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The Structure of Materials | Definitions
Cation (+ve)
An atom that has lost an electron. Metals tend to form cations as they are prone to losing their outer electron to other ions.
Anion (-ve)
An atom that has gained an electron. Non-metals tend to gain electrons (electron acceptors).
Metallic Bond
Metals have atoms from which electrons are readily released. Positive ions are bonded together by a cloud of free electrons. These bonds are generally weaker than ionic. The metallic bond consists of positive atoms surrounded by a cloud of electrons. These free electrons are easily excited resulting in the material being very conductive of heat and electricity. Main properties of a metallic bond include ductility, malleability, high melting point, insoluble in water, good conductor of heat and electricity.
Ionic Bond
For an ionically bonded material, an atom of one element gives an electron to an atom of another element. As a result, one element consists of positive ions and the other, negative ions. Since unlike charged bodies attract, there is a force of attraction between the atoms forming an ionic bond. Sodium chloride is an example.
Covalent Bonding
In a covalent bond, atoms bond together by sharing electrons. The bonds between the atoms in an ethylene molecule are covalent bonds.
BCC Crystal Structure
The body-centred cubic (BCC) structure is not a close-packed arrangement, which allows easier activation and movement of atoms across each other. BCC metals tend to be more brittle. The metals based on this BCC structure include iron, tungsten, chromium, vanadium, sodium, potassium and molybdenum.
FCC Crystal Structure
The face-centred cubic (FCC) structure is close-packed and FCC metals are ductile.
Point Defect
A point defect is where one atom is missing in the crystal lattice structure.
Line Defect
A line defect is where a full line of atoms is missing.
Dendritic Growth
When small seed-like cells begin to form tree-like cells on cooling, the process is referred to as dendritic growth.
Sacrificial Protection
Sacrificial protection occurs when a reactive metal like zinc comes in contact with a metal to be protected, e.g. steel. The zinc becomes the anode and the steel, the cathode. Corrosion occurs at the anode and this is sacrificed, thus protecting the cathode. This means that the anode has to be replaced from time to time.
Ferrite
Iron that contains less than 0.02% carbon dissolved in solution is almost pure. It has a body-centred cubic structure.
Pearlite
A mixture of alternate layers of ferrite and cementite. It is mostly ferrite and gives off sheen similar to mother of pearl. Formed at 0.83% carbon.
Eutectic Point
A liquid to solid change occurs at this point. It happens at 1140˚C for the iron carbon alloy with 4.3% carbon. Liquid steel changes to solid austenite and cementite.
Eutectoid Point
A reaction that occurs in the solid state when solid austenite changes to solid pearlite. It happens at 723˚C for the iron carbon alloy with 0.83% carbon.
Allotropy
Allotropy is the ability of a material to exist in more than one crystalline structure. Steel is a good example as it exists as BCC when cold and FCC austenite when heated above its upper critical temperature. Carbon is another example as it exists as graphite or as diamond under extremely high pressure.
Allotropic
Any metal that can exist in more than one form is called an allotropic metal. Steel is in the body-centred cubic form at room temperature but changes to face-centred cubic above its critical temperature.
Partial Solubility Alloy
An alloy of two metals will dissolve in each other to a limited degree. The lead-tin alloy is an example.
Solid Solution Alloy
When two metals are completely soluble in each other in both the liquid and solid states. When viewed under a microscope, a solid solution appears like a pure metal, e.g. copper-nickel and iron-chromium.
Eutectic Alloy
A mixture of metals that is completely soluble in the liquid state but insoluble in the solid state, e.g. the cadmium and bismuth combination.
Solvus
The transition from one solid form to another solid form of an alloy is called the solvus line. It is shown on the lead-tin diagram.
Solidus
A solidus line represents the end of solidification in an alloy.
Re-Crystallisation
During cold working, distorted nuclei are formed and then replaced by new crystals during re-crystallisation. This is achieved by heating and cooling the component during annealing and allowing large grains to form.
Optical Pyrometer
This method compares the intensity of light from the filament of a lamp. Current flow from the lamp can be adjusted, using a variable resistor, to match the light from the furnace. When the filament seems to ‘disappear’, a temperature reading can be taken.
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The Structure of Materials | Definitions
Cation (+ve)
An atom that has lost an electron. Metals tend to form cations as they are prone to losing their outer electron to other ions.
Anion (-ve)
An atom that has gained an electron. Non-metals tend to gain electrons (electron acceptors).
