🔬 Materials Science & Metallurgy Terminology Glossary: 55 Essential Terms Explained

Why These Terms Matter

Materials science and metallurgy sit underneath nearly every other engineering discipline — the steel in a structural beam, the aluminum in an aircraft skin, the polymer in a gasket, and the ceramic in a turbine coating are all selected and specified using a shared, precise vocabulary. Getting these terms right is not academic pedantry: specifying "yield strength" when a design actually requires "ultimate tensile strength," or confusing "hardness" with "toughness," can lead to a part that meets a spec sheet number but fails catastrophically in service.

Much of this vocabulary is standardized by ASTM International, ISO, and SAE test methods, so a term like "Charpy impact energy" or "Rockwell C hardness" refers to a specific, repeatable test procedure — not just a general concept. This glossary covers 55 of the most important terms an engineer, technician, or student encounters when reading a materials data sheet, a mill certification, a failure analysis report, or a materials selection standard, organized alphabetically with standard references and plain-language explanations.

A

Alloy — general metallurgy

A metallic material composed of two or more elements, at least one of which is a metal, combined to produce properties superior to those of the individual constituent elements. Steel (iron + carbon, often with chromium, nickel, or molybdenum) and brass (copper + zinc) are alloys. Alloying elements are added deliberately to improve strength, corrosion resistance, machinability, or hardenability.

Anisotropy — general materials science

The property of a material having different mechanical or physical properties depending on the direction of measurement. Rolled sheet metal is anisotropic — its yield strength and ductility differ between the rolling direction and the transverse direction. Additively manufactured (3D-printed) metal parts are also strongly anisotropic, with weaker properties in the build direction than in-plane, because of the layer-by-layer solidification process.

Annealing — ASTM A1077 / general heat treatment

A heat treatment process in which a material is heated above a critical temperature, held to allow internal structure to reorganize, and then cooled slowly (usually in the furnace) to relieve internal stresses, soften the material, and improve ductility. Full annealing of steel involves heating above the upper critical temperature (A3 or Acm) and furnace-cooling; the resulting microstructure is coarse pearlite, which is soft and easily machined but has reduced strength compared to normalized or hardened conditions.

Austenite — Fe-C phase diagram terminology

The face-centered cubic (FCC) phase of iron, stable at high temperature in plain carbon steel (above roughly 727°C / 1341°F depending on carbon content), capable of dissolving significantly more carbon than ferrite. Austenite is the parent phase from which martensite, bainite, and pearlite form on cooling — its cooling rate and transformation temperature govern the final microstructure and hardness of a heat-treated steel. Austenitic stainless steels (like 304 and 316) retain the FCC austenite structure at room temperature due to nickel additions.

B

Bainite — Fe-C phase diagram terminology

A microstructure formed when austenite is cooled at an intermediate rate — faster than needed to form pearlite but slower than needed to form martensite — typically between about 250°C and 550°C. Bainite consists of fine ferrite plates with carbide precipitates and offers a useful combination of strength and toughness that is often superior to tempered martensite of the same hardness, which is why it is deliberately targeted in austempering and some pipeline and gear steels.

Brinell Hardness (HB) — ASTM E10

A hardness test in which a hardened steel or tungsten carbide ball (typically 10 mm diameter) is pressed into a material surface under a specified load, and hardness is calculated from the diameter of the resulting indentation. Brinell testing is well suited to coarse-grained or heterogeneous materials like castings because the large indentation averages over local variations. Brinell hardness correlates approximately with tensile strength for steels (tensile strength in ksi ≈ 0.5 × HB).

C

Case Hardening — general heat treatment

A family of surface-hardening processes (carburizing, nitriding, carbonitriding, induction hardening) that produce a hard, wear-resistant outer layer (the "case") while leaving a tougher, more ductile core. Case-hardened gears and shafts resist surface wear and rolling contact fatigue while retaining the impact toughness needed in the core to survive shock loading.

