A comprehensive reference glossary of metallurgical terms and definitions, written for engineers, students, and materials professionals. Use the alphabetical index below to find any term quickly.

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A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · Q · R · S · T · U · V · W · X · Y · Z

A

Age Hardening (Precipitation Hardening)
A heat treatment process that increases the yield strength of an alloy by causing the precipitation of fine particles within the metal’s grain structure. Commonly applied to aluminium, nickel, and titanium alloys. See also: Precipitation Hardening.
Alloy
A metallic material composed of two or more elements, at least one of which is a metal, combined to improve properties such as strength, corrosion resistance, or hardness compared to the pure base metal. Examples: steel (Fe + C), brass (Cu + Zn), Inconel (Ni + Cr + Fe).
Annealing
A heat treatment process involving heating a metal to a specific temperature, holding, and slow cooling — used to soften the material, relieve internal stresses, refine grain size, and improve ductility and toughness. Types include full annealing, process annealing, and stress-relief annealing.
Austenite
A face-centred cubic (FCC) solid solution of carbon in iron, stable above the A1 transformation temperature (~727°C for plain carbon steel). Austenite is the starting microstructure for most steel heat treatments (quenching, normalising, carburising).
Austenitising
The process of heating steel to above its upper critical temperature (Ac3) to fully transform the microstructure to austenite prior to quenching or controlled cooling. Typical austenitising temperatures range from 820°C to 950°C depending on steel grade.

B

Bainite
A microstructure formed in steel by isothermal transformation of austenite at temperatures between the pearlite and martensite transformation ranges (approximately 250–550°C). Bainite offers a good combination of strength and toughness. Upper bainite (feathery) forms at higher temperatures; lower bainite (acicular) forms at lower temperatures.
BCC (Body-Centred Cubic)
A crystal structure in which atoms are located at each corner of a cube and one atom at the centre of the cube. Iron (ferrite) below 912°C adopts the BCC structure. BCC metals typically have lower ductility than FCC metals at low temperatures (ductile-to-brittle transition).
Brittle Fracture
A fracture mode characterised by rapid crack propagation with little or no plastic deformation prior to failure. Common in BCC metals at low temperatures, in high-strength steels, and in hydrogen-embrittled materials. Fracture surfaces typically appear flat and crystalline (cleavage or intergranular).

C

Carbon Equivalent (CE)
A formula used to assess the weldability of steel by expressing the combined effect of carbon and other alloying elements on hardenability. The IIW formula is: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. A CE below 0.40 generally indicates good weldability without preheat.
Carbide
A compound formed between carbon and a metallic element (e.g., iron carbide Fe₃C — cementite; chromium carbide Cr₂₃C₆; titanium carbide TiC). Carbides increase hardness and wear resistance but can reduce toughness and, if precipitated at grain boundaries (sensitisation), reduce corrosion resistance.
Cementite (Fe₃C)
Iron carbide — a hard, brittle intermetallic compound containing 6.67 wt% carbon, found in steels and cast irons. Cementite is a component of pearlite (alternating lamellae of ferrite and cementite) and forms as a network around prior austenite grain boundaries in hypereutectoid steels.
Charpy Impact Test
A standardised test (ISO 148 / ASTM E23) that measures the energy absorbed by a notched specimen when fractured by a swinging pendulum. Used to assess toughness and ductile-to-brittle transition temperature. Results expressed in Joules (J). Common specimen sizes: Charpy V-notch (CVN) 10×10×55 mm.
Corrosion
The degradation of a metal through electrochemical or chemical reaction with its environment. Common forms include uniform corrosion, pitting, crevice corrosion, galvanic corrosion, stress corrosion cracking (SCC), and microbiologically influenced corrosion (MIC).
Creep
The time-dependent plastic deformation of a material under sustained stress at elevated temperatures (typically above 0.4 Tm, where Tm is the absolute melting temperature). Critical consideration in high-temperature applications such as gas turbines, boilers, and pressure vessels.

