Metallurgy Terms - Industrial Metallurgists

Metallurgy Terms

List of metallurgy terms, definitions, and courses we offer to learn more

A1 critical temperature: Applies to the iron-carbon alloy and phase diagram. The temperature at which transformations from austenite to ferrite + cementite during cooling and from cementite + ferrite to austenite occur under controlled, very slow cooling and heating (i.e., equilibrium) conditions. More rapid cooling and heating rates, like those encountered during normal steel processing, change this critical temperature downward and upward, respectively. Additions of other alloying elements also will shift the A1 critical temperature. To learn more see our Steel Metallurgy course.

A3 critical temperature: Applies to the iron-carbon alloy and phase diagram. The temperature at which steel with less than 0.78% carbon becomes 100% austenite during heating and at which austenite starts to transform to ferrite during cooling. This applies to controlled, very slow heating and cooling (i.e., equilibrium) conditions. More rapid heating and cooling rates, like those encountered during normal steel processing, change this critical temperature upward and downward, respectively. Additions of other alloying elements also will shift the A3 critical temperature. To learn more see our Steel Metallurgy course.

Acm critical temperature: Applies to the iron-carbon alloy and phase diagram. The temperature at which steel with more than 0.78% carbon becomes 100% austenite during heating and at which austenite starts to transform to cementite during cooling. This applies to controlled, very slow heating and cooling (i.e., equilibrium) conditions. More rapid heating and cooling rates, like those encountered during normal steel processing, change this critical temperature upward and downward, respectively. Additions of other alloying elements also will shift the Acm critical temperature. To learn more see our Steel Metallurgy course.

Activation polarization: To learn more see our Metal Corrosion course.

Active metal: A metal with a tendency to corrode when exposed to a corrosive environment. The electrode potential of a metal exposed to a specific aqueous solution can be measured to determine whether the metal is electrochemically active or noble. Learn more from our Metal Corrosion course.

Aging: A heat treatment process that results in the transformation of a supersaturated solid solution to precipitate particles that strengthen an alloy. Aging is used with certain aluminum alloys, PH stainless steels, copper-beryllium alloys, and other alloys. To learn more see our Principles of Metallurgy, Aluminum Metallurgy, and Precipitation Strengthening courses.

Anneal: heat treatment used to reduce metal strength and increase metal ductility. All metals can be annealed after cold working. Carbon, low-alloy, and tool steels can also be full-annealed and spheroidize annealed. To learn more see our Principles of Metallurgy and our Steel Metallurgy courses.

Anode: In a corrosion cell, the anode is the metal or site on a metal where oxidation (loss of electrons) occurs. The anode has a more negative electrode potential than the cathode. To learn more see our Metal Corrosion course.

Aqueous corrosion: Metal Corrosion

Artificial aging: Elevated temperature aging of aluminum alloys to form strengthening precipitate particles. See "aging" definition. To learn more see our Principles of Metallurgy, Aluminum Metallurgy, and Precipitation Strengthening courses.

Austenite: A metallic, non-magnetic solid solution of iron with carbon and other alloying elements. In plain-carbon steel, austenite exists above 727 °C. Other steel alloys have different eutectoid temperatures. Austenite exists at room temperature in some stainless steels due to the presence of nickel which stabilizes austenite at lower temperatures. To learn more see our Steel Metallurgy course Stainless Steel Metallurgy video.

Austenitize: The process of heating steel to form 100% austenite. Involves heating steel above the Acm or A3 critical temperature. To learn more see our Steel Metallurgy course.

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Bainite: Combination of ferrite and cementite in steel. The ferrite has a plate or needle shape and cementite is present as small particles. Bainite forms when carbon, low-alloy, and tool steels are cooled fast enough to avoid forming pearlite (plates of cementite and ferrite between the plates). To learn more about bainite see our Steel Metallurgy course.

Barrier coating: Physical barrier between a metal and its environment. The purpose of a barrier coating is to prevent a corrosive element from contacting the substrate. Barrier coatings include a wide variety of metals, polymers, and ceramics that are applied to a substrate using a wide variety of techniques. To learn more see our Corrosion of Metals course.

Body-centered cubic: An arrangement of atoms in a unit cell, where atoms occupy sites at the corner of a cube and at the center of the cube. Examples of body centered cubic metals include iron and chromium. To learn more see our Principles of Metallurgy course.