Charpy Impact Test — ASTM E23

A standardized test in which a notched specimen is struck by a swinging pendulum, and the energy absorbed in fracturing the specimen is measured (typically in joules or ft-lb). The Charpy V-notch (CVN) test is the most common variant. Results are used to determine a material's ductile-to-brittle transition temperature (DBTT) — critical for selecting steels used in low-temperature service, such as pressure vessels and structural steel in cold climates, where NFPA and ASME codes reference minimum CVN values at the lowest anticipated service temperature.

Cold Working — general metallurgy

Plastic deformation of a metal performed below its recrystallization temperature (for most metals, this means at or near room temperature), such as by rolling, drawing, or forging. Cold working increases strength and hardness through strain hardening (work hardening) while reducing ductility, and it introduces residual stresses and directional (anisotropic) grain structure that can later be relieved by annealing.

Corrosion — general materials science / NACE / ISO 8044

The deterioration of a material, usually a metal, through chemical or electrochemical reaction with its environment. Common forms include uniform (general) corrosion, pitting, crevice corrosion, galvanic corrosion, stress corrosion cracking (SCC), and intergranular corrosion. Corrosion control strategies include material selection, coatings, cathodic protection, and corrosion inhibitors, and are frequently specified per NACE/AMPP standards.

Creep — ASTM E139

The time-dependent, permanent deformation of a material under sustained load, typically at elevated temperature (generally above about 0.4 times the absolute melting temperature). Creep progresses through primary (decreasing rate), secondary (steady-state, minimum rate), and tertiary (accelerating rate leading to rupture) stages. Creep governs the design life of components operating at high temperature for extended periods, such as gas turbine blades, boiler tubes, and high-temperature piping in power plants.

Crystal Structure — general materials science

The ordered, repeating three-dimensional arrangement of atoms in a solid material. The most common metallic crystal structures are body-centered cubic (BCC, e.g., ferrite, chromium, tungsten), face-centered cubic (FCC, e.g., austenite, aluminum, copper, nickel), and hexagonal close-packed (HCP, e.g., titanium, magnesium, zinc). Crystal structure strongly influences ductility — FCC metals generally deform more easily at low temperature than BCC metals, which is one reason BCC steels can exhibit a ductile-to-brittle transition while FCC austenitic stainless steels do not.

D

Dislocation — general materials science

A line defect in a crystal lattice where the regular atomic arrangement is disrupted. Dislocations are the primary mechanism by which metals deform plastically — they move through the crystal lattice under applied stress, allowing planes of atoms to slip past one another at stresses far lower than would be required to break all atomic bonds simultaneously. Strengthening mechanisms like strain hardening, grain refinement, and precipitation hardening all work by impeding dislocation motion.

Ductility — ASTM E8

The ability of a material to undergo significant plastic (permanent) deformation before fracture, commonly quantified as percent elongation or percent reduction in area measured in a tensile test. Ductile materials (most structural steels, aluminum alloys) give visible warning — necking and stretching — before failure, while brittle materials (cast iron, ceramics, hardened but untempered steel) fracture with little or no prior deformation, which is a major safety concern in structural and pressure-boundary applications.

E

Elastic Modulus (Young's Modulus, E) — ASTM E111

The ratio of stress to strain in the linear elastic region of a material's stress-strain curve, representing its stiffness — resistance to elastic (recoverable) deformation. Young's modulus is a fundamental material property (about 200 GPa / 29,000 ksi for steel, about 69 GPa / 10,000 ksi for aluminum) that does not change with heat treatment or alloying in most practical ranges, unlike yield or tensile strength, because it is governed by interatomic bonding forces rather than microstructure.

Elongation, Percent — ASTM E8

The permanent increase in gauge length of a tensile test specimen after fracture, expressed as a percentage of the original gauge length. Percent elongation is a standard measure of ductility reported on mill certifications and material data sheets; higher elongation indicates greater capacity for plastic deformation before fracture.