D

Decarburisation
The loss of carbon from the surface layer of steel during heating in an oxidising or decarburising atmosphere (air, CO₂, water vapour). Results in a soft surface layer with reduced hardness and fatigue resistance. Prevented by use of controlled protective atmospheres or vacuum furnaces.
Dendrite
A tree-like crystal structure that forms during solidification of metals as the primary solid phase grows into the liquid. Dendritic solidification leads to chemical segregation (coring) and directionality of properties that must be addressed by subsequent homogenisation heat treatment or hot working.
Ductility
The ability of a material to undergo plastic deformation before fracture, measured as percentage elongation or reduction in area in a tensile test. High ductility enables forming operations and provides warning before fracture. Ductility decreases with increasing strength, decreasing temperature, and hydrogen embrittlement.

E

Elongation (%El)
A measure of ductility expressed as the percentage increase in gauge length of a tensile specimen at fracture. Measured per ASTM E8 or ISO 6892-1. Typical values: structural steel ~20–30%; high-strength steel ~8–14%; cast iron ~0–2%.
Eutectic
A specific composition in a binary (or higher) alloy system at which the liquid transforms directly to two (or more) solid phases simultaneously at a single temperature (the eutectic temperature). The eutectic point represents the lowest melting point in the system. Example: Fe-C eutectic at 4.3 wt% C, 1147°C (ledeburite).
Eutectoid
A solid-state transformation analogous to the eutectic, where one solid phase transforms to two different solid phases at a specific composition and temperature. Example: the Fe-C eutectoid at 0.77 wt% C, 727°C (austenite → pearlite = ferrite + cementite).

F

FCC (Face-Centred Cubic)
A crystal structure with atoms at each corner and at the centre of each face of the unit cell. Metals with FCC structure (austenite, copper, aluminium, nickel) typically have good ductility and toughness at low temperatures due to the large number of available slip systems (12).
Ferrite (α-iron)
The BCC solid solution of carbon in iron, stable below 912°C. Ferrite is soft (HV 70–100), highly ductile, and magnetic. In carbon steels, ferrite forms during cooling from austenite and is the matrix phase in low-carbon steels and the majority phase in structural steels.
Fracture Toughness (KIc)
A material property measuring resistance to crack propagation under plane-strain conditions. Expressed in MPa√m. Measured per ASTM E399. A key parameter in damage-tolerance design and fitness-for-service assessments. Higher KIc = better resistance to catastrophic fracture in the presence of cracks.

G

Galvanic Corrosion
Accelerated corrosion of the less noble (more anodic) metal when two dissimilar metals are in electrical contact in a conductive electrolyte. Severity depends on the potential difference between the metals (galvanic series), area ratio, and conductivity of the electrolyte. Prevented by insulation, coatings, or material selection.
Grain Boundary
The interface between adjacent grains of different crystallographic orientation in a polycrystalline metal. Grain boundaries affect mechanical properties (Hall-Petch relationship), diffusion rates, corrosion behaviour (intergranular corrosion, sensitisation), and creep behaviour. Grain boundary engineering is used to optimise these properties.
Grain Size
A measure of the average size of crystalline grains in a polycrystalline metal, measured by the ASTM grain size number (G) per ASTM E112, or by mean intercept length. Finer grain size generally increases yield strength (Hall-Petch), toughness, and fatigue resistance, but reduces creep resistance at high temperatures.

H

HAZ (Heat Affected Zone)
The region of the base metal adjacent to a weld that has not melted but has experienced microstructural changes due to the heat cycle. The HAZ is divided into sub-zones (coarse-grained HAZ, fine-grained HAZ, intercritical HAZ, subcritical HAZ) each with distinct properties. The CGHAZ is typically the most critical for toughness.
Hardenability
The ability of a steel to be hardened by quenching, expressed as the depth of hardening achievable under given quenching conditions. Assessed by the Jominy end-quench test (ASTM A255 / ISO 642). Higher alloying (Mn, Cr, Mo, Ni, B) increases hardenability. Not to be confused with hardness (which is a property of the hardened surface).
Heat Treatment
The controlled heating and cooling of a metal to achieve desired microstructure and properties without changing the overall shape. Key processes: annealing, normalising, hardening (quenching), tempering, solution treatment, ageing, case hardening (carburising, nitriding).
HCP (Hexagonal Close-Packed)
A crystal structure in which atoms are arranged in layers with hexagonal symmetry, with atoms in alternating positions. HCP metals (titanium at low temperature, zinc, magnesium, cobalt) have fewer slip systems than FCC metals, leading to lower ductility and stronger deformation anisotropy.