Cathode: In a corrosion cell, the cathode is the metal or site on the metal where reduction (gain of electrons) occurs. The cathode has a more positive or less negative electrode potential than the anode. To learn more see our Corrosion of Metals course.

Cathodic protection: Cathodic protection is used to control the corrosion of a metal by making it the cathode of an electrochemical cell. One method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the protected metal. An example is applying a zinc coating over a steel component, with the steel being cathodically protected by the zinc. For structures, such as pipelines and storage tanks, an external DC electrical power source is used to provide sufficient current between the anode and cathode to ensure protections of the structure. To learn more see our Corrosion of Metals course.

Cementite (iron carbide): A compound of iron and carbon with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It is a hard, brittle material and has an orthorhombic crystal structure. It is a metallurgical phase in steels and cast irons, and can be present as plates or particles, depending on how a steel or cast iron is processed. To learn more about cementite see our Principles of Metallurgy and Steel Metallurgy courses.

Chromium carbide precipitates: Chromium carbide precipitates form in austenitic stainless steels exposed to between about 425 and 870 C in some ferritic stainless steels exposed to over about 925 C. The precipitates form on the grain boundaries. Their formation enables corrosion by intergranular corrosion when an alloy is exposed to a corrosive environment, such as environments that contain chloride ions. To learn more see our Corrosion of Metals course and Stainless Steel Metallurgy video.

Coarsening: Precipitation Strengthening, Principles of Metallurgy

Cold rolling: A metal forming process in which metal plate or sheet, at room temperature, is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The temperature of the metal is below its recrystallization temperature. To learn more see our Principles of Metallurgy course.

Cold working: Any metalworking process in which metal is shaped below its recrystallization temperature, usually at the ambient temperature. This includes rolling, drawing, bending, and shearing processes. Cold working causes metal grains to distort in the direction of the flow of the metal and causes work hardening, making the metal stronger and harder while reducing its ductility. To learn more see our Principles of Metallurgy course.

Concentration cell: Water and other aqueous solutions in contact with a metal will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points. Corrosion will occur at the area of low-oxygen concentration, which is anodic to the area of higher oxygen concentration. Crevice corrosion occurs due to the formation of a concentration cell.To learn more see our Corrosion of Metals course.

Corrosion cell: In a corrosion cell, metal ions formed from metal oxidation migrate from the anode to the cathode through the electrolyte. The electrons given off by this oxidation reaction move from the anode to the cathode through the electrical connection. At the cathode the electrons are consumed in reduction reactions. To learn more see our Corrosion of Metals course.

Corrosion reactions: Oxidation and reduction reactions take place in a corrosion cell. Oxidation occurs at the corrosion cell cathode and involves loss of electrons of the cathode metal atoms, and corrosion of the metal. Reduction occurs at the corrosion cell anode and involves ions in the electrolyte gaining electrons. To learn more see our Corrosion of Metals course.

Crevice corrosion: Crevice corrosion occurs in crevices between components and under surface debris, polymer coatings, adhesives. The driving force for the corrosion is the difference between the oxygen concentration inside the crevice and outside the crevice. To learn more see our Corrosion of Metals course.

Crystal structure: Describes the arrangement of atoms in a unit cell and the spacing between the atoms. Examples of arrangements of atoms include body-centered cubic (BCC) and face-centered cubic (FCC).  To learn more see our Principles of Metallurgy course.

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Dealloying: The selective leaching of the atoms of one element from an alloy. This results in the formation of a porous structure that is not strong enough to support the applied mechanical loads. One common example is dezincification of brass alloys used for plumbing, where the zinc is leached out of the alloy. To learn more see our Corrosion of Metals course.

Dezincification: A form of dealloying and occurs in brass alloys that contain more than 15% zinc. The zinc atoms leach out of the brass, resulting in the formation of a porous metal and reduction of the metal's strength. To learn more see our Corrosion of Metals course.