Endurance Limit (Fatigue Limit) — ASTM E466

The stress amplitude below which a material can theoretically withstand an infinite number of load cycles without fatigue failure. Many steels exhibit a true endurance limit (typically around 40-50% of ultimate tensile strength) visible as a plateau on the S-N (stress-life) curve, while aluminum alloys and most nonferrous metals do not — their fatigue strength continues to decrease with increasing cycles, so a fatigue strength at a specified number of cycles (e.g., 10^7 or 10^8) is reported instead.

F

Fatigue — ASTM E466 / E606

Progressive, localized structural damage that occurs when a material is subjected to cyclic (repeated) loading, even at stress levels well below the material's static yield strength. Fatigue failure proceeds through crack initiation (often at a surface defect, notch, or stress concentration), stable crack propagation (visible on a fracture surface as "beach marks"), and final rapid fracture once the remaining cross-section can no longer support the load. Fatigue is responsible for a large share of in-service mechanical failures, which is why rotating and vibrating machinery components are designed against S-N curves or fracture mechanics fatigue crack growth models rather than static strength alone.

Ferrite — Fe-C phase diagram terminology

The body-centered cubic (BCC) phase of iron, stable at room temperature in plain carbon and low-alloy steels, capable of dissolving only a very small amount of carbon (about 0.02% maximum). Ferrite is relatively soft and ductile compared to the other steel microstructures (pearlite, bainite, martensite) and is the matrix phase in low-carbon steels.

Fracture Toughness (K_IC) — ASTM E399

A material property, expressed in units of MPa·√m or ksi·√in, that quantifies a material's resistance to crack propagation in the presence of a pre-existing flaw. Fracture toughness is central to fracture mechanics design (as used in ASME Section XI and API 579 fitness-for-service assessments) because it allows engineers to calculate the critical flaw size that will cause unstable fracture at a given applied stress — a fundamentally different and more conservative approach than simply comparing applied stress to yield strength.

G

Galvanic Corrosion — ASTM G82

Accelerated corrosion of a less noble (more anodic) metal that occurs when it is in electrical contact with a more noble (more cathodic) metal in the presence of an electrolyte (such as moisture or seawater). The relative nobility of common metals is summarized in the galvanic series; aluminum in direct contact with stainless steel fasteners in a marine environment is a classic galvanic corrosion risk, which is why dielectric isolation, compatible fastener selection, or sacrificial anodes are used to mitigate it.

Grain Boundary — general materials science

The interface between two crystal grains of differing orientation within a polycrystalline material. Grain boundaries impede dislocation motion (contributing to strength per the Hall-Petch relationship — finer grain size generally means higher strength) but can also be preferential sites for corrosion (intergranular corrosion) and crack propagation at elevated temperature (creep cavitation) if embrittling phases or impurities segregate there.

Grain Size — ASTM E112

The average size of the individual crystals (grains) making up a polycrystalline material, commonly reported as an ASTM grain size number (higher number = finer grain). Grain refinement is one of the few strengthening mechanisms that simultaneously increases strength and toughness (unlike cold working or precipitation hardening, which trade ductility for strength), which is why controlled rolling and normalizing treatments that produce fine grain structure are valued in structural and pressure vessel steels.

H

Hardenability — ASTM A255 (Jominy end-quench test)

A measure of the depth to which a steel can be hardened (transformed to martensite) upon quenching, as distinct from the maximum hardness it can achieve (which depends primarily on carbon content). Hardenability is controlled largely by alloying content (chromium, molybdenum, manganese, nickel) and is quantified using the Jominy end-quench test, which measures hardness as a function of distance from a quenched end. Thick sections require higher-hardenability alloy steels to through-harden, since the interior cools more slowly than the surface.

Hardness — general materials science

A material's resistance to localized plastic deformation, typically measured by indentation testing (Brinell, Rockwell, Vickers, or Knoop). Hardness correlates loosely with tensile strength and wear resistance but is not itself a fundamental design property — it is widely used for quality control, incoming material verification, and quick field assessment because hardness tests are fast, low-cost, and often nondestructive (portable hardness testers can check hardness on an installed component).