I

Impact Toughness
The ability of a material to absorb energy during rapid (impact) loading before fracture. Measured by Charpy (CVN) or Izod tests. Critical for applications subject to dynamic loading, low-temperature service, or shock. Minimum CVN requirements are specified in many pressure vessel, pipeline, and structural steel standards.
Intergranular Corrosion
Preferential corrosion attack along grain boundaries, typically due to chromium depletion (sensitisation) in austenitic stainless steels heated in the 425–850°C range. Prevented by low-carbon grades (304L, 316L), stabilised grades (321, 347), or solution annealing after welding.

J

Jominy Test
A standardised end-quench test (ASTM A255 / ISO 642) for measuring hardenability of steel. A round bar specimen is austenitised, then quenched at one end with a water jet. Hardness is measured along the bar length, producing a hardenability curve (Jominy curve). Used to predict through-hardening behaviour in actual components.

L

Liquidus
In a phase diagram, the liquidus line represents the temperature above which an alloy is completely liquid. During cooling, solidification begins when the liquidus temperature is reached. The region between the liquidus and solidus is the two-phase (liquid + solid) mushy zone.

M

Martensite
A very hard, body-centred tetragonal (BCT) microstructure formed by diffusionless transformation of austenite during rapid quenching. Carbon atoms are trapped in the distorted lattice, producing extreme hardness (up to ~65 HRC in high-carbon steels) but brittleness. Tempering is always required after quenching to restore toughness.
Martensite Start Temperature (Ms)
The temperature at which martensite starts to form upon quenching of austenite. A function of composition — carbon, manganese, chromium, nickel, molybdenum all lower Ms. Significant for understanding distortion, cracking risk during quenching, and retained austenite content.
Microstructure
The structural features of a metal observable under a microscope, including grain size and morphology, phase distribution, precipitates, inclusions, and defects. Microstructure is the direct result of composition and processing history, and determines all mechanical and physical properties.

N

Normalising
A heat treatment involving austenitising followed by air cooling. Used to refine grain size, homogenise microstructure, and improve mechanical properties after hot working. Produces a finer, more uniform microstructure than full annealing and results in somewhat higher strength.

P

Pearlite
A lamellar (layered) microstructure consisting of alternating plates of ferrite and cementite (Fe₃C), formed by the eutectoid transformation of austenite at ~727°C. Fine pearlite (fast cooling) is harder and stronger than coarse pearlite (slow cooling). Pearlite has good strength and wear resistance.
Phase Diagram
A graphical representation of the stable phases present in a material system as a function of temperature and composition (and sometimes pressure). The iron-carbon (Fe-C) equilibrium diagram is the most important in ferrous metallurgy. Phase diagrams guide alloy design, heat treatment selection, and welding metallurgy.
Precipitation Hardening
See Age Hardening. A strengthening mechanism involving the controlled precipitation of fine coherent particles within the matrix to impede dislocation movement. The three stages are: solution treatment → quenching → ageing (natural or artificial).
PWHT (Post Weld Heat Treatment)
Heat treatment applied after welding to reduce residual stresses, temper hard microstructures in the HAZ, reduce hydrogen risk, and improve toughness and dimensional stability. Typically involves stress-relief tempering at 580–720°C for carbon and low-alloy steels. Required by many pressure vessel and pipeline standards (ASME VIII, AS 4458, BS PD 5500).

Q

Quenching
Rapid cooling of an austenitised steel (or other alloy) to suppress diffusion-controlled transformations and produce martensite (or other non-equilibrium microstructures). Quench media include water, brine, oil, polymer solutions, and gas, in order of decreasing severity. Quench cracking and distortion risk increase with section size and hardenability.

R

Recrystallisation
The formation of a new strain-free grain structure from a deformed (cold-worked) metal upon heating above the recrystallisation temperature (typically 0.3–0.5 Tm). Recrystallisation restores ductility, reduces strength and hardness, and is the basis of annealing after cold working. Grain size after recrystallisation depends on deformation amount and annealing temperature.
Residual Stress
Internal stresses remaining in a material after manufacturing or processing (welding, quenching, machining, forming) in the absence of external loading. Tensile residual stresses are detrimental — they promote fatigue crack initiation, stress corrosion cracking, and distortion. Reduced by PWHT, vibratory stress relief, or shot peening (which introduces beneficial compressive surface stresses).

