooling, a liquid phase transforms isothermally and reversibly into two intimately mixed solid phases.

• eutectic structure: A two-phase microstructure resulting from the solidification of a liquid having the

eutectic composition; the phases exist as lamellae that alternate with one another.

• eutectic phase: One of the two phases found in the eutectic structure.

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

• microconstituent: An element of the microstructure that has an identifiable and characteristic structure. It may consist of more than one phase, such as with pearlite.

• terminal solid solution: A solid solution that exists over a composition range extending to either composition extreme of a binary phase diagram.

• 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 metals that has a distinct chemical formula. On a phase diagram it appears as an intermediate phase that exists over a very narrow range of compositions.

• eutectoid reaction: A reaction in which, upon cooling, one solid phase transforms isothermally and reversibly into two new solid phases that are intimately mixed.

• peritectic reaction: A reaction in which, upon cooling, a solid and a liquid phase transform isothermally and reversibly to a solid phase having a different composition.

• congruent transformation: A transformation of one phase to another of the same composition.

• Gibbs phase rule: For a system at equilibrium, an equation that expresses the relationship between the number of phases present and the number of externally controllable variables.

• ferrite (ceramic): Ceramic oxide materials composed of both divalent and trivalent cations (e.g., Fe2+ and Fe3+), some of which are ferrimagnetic.

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

• cementite: Iron carbide (Fe3C).

• 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.

• hypoeutectoid alloy: For an alloy system displaying a eutectoid, an alloy for which the concentration of solute is less than the eutectoid composition.

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

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

• transformation rate: The reciprocal of the time necessary for a reaction to proceed halfway to its completion.

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

• nucleation: The initial stage in a phase transformation. It is evidenced by the formation of small particles (nuclei) of the new phase that are capable of growing.

• growth (particle): During a phase transformation and subsequent to nucleation, the increase in size of a particle of a new phase.

• free energy: A thermodynamic quantity that is a function of both the internal energy and entropy (or randomness) of a system. At equilibrium, the free energy is at a minimum.

• kinetics: The study of reaction rates and the factors that affect them.

• thermally activated transformation: A reaction that depends on atomic thermal fluctuations; the atoms having energies greater than an activation energy spontaneously react or transform.

• 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.

• isothermal transformation or time-temperature-transformation (TTT) diagram: A plot of temperature versus the logarithm of time for a steel alloy of definite composition. Used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously

austenitized alloy.

• coarse pearlite: Pearlite for which the alternating ferrite and cementite layers are relatively thick.

• fine pearlite: Pearlite in which the alternating ferrite and cementite layers are relatively thin.

• bainite: An austenitic transformation product found in some steels and cast irons. It forms at temperatures between those at which pearlite and martensite transformations occur. The microstructure consists of - ferrite and a fine dispersion of cementite.

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

• martensite: A metastable iron phase supersaturated in carbon that is the product of a diffusionless (athermal) transformation from austenite.

• athermal transformation: A reaction that is not thermally activated, and usually diffusionless, 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.

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

• alloy 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). These alloying elements are usually added to improve mechanical and corrosion-resistance properties.

• continuous-cooling transformation (CCT) diagram: A plot of temperature versus the logarithm of time for a steel alloy of definite composition. Used to indicate when transformations occur as the initially austenitized material is continuously cooled at a specified rate; in addition, the final microstructure and mechanical characteristics may be predicted.

• tempered martensite: The microstructural product resulting from a tempering heat treatment of a martensitic steel. The microstructure consists of extremely small and uniformly dispersed cementite particles embedded within a continuous -ferrite matrix. Toughness and ductility are enhanced significantly by tempering.

• ferrous alloy: A metal alloy for which iron is the prime constituent.

• high-strength, low-alloy (HSLA) steel: Relatively strong, low-carbon steels, with less than about 10 wt% total of alloying elements.

• 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, including nickel and molybdenum, are also possible.

• cast iron: Generically, a ferrous alloy, the carbon content of which is greater than the maximum solubility in austenite at the eutectic temperature. Most commercial cast irons contain between 3.0 and 4.5 wt% C and between 1 and 3 wt% Si.

• gray cast iron: A cast iron alloyed with silicon in which the graphite exists in the form of flakes. A fractured surface appears gray.

• ductile iron: A cast iron alloyed with silicon and a small concentration of magnesium and/or cerium and in which the free graphite exists in nodular form. Sometimes called nodular iron.

• white cast iron: A low-silicon and very brittle cast iron in which the carbon is in combined form as cementite; a fractured surface appears white.

• malleable cast iron: White cast iron that has been heat-treated to convert the cementite into graphite clusters; a relatively ductile cast iron.

• compacted graphite iron: A cast iron alloyed with silicon and a small amount of magnesium, cerium, or other additives, in which the graphite exists as wormlike particles.

• wrought alloy: A metal alloy that is relatively ductile and amenable to hot working or cold working during fabrication.

• brass: A copper-rich copper-zinc alloy.

• bronze: A copper-rich copper-tin alloy; aluminum, silicon, and nickel bronzes are also possible.

• temper designation: A letter-digit code used to designate the mechanical and/or thermal treatment to which a metal alloy has been subjected.

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

• nonferrous alloys: A metal alloy of which iron is not the prime constituent.

• temper designation: A letter-digit code used to designate the mechanical and/or thermal treatment to which a metal alloy has been subjected.

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

• nonferrous alloys: A metal alloy of which iron is not the prime constituent.

• hot working: Any metal-forming operation performed above a metal's recrystallization temperature.

• cold working: The plastic deformation of a metal at a temperature below that at which it recrystallizes.

• forging: Mechanical forming of a metal by heating and hammering.

• rolling: A metal-forming operation that reduces the thickness of sheet stock; elongated shapes may be fashioned using grooved circular rolls.

• extrusion: A forming technique by which a material is forced, by compression, through a die orifice.

• drawing (metals): A forming technique used to fabricate metal wire and tubing. Deformation is

accomplished by pulling the material through a die by means of a tensile force applied on the exit side.

• 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.

• welding: A technique for joining metals in which actual melting of the pieces to be joined occurs in the vicinity of the bond. A filler metal may be used to facilitate the process.

• annealing: A generic term used to denote a heat treatment in which 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.

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

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

• lower critical temperature: For a steel alloy, the temperature below which, under equilibrium conditions, all austenite has transformed into ferrite and cementite phases.

• upper critical temperature: For a steel alloy, the minimum temperature above which, under equilibrium conditions, only austenite is present.

• normalizing: For ferrous alloys, austenitizing above the upper critical temperature, then cooling in air. The objective of this heat treatment is to enhance toughness by refining the grain size.

• austenitizing: Forming austenite by heating a ferrous alloy above its upper critical temperature—to within the austenite phase region from the phase diagram.

• full annealing: For ferrous alloys, austenitizing, followed by cooling slowly to room temperature.

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

• hardenability: A measure of the depth to which a specific ferrous alloy may be hardened by the formation of martensite upon quenching from a temperature above the upper critical temperature.

• Jominy end-quench test: A standardized laboratory test used to assess the hardenability of ferrous alloys.

• 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.

• 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.

• 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.

• overaging: During precipitation hardening, aging beyond the point at which strength and hardness are at their maxima.

• natural aging: For precipitation hardening, aging at room temperature.

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