Heat-Affected Zone (HAZ) — AWS D1.1 / welding metallurgy

The region of base metal adjacent to a weld that did not melt but whose microstructure and mechanical properties were altered by the heat of welding. In hardenable steels, the HAZ can form untempered martensite (brittle and crack-prone) if cooling is too rapid, which is why preheat, interpass temperature control, and post-weld heat treatment (PWHT) are specified to control HAZ hardness and reduce the risk of hydrogen-induced cracking.

I

Impact Toughness — ASTM E23

The energy a material absorbs during rapid (impact) loading before fracture, typically measured by the Charpy V-notch test and reported in joules or ft-lb. Impact toughness is distinct from static fracture toughness (K_IC) — it is a comparative, notch-sensitive screening test rather than a direct fracture-mechanics design input, but it is widely specified in codes (ASME, API, AWS) as a minimum acceptance criterion for materials used in low-temperature or impact-prone service.

Intergranular Corrosion — ASTM A262

Preferential corrosive attack along grain boundaries, without significant attack of the grain interiors. In austenitic stainless steels, intergranular corrosion is often caused by "sensitization" — chromium carbide precipitation at grain boundaries during welding or improper heat treatment that locally depletes chromium below the level needed for passivation, making the depleted zone susceptible to attack. Low-carbon ("L" grade, e.g., 304L, 316L) stainless steels are used specifically to reduce sensitization risk in welded structures.

Isotropic — general materials science

Having identical physical and mechanical properties regardless of the direction of measurement. Most cast and annealed wrought metals are approximately isotropic, while cold-rolled sheet, forgings, and additively manufactured parts are typically anisotropic. Isotropy is often an implicit assumption in simplified engineering stress calculations and should be verified before relying on it for directional or fatigue-critical designs.

M

Martensite — Fe-C phase diagram terminology

A hard, brittle, supersaturated body-centered tetragonal (BCT) phase formed when austenite is cooled (quenched) so rapidly that carbon atoms cannot diffuse out of solution, producing a highly strained crystal lattice. Martensite is the hardest microstructure achievable in steel but is typically too brittle for direct use, so it is almost always tempered (reheated to an intermediate temperature) afterward to restore some ductility and toughness at a controlled sacrifice of maximum hardness.

Metallography — ASTM E3 / E407

The scientific discipline and laboratory practice of preparing (cutting, mounting, grinding, polishing, and etching) and examining the microstructure of a material, typically using optical or electron microscopy. Metallographic examination is used in failure analysis to identify the microstructure present (confirming heat treatment was correct), detect defects like inclusions or porosity, and reveal evidence of the failure mechanism (fatigue striations, intergranular fracture, etc.).

Modulus of Elasticity — see Elastic Modulus (Young's Modulus)

See Elastic Modulus (Young's Modulus, E).

N

Necking — ASTM E8

The localized reduction in cross-sectional area that occurs in a ductile tensile specimen once it reaches its ultimate tensile strength and begins unstable, localized plastic deformation. Necking is a visible warning sign of impending ductile fracture and is why ultimate tensile strength (based on original cross-sectional area) is technically an engineering stress value rather than the true stress the material experiences at the neck at the moment of fracture.

Nondestructive Testing (NDT) — ASNT SNT-TC-1A / ASTM E-series

A family of inspection methods used to evaluate material or component integrity without causing damage, including visual testing (VT), liquid penetrant testing (PT), magnetic particle testing (MT), radiographic testing (RT), ultrasonic testing (UT), and eddy current testing (ET). NDT methods are selected based on the defect type of concern (surface vs. subsurface), material type (MT requires ferromagnetic material), and access constraints, and are widely required by ASME, API, and AWS codes for weld and casting acceptance.

Normalizing — ASTM A1077

A heat treatment in which steel is heated above its upper critical temperature and then air-cooled (rather than furnace-cooled as in annealing or quenched as in hardening). Normalizing refines grain size and produces a more uniform microstructure than as-rolled steel, with somewhat higher strength than fully annealed steel but better machinability and lower residual stress than quenched-and-tempered steel.