S

SCC (Stress Corrosion Cracking)
A form of environmentally assisted cracking caused by the combined action of tensile stress (applied or residual) and a specific corrosive environment. Classic examples: austenitic stainless steel in chloride environments; brass in ammonia; high-strength steels in H₂S (sulphide stress cracking). Prevention: material selection, stress relief, coatings, cathodic protection.
Sensitisation
The precipitation of chromium carbides at grain boundaries in austenitic stainless steels when heated in the 425–850°C sensitisation range (e.g., during welding). This depletes chromium at grain boundaries below the ~11% minimum needed for passivity, causing susceptibility to intergranular corrosion. Prevented by using low-carbon (L-grade) or stabilised stainless steels.
Solidus
In a phase diagram, the solidus line represents the temperature below which an alloy is completely solid. Above the solidus, some liquid is present. The solidus temperature is critical for hot working limits and casting processes.
Solid Solution Strengthening
A strengthening mechanism in which solute atoms dissolve in the metal lattice and impede dislocation movement by causing lattice distortion. Substitutional (same lattice site, e.g. Mn, Cr in iron) and interstitial (between atoms, e.g. C, N in iron) solid solutions both increase strength, with interstitial elements having a larger effect per atom.

T

Tempering
A heat treatment applied after quenching to reduce brittleness and internal stresses in martensite by reheating to a temperature below Ac1 (typically 150–700°C). Tempering reduces hardness and strength while improving ductility and toughness. Higher tempering temperatures produce greater toughness at the expense of strength (tempered martensite embrittlement must be avoided at 300–350°C).
TTT Diagram (Time-Temperature-Transformation)
An isothermal transformation diagram showing the start and finish of austenite transformation to various products (pearlite, bainite, martensite) as a function of temperature and time. Used to design heat treatment cycles and predict microstructures during quenching. The continuous-cooling transformation (CCT) diagram is more directly applicable to practical heat treatments.
Toughness
The ability of a material to absorb energy and plastically deform without fracturing — a combination of strength and ductility. Measured as the area under the stress-strain curve (modulus of toughness) or by impact tests (Charpy CVN). Toughness is reduced by low temperature, high strain rates, stress concentrations, and hydrogen embrittlement.

U

UTS (Ultimate Tensile Strength)
The maximum stress a material can sustain under uniaxial tensile loading before necking begins. Expressed in MPa or N/mm². Measured per ASTM E8 or ISO 6892-1. UTS is used in many design codes (pressure vessels, lifting equipment) as a basis for allowable stress. Not to be confused with yield strength (the stress at the onset of plastic deformation).

W

Weldability
The ability of a metal to be welded under specific conditions to produce a joint meeting required mechanical and integrity properties. Influenced by composition (carbon equivalent), thickness, heat input, preheat, and post-weld treatment. Assessed by weldability tests (Implant test, CTS test, Tekken test) or predicted from CE formulae.
Work Hardening (Strain Hardening)
The increase in strength and hardness of a metal as a result of plastic deformation at temperatures below the recrystallisation temperature. Caused by increasing dislocation density and entanglement. Exploited in cold-working processes; reversed by annealing. Rate of work hardening expressed by the strain-hardening exponent (n) in the Hollomon equation: σ = Kεⁿ.

Y

Yield Strength (YS / Rp0.2)
The stress at which a material begins to deform plastically. For materials without a clear yield point (most non-ferrous alloys and high-strength steels), the 0.2% proof stress (Rp0.2) is used — the stress causing 0.2% permanent strain. The primary design parameter for structural and pressure vessel applications. Measured per ASTM E8 or ISO 6892-1.
Young’s Modulus (E)
The ratio of stress to strain in the elastic (linear) region of the stress-strain curve — a measure of stiffness. For steel, E ≈ 200–210 GPa; aluminium ≈ 69 GPa; titanium ≈ 116 GPa. Young’s modulus is relatively insensitive to heat treatment and microstructure — it depends primarily on atomic bonding.

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