Differential aeration cell: A corrosion cell that arises due to differences of dissolved gas or ions in an electrolyte at adjacent areas on a piece of metal. A common form of a differential aeration cell is the oxygen concentration cell, which is caused by a difference in electrolyte oxygen concentration between two adjacent areas on a metal. When there is a difference in the concentration of dissolved oxygen at two points on a metal surface, the area in contact with electrolyte containing the higher concentration of dissolved oxygen will become cathodic to areas in contact with electrolyte with a lower concentration of dissolved oxygen. As a result, the anodic surface undergoes accelerated corrosion. Crevices between metal are susceptible to oxygen concentration cell formation. To learn more see our Corrosion of Metals course.

Diffusion: The motion of atoms through a metal. Diffusion is important for heat treating as the motion of atoms results in changes to a metal's microstructure, which results in changes to a metal's properties. To learn more see our Principles of Metallurgy course and Five Key Metallurgy Concepts video.

Diffusivity: The speed of atom motion through a metal during diffusion. Depends on the primary element in a metal, the element of the atom diffusing, and the metal's temperature. To learn more see our Principles of Metallurgy course.

Dislocation: A dislocation is a linear crystallographic defect or irregularity within a metal's crystal structure. The presence of dislocations strongly influences the mechanical properties of materials. Dislocations move when a stress is applied. The motion of dislocations allows plastic deformation to occur. Metal strength is modified by modifying ease of dislocation motion through a metal by modifying alloy composition and microstructure. To learn more see our Principles of Metallurgy course.

Drawing: Drawing is a manufacturing process that uses tensile forces to elongate metal. As the material is drawn (pulled), it stretches and becomes thinner, achieving a desired shape and thickness. Drawing is classified into two types: sheet metal drawing and wire, bar, and tube drawing. Sheet metal drawing involves plastic deformation over a curved axis. For wire, bar, and tube drawing, the starting stock is drawn through a die to reduce its diameter and increase its length. Drawing is usually performed at room temperature and is thus classified as a cold working process. During cold working, metal strength increases due to an increase of dislocation density. To learn more see our Principles of Metallurgy course.

Elastic deformation: Non-permanent and reversible deformation of a material under an applied load. When the applied load is removed the material returns to its original shape. Elastic deformation occurs when a material is exposed to an applied load that is less than the material's yield strength. Elastic deformation of metals and ceramics occurs at low strains. To learn more see our Tensile Testing course.

Elastic modulus: A material's resistance to non-permanent, or elastic, deformation. It is also known as Modulus of Elasticity. A metal's elastic modulus is measured based on the elastic portion of a stress-strain curve for a tensile specimen made of the metal of interest. To learn more see our Tensile Testing course.

Electrochemical reaction: An electrochemical reaction involves chemical reactions (oxidation and reduction) and the transfer of electrons (current flow) from the oxidizing atoms to the reducing atoms. To learn more see our Corrosion of Metals course.

Electrode potential: The voltage difference between a metal immersed in an electrolyte and a standard reference electrode or an electrode which has a stable electrode potential in the electrolyte. Electrode potential is a measure of the tendency of a metal to corrode in the electrolyte. To learn more see our Corrosion of Metals course.

Electrolyte: An electrolyte is a medium containing ions that are electrically conductive through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dissolving, the substance separates into cations and anions, which disperse uniformly throughout the solvent. Corrosion of a metal requires the metal be in contact with an electrolyte. To learn more see our Corrosion of Metals course.

Elongation: Measure of the total amount of deformation of a tensile specimen during a tensile test. It is a calculated value, based on the fractional or percent change in the specimen length during a tensile test. Elongation is an indication of a metal's ductility - greater elongation indicates greater ductility. To learn more see our Tensile Testing course.

Energy dispersive x-ray spectroscopy (EDS): An analytical technique used for the elemental analysis or chemical characterization of a sample. When used with a scanning electron microscope, EDS is used to measure and analyze x-rays emitted from a sample due to the interaction between the electron beam and the atoms at the sample's surface. Its characterization capabilities are due to every element emitting a unique set of peaks on its electromagnetic emission spectrum. To learn more see our Metal Failure Analysis course.

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Face centered cubic: An arrangement of atoms in a unit cell, where atoms occupy sites at the corner of a cube and at the center of each face of the cube. Examples of face centered cubic metals include aluminum, copper, and gold. To learn more see our Principles of Metallurgy course.

Ferrite: A steel phase consisting of a body-centered-cubic crystal structure of iron atoms. The carbon solubility in ferrite is 0.022% in iron-carbon alloys. To learn more see our Principles of Metallurgy and Steel Metallurgy course.