O

Oxidation — general materials science

A chemical reaction in which a material loses electrons, most commonly reacting with oxygen to form an oxide. In metals, oxidation is the fundamental chemical process underlying most corrosion; some metals (aluminum, chromium, titanium) form a thin, adherent, self-healing oxide layer that passivates the surface and dramatically slows further attack, which is the basis of stainless steel's corrosion resistance (the chromium oxide passive film).

P

Passivation — ASTM A967

The formation (naturally or through chemical treatment, typically nitric or citric acid) of a thin, protective, chromium-rich oxide film on stainless steel that renders the surface chemically inert (passive) and resistant to corrosion. Passivation treatments per ASTM A967 are commonly specified after machining stainless steel components to remove embedded free iron particles that could otherwise initiate localized corrosion.

Pearlite — Fe-C phase diagram terminology

A lamellar (layered) microstructure of alternating ferrite and cementite (iron carbide, Fe3C) plates that forms when austenite cools slowly through the eutectoid transformation. Pearlite provides a balance of moderate strength and ductility and is the dominant microstructure in slowly cooled or normalized plain carbon steels of near-eutectoid composition.

Phase Diagram — general materials science (e.g., Fe-Fe3C diagram)

A graphical map showing the equilibrium phases present in a material system as a function of composition and temperature. The iron-carbon (Fe-Fe3C) phase diagram is the foundational reference for steel heat treatment, showing the critical temperatures (A1, A3, Acm) that define the boundaries between austenite, ferrite, cementite, and their transformation products, and is used to select proper austenitizing, annealing, and normalizing temperatures for a given carbon content.

Pitting Corrosion — ASTM G46 / G48

A localized form of corrosion that produces small, deep cavities (pits) in an otherwise largely unaffected surface, typically initiated by a local breakdown of a passive film (often at a chloride-induced defect). Pitting is particularly dangerous because it can perforate a wall thickness with minimal overall material loss and little visible warning, and it is often the initiation site for stress corrosion cracking or corrosion fatigue.

Poisson's Ratio — ASTM E132

The ratio of transverse (lateral) strain to axial strain when a material is loaded elastically in one direction, describing how much a material contracts perpendicular to the direction of an applied tensile load. Poisson's ratio is approximately 0.30 for most structural steels and 0.33 for aluminum, and is a required input for finite element analysis and for calculating multiaxial stress states from measured strains (as in strain gauge rosette analysis).

Precipitation Hardening (Age Hardening) — general metallurgy

A strengthening mechanism in which a supersaturated solid solution is aged (either at room temperature — natural aging — or at an elevated temperature — artificial aging) to form fine, uniformly distributed second-phase precipitates that impede dislocation motion. Precipitation hardening is the primary strengthening mechanism for many aluminum alloys (2xxx and 7xxx series), nickel superalloys (via gamma-prime precipitates), and 17-4 PH stainless steel.

Proportional Limit — ASTM E8

The stress level up to which a material's stress-strain curve remains linear, i.e., stress is directly proportional to strain following Hooke's law. The proportional limit is closely related to but not identical to the elastic limit; in practice, the 0.2% offset yield strength is used instead of either for engineering design because the proportional limit is difficult to determine precisely from test data.

Q

Quenching — general heat treatment

Rapid cooling of a heated material (from the austenitizing temperature, in the case of steel), typically by immersion in water, oil, polymer solution, or by forced air, to suppress diffusion-controlled transformations and produce martensite. The quench severity (cooling rate) must be matched to the steel's hardenability and section size — too slow a quench for a given steel produces soft, non-martensitic microstructure, while too severe a quench in a complex-geometry part can cause quench cracking from thermal and transformation stresses.

R

Residual Stress — general materials science

Internal stress that remains in a material or component after the external load or thermal gradient that caused it has been removed, arising from non-uniform plastic deformation, differential cooling, welding, or phase transformation. Residual stresses add algebraically to applied service stresses and can significantly reduce fatigue life or promote stress corrosion cracking if tensile at the surface; stress-relieving heat treatments, shot peening (which deliberately introduces beneficial compressive residual stress), and vibratory stress relief are common mitigation methods.