Filiform corrosion: A form of corrosion specific to painted or lacquered steel, aluminum and magnesium surfaces involving the formation and growth of filaments under the paint or lacquer. A filament consists of a head and a tail, which constitute a mobile electrochemical cell on the substrate surface. To learn more see our Corrosion of Metals course.

Full anneal: A heat treatment used with carbon, low-alloy, and tool steels to form a ferrite + coarse pearlite microstructure. The heat treatment involved heating the steel to form austenite and then slow cooling the steel to form ferrite + coarse pearlite. To learn more see our Steel Metallurgy course.

Galvanic corrosion: Galvanic corrosion occurs near the junction between two dissimilar metals.The driving force for the corrosion reaction is the difference in electrode potentials between the two metals. To learn more see our Corrosion of Metals course.

Galvanic series: List of metals and their electrode potentials in a specific electrolyte. The metals are listed in order of decreasing electrode potential. A galvanic series is used to predict the galvanic corrosion compatibility between two metals placed in contact in the electrolyte. To learn more see our Corrosion of Metals course.

General corrosion: General corrosion is also referred to as uniform corrosion. It is a form of corrosion that occurs over the entire exposed surface of a metal. Rust on a steel structure or the green patina on a copper roof are examples of uniform corrosion. The driving force for this type of corrosion is the electrochemical activity of the metal in the environment to which the metal is exposed. To learn more see our Corrosion of Metals course.

Grain boundary: The interface between two grains in a polycrystalline material. To learn more see our Principles of Metallurgy course.

Grain growth: Occurs during recrystallization anneal of a cold-worked metal. Grain growth occurs after the cold-worked grains are completely replaced by the newly formed grains. To learn more see our Principles of Metallurgy course and our Cold Working and Annealing video.

Grain: Individual crystal with a crystal structure that depends on the composition of the metal. Commercial metals are comprised of many grains (polycrystalline). To learn more see our Principles of Metallurgy course.

Impurity: Principles of Metallurgy

Intergranular corrosion: Interganular corrosion involves corrosion along the grain boundaries of the affected metal. The result is that the metal grains fall away and the metal is weakened. Austenitic stainless steels and precipitation strengthened aluminum alloys such as 2xxx alloys are examples of metals that can suffer from intergranular corrosion if the alloys are not properly processed and if they are exposed to corrosive environments. To learn more see our Corrosion of Metals course.

Interstitial diffusion: Diffusion of interstitial atoms in a metal. For example, carbon in steel. To learn more see our Principles of Metallurgy course.

Interstitial site: The space between lattice site atoms in a metal crystal. To learn more see our Principles of Metallurgy course.

Iron-Carbon Phase Diagram: Shows the equilibrium phases present in iron-carbon alloys at different temperatures. Used for understanding and predicting the phases that form in steel during heat treating. To learn more see our Principles of Metallurgy course.

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Localized corrosion: To learn more see our Corrosion of Metals course.

Low-alloy carbon steel: Steel alloys that exhibit mechanical properties superior to plain carbon steels through the addition of alloying elements such as chromium, molybdenum, and nickel. To learn more see our Steel Metallurgy course.

Martensite finish temperature: The temperature at which the transformation from austenite to martensite is complete when steel is cooled from austenitizing temperatures. To learn more see our Steel Metallurgy course.

Martensite start temperature: The temperature at which martensite starts to form when steel is cooled from austenitizing temperatures. To learn more see our Steel Metallurgy course.

Martensite: A metastable metallurgical phase formed using a heat treatment that prevents the formation of stable phases. In steels, martensite is a strong, hard phase with a body-centered tetragonal crystal structure. To learn more see our Principles of Metallurgy and Steel Metallurgy courses and our Steel Metallurgy Fundamentals – Steel Phases video.

Microstructure: The very small scale structure of a material and includes the phases present, their shape, and relative amounts and the grain structure. The microstructure of a metal strongly influences metal physical properties such as strength, toughness, ductility, hardness, fatigue resistance, and corrosion resistance. To learn more see our Principles of Metallurgy course.

Natural aging: Precipitation Strengthening, Principles of Metallurgy

Noble metal: To learn more see our Corrosion of Metals course.