Rockwell Hardness (HRC, HRB) — ASTM E18

A hardness test method that measures the depth of penetration of an indenter (a diamond cone for the C scale, a steel ball for the B scale) under a major load after a minor preload, with hardness read directly from the test machine. Rockwell C (HRC) is used for hardened steels (roughly HRC 20-70); Rockwell B (HRB) is used for softer materials like annealed steel, aluminum, and brass. Rockwell testing is fast and widely used for in-process quality control.

S

S-N Curve (Wöhler Curve) — ASTM E468

A plot of cyclic stress amplitude (S) versus the number of cycles to failure (N), typically on a log-log or semi-log scale, generated from fatigue testing of multiple specimens at different stress levels. The S-N curve is the foundational data set for stress-life (S-N) fatigue design, the traditional approach for high-cycle fatigue analysis of components subject to many millions of load cycles at stresses below yield.

Sensitization — ASTM A262

See Intergranular Corrosion — the metallurgical condition (chromium carbide precipitation at grain boundaries, typically from prolonged exposure in the 425-815°C / 800-1500°F range such as during welding) that renders austenitic stainless steel susceptible to intergranular corrosion.

Stainless Steel — ASTM A240 / general metallurgy

A family of iron-based alloys containing a minimum of approximately 10.5% chromium by weight, which forms a thin, self-healing passive chromium oxide film that provides corrosion resistance. Major families include austenitic (300 series, non-magnetic, most corrosion-resistant and weldable), ferritic (400 series, magnetic, lower cost, more limited corrosion resistance), martensitic (400 series, hardenable by heat treatment, used for cutlery and shafts), and precipitation-hardening (17-4 PH and similar, combining high strength with moderate corrosion resistance).

Strain — general materials science

The dimensionless measure of deformation representing the change in length divided by the original length (engineering strain), or the natural logarithm of the ratio of instantaneous to original length (true strain). Strain is the deformation-side counterpart to stress in the stress-strain relationship that defines a material's mechanical behavior.

Strain Hardening (Work Hardening) — general metallurgy

The increase in a metal's strength and hardness (with a corresponding decrease in ductility) resulting from plastic deformation at temperatures below the recrystallization temperature, caused by the accumulation and interaction of dislocations that increasingly impede further dislocation motion. Strain hardening is why a wire that has been repeatedly bent back and forth becomes progressively stiffer and eventually fractures.

Stress — general materials science

The internal force per unit area within a material resulting from an externally applied load, typically expressed in units of MPa or psi/ksi. Engineering stress is calculated using the original cross-sectional area, while true stress accounts for the instantaneous (reduced) cross-sectional area during plastic deformation — the two diverge significantly after necking begins in a tensile test.

Stress Concentration — general materials science

A localized increase in stress at a geometric discontinuity (a hole, notch, fillet, sharp corner, or thread root) relative to the nominal (average) stress in the surrounding section, quantified by a stress concentration factor (K_t). Stress concentrations are frequently the initiation sites for fatigue cracks, which is why generous fillet radii, smooth transitions, and controlled surface finish are standard design practices at points of geometric change in fatigue-loaded components.

Stress Corrosion Cracking (SCC) — ASTM G36 / NACE standards

The initiation and propagation of cracks in a susceptible material caused by the combined, simultaneous action of a tensile stress (applied or residual) and a specific corrosive environment, at stress levels well below the material's yield strength. Classic examples include chloride SCC in austenitic stainless steel and caustic embrittlement in carbon steel boiler tubing; SCC is particularly insidious because it can occur with minimal overall corrosion or visible warning before sudden brittle-appearing fracture.

T

Tempering — general heat treatment

A heat treatment in which quenched (martensitic) steel is reheated to a temperature below the lower critical temperature (typically 150-650°C / 300-1200°F) and held, then cooled, to reduce brittleness and residual stress at a controlled cost to maximum hardness. Tempering temperature is selected to hit a target combination of hardness, strength, and toughness — higher tempering temperatures produce lower hardness but greater ductility and impact toughness.