Overage: For precipitation strengthening. Aging a metal past its peak strength. To learn more see our Principles of Metallurgy, Aluminum Metallurgy, and Precipitation Strengthening courses.

Oxidation: To learn more see our Corrosion of Metals course.

Passivation: A surface treatment process used with stainless steel to remove light surface contamination from machined stainless steel parts, including shop dirt, iron particles from cutting tools, and machining lubricants. This improves the steel's corrosion resistance. During the process the metal is immersed in hot acid to remove the contamination from the steel's surface.

Passive: To learn more see our Corrosion of Metals course.

Passivity: To learn more see our Corrosion of Metals course.

Pearlite: A steel microstructure composite consisting of parallel cementite plates with ferrite between the plates. To learn more see our Steel Metallurgy course.

Phase field boundary: Steel Metallurgy

Phase transformation: Principles of Metallurgy

Phases:  Principles of Metallurgy

Pitting corrosion: A localized form of corrosion by which pits form in the metal. Pitting occurs when a passive metal has local loss of passivity and the metal is attacked. To learn more see our Corrosion of Metals course.

Plain carbon steel: Steel Metallurgy

Plastic deformation: permanent, irreversible deformation or change in shape of a metal under the action of an applied force. For example, a piece of metal being bent or pounded into a new shape undergoes plastic deformation. To learn more see our Tensile Testing course.  

Polarization: To learn more see our Corrosion of Metals course.

Polycrystalline:  Principles of Metallurgy

Precipitate: Precipitation Strengthening, Principles of Metallurgy

Precipitation strengthening: Precipitation Strengthening, Principles of Metallurgy

Process anneal: Annealing

Quench: Steel Metallurgy

Recrystallization anneal: Annealing, Principles of Metallurgy

Recrystallization: Principles of Metallurgy

Reduction: To learn more see our Corrosion of Metals course.

Rephosphorized steel: Steel in which phosphorous is intentionally added as an alloying element for the purpose of improving the machinability of the steel. In steels that are not rephosphorized, phosphorous is an impurity. To learn more see our Steel Metallurgy course.   

Resulfurized steel: Steel in which sulfur is intentionally added as an alloying element for the purpose of improving the machinability of the steel. In steels that are not resulfurized, sulfur is an impurity. To learn more see our Steel Metallurgy course.

Rockwell hardness: Failure Analysis of Metal Fractures 

Sacrificial anode: To learn more see our Corrosion of Metals course.

Sacrificial coating: To learn more see our Corrosion of Metals course.

Scanning electron microscope: Failure Analysis of Metal Fractures

SEM: Failure Analysis of Metal Fractures

Sensitization: Intergranular Corrosion

Sensitized: Intergranular Corrosion

Single crystal: Principles of Metallurgy

Solid solution strengthening: Principles of Metallurgy

Solid solution: Principles of Metallurgy

Solution heat treatment: Precipitation Strengthening, Principles of Metallurgy

Spheroidize anneal: Annealing

Spheroidized cementite: Annealing

Strain: Tensile Testing

Stress corrosion cracking (SCC): The formation and growth of cracks due to the simultaneous action of a stress and a corrosive environment on a metal. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. To learn more see our Corrosion of Metals course.

Stress: Tensile Testing

Substitution: Principles of Metallurgy

Super-saturated solid solution: Precipitation Strengthening, Principles of Metallurgy

Temper: Steel Metallurgy

Tensile strength: This is the maximum stress that a tensile test specimen sustains during the test. See our Tensile Testing course to learn more.

Tensile test: A mechanical test during which a force is applied to a material specimen in order to measure the material's response to tensile stress. This test provides insight into the mechanical properties of a material including yield strength, tensile strength, and elongation. To learn more see our Tensile Testing course and this page.

Threshold stress: Stress Corrosion Cracking

Time-temperature-transformation diagram: Steel Metallurgy

Underage: Precipitation Strengthening, Principles of Metallurgy

Uniform corrosion: To learn more see our Corrosion of Metals course.

Unit cell: Principles of Metallurgy

Upper yield strength: Tensile Testing

Vacancy diffusion: Principles of Metallurgy

Vacancy: Principles of Metallurgy

Yield strength: Tensile Testing, Principles of Metallurgy

Yield stress: Principles of Metallurgy

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