Tensile Strength (Ultimate Tensile Strength, UTS) — ASTM E8

The maximum engineering stress a material can withstand while being stretched or pulled before necking and eventual fracture, calculated as the peak load divided by the original cross-sectional area. Tensile strength is a standard property reported on every mill certification and material data sheet, but it is generally not the controlling design property for ductile materials — yield strength (which governs the onset of permanent deformation) is typically used for design, with tensile strength serving as a secondary check and a measure of margin against fracture.

Toughness — general materials science

A material's ability to absorb energy and deform plastically before fracturing, combining both strength and ductility. Toughness can be quantified as the area under a stress-strain curve (a measure of total energy absorption to fracture) or via impact testing (Charpy) and fracture mechanics (K_IC) methods. A material can be strong but not tough (high-carbon hardened steel, cast iron) or tough but not especially strong (some low-carbon steels), which is why toughness and strength must both be specified for safety-critical applications.

U

Ultrasonic Testing (UT) — ASTM E164 / E797

A nondestructive testing method that uses high-frequency sound waves transmitted into a material to detect internal discontinuities (cracks, laminations, voids, inclusions) and to measure material thickness, based on the reflection of sound waves at material interfaces or defects. UT is widely used for weld inspection, in-service corrosion thickness monitoring on pressure vessels and piping, and forging/casting quality verification.

V

Vickers Hardness (HV) — ASTM E384

A microhardness (or macrohardness, depending on load) test method using a square-based pyramidal diamond indenter, with hardness calculated from the diagonal length of the resulting indentation. Vickers testing works across a very wide hardness range on a single continuous scale (unlike Rockwell, which uses different scales for different hardness ranges) and, at low loads, can measure hardness of individual microstructural features, thin coatings, and case-hardened layers with high spatial resolution.

W

Weldability — AWS D1.1 / general welding metallurgy

The relative ease with which a material can be welded to produce a sound joint free of unacceptable defects and with adequate mechanical properties in both the weld metal and heat-affected zone, without excessive precautions (preheat, controlled interpass temperature, post-weld heat treatment). Weldability generally decreases with increasing carbon equivalent (a formula combining carbon content with other hardenability-boosting alloying elements), which is why high-carbon and highly alloyed steels typically require preheat and controlled cooling to avoid hydrogen-induced HAZ cracking.

Y

Yield Strength — ASTM E8

The stress at which a material begins to deform plastically (permanently), most commonly reported using the 0.2% offset method — the stress corresponding to the intersection of the stress-strain curve with a line parallel to the elastic portion, offset by 0.2% strain. Yield strength is the primary design property for most structural and mechanical engineering applications, since exceeding it produces permanent, unrecoverable deformation of the component even though the part has not yet fractured.

Young's Modulus — see Elastic Modulus

See Elastic Modulus (Young's Modulus, E).

Quick Reference Table — Selected Terms

Term	Primary Standard	Key Point
Tensile Strength (UTS)	ASTM E8	Peak engineering stress before fracture
Yield Strength	ASTM E8	0.2% offset; governs onset of permanent deformation
Elastic Modulus (E)	ASTM E111	Stiffness; unaffected by heat treatment
Hardness (Brinell/Rockwell/Vickers)	ASTM E10/E18/E384	Resistance to indentation; QC screening tool
Fracture Toughness (K_IC)	ASTM E399	Critical flaw-size design input
Charpy Impact Energy	ASTM E23	Screening for ductile-to-brittle transition
Fatigue / S-N Curve	ASTM E466/E468	Cyclic loading below static yield can still fail
Creep	ASTM E139	Time-dependent deformation at high temperature
Austenite / Ferrite / Martensite / Pearlite / Bainite	Fe-C phase diagram	Steel microstructures from different cooling rates
Stress Corrosion Cracking	ASTM G36	Tensile stress + corrosive environment, below yield
Sensitization	ASTM A262	Chromium carbide at grain boundaries from welding
Ultrasonic Testing	ASTM E164/E797	Sound-wave NDT for internal flaws and thickness