Metallic Bond
Metals have atoms from which electrons are readily released. Positive ions are bonded together by a cloud of free electrons. These bonds are generally weaker than ionic. The metallic bond consists of positive atoms surrounded by a cloud of electrons. These free electrons are easily excited resulting in the material being very conductive of heat and electricity. Main properties of a metallic bond include ductility, malleability, high melting point, insoluble in water, good conductor of heat and electricity.
Ionic Bond
For an ionically bonded material, an atom of one element gives an electron to an atom of another element. As a result, one element consists of positive ions and the other, negative ions. Since unlike charged bodies attract, there is a force of attraction between the atoms forming an ionic bond. Sodium chloride is an example.
Covalent Bonding
In a covalent bond, atoms bond together by sharing electrons. The bonds between the atoms in an ethylene molecule are covalent bonds.
BCC Crystal Structure
The body-centred cubic (BCC) structure is not a close-packed arrangement, which allows easier activation and movement of atoms across each other. BCC metals tend to be more brittle. The metals based on this BCC structure include iron, tungsten, chromium, vanadium, sodium, potassium and molybdenum.
FCC Crystal Structure
The face-centred cubic (FCC) structure is close-packed and FCC metals are ductile.
Point Defect
A point defect is where one atom is missing in the crystal lattice structure.
Line Defect
A line defect is where a full line of atoms is missing.
Dendritic Growth
When small seed-like cells begin to form tree-like cells on cooling, the process is referred to as dendritic growth.
Sacrificial Protection
Sacrificial protection occurs when a reactive metal like zinc comes in contact with a metal to be protected, e.g. steel. The zinc becomes the anode and the steel, the cathode. Corrosion occurs at the anode and this is sacrificed, thus protecting the cathode. This means that the anode has to be replaced from time to time.
Ferrite
Iron that contains less than 0.02% carbon dissolved in solution is almost pure. It has a body-centred cubic structure.
Pearlite
A mixture of alternate layers of ferrite and cementite. It is mostly ferrite and gives off sheen similar to mother of pearl. Formed at 0.83% carbon.
Eutectic Point
A liquid to solid change occurs at this point. It happens at 1140˚C for the iron carbon alloy with 4.3% carbon. Liquid steel changes to solid austenite and cementite.
Eutectoid Point
A reaction that occurs in the solid state when solid austenite changes to solid pearlite. It happens at 723˚C for the iron carbon alloy with 0.83% carbon.
Allotropy
Allotropy is the ability of a material to exist in more than one crystalline structure. Steel is a good example as it exists as BCC when cold and FCC austenite when heated above its upper critical temperature. Carbon is another example as it exists as graphite or as diamond under extremely high pressure.
Allotropic
Any metal that can exist in more than one form is called an allotropic metal. Steel is in the body-centred cubic form at room temperature but changes to face-centred cubic above its critical temperature.
Partial Solubility Alloy
An alloy of two metals will dissolve in each other to a limited degree. The lead-tin alloy is an example.
Solid Solution Alloy
When two metals are completely soluble in each other in both the liquid and solid states. When viewed under a microscope, a solid solution appears like a pure metal, e.g. copper-nickel and iron-chromium.
Eutectic Alloy
A mixture of metals that is completely soluble in the liquid state but insoluble in the solid state, e.g. the cadmium and bismuth combination.
Solvus
The transition from one solid form to another solid form of an alloy is called the solvus line. It is shown on the lead-tin diagram.
Solidus
A solidus line represents the end of solidification in an alloy.
Re-Crystallisation
During cold working, distorted nuclei are formed and then replaced by new crystals during re-crystallisation. This is achieved by heating and cooling the component during annealing and allowing large grains to form.
Optical Pyrometer
This method compares the intensity of light from the filament of a lamp. Current flow from the lamp can be adjusted, using a variable resistor, to match the light from the furnace. When the filament seems to ‘disappear’, a temperature reading can be taken.
Cut dotted horizontal lines. Fold vertical line.

Cation (+ve)
An atom that has lost an electron. Metals tend to form cations as they are prone to losing their outer electron to other ions.
Anion (-ve)
An atom that has gained an electron. Non-metals tend to gain electrons (electron acceptors).
Metallic Bond
Metals have atoms from which electrons are readily released. Positive ions are bonded together by a cloud of free electrons. These bonds are generally weaker than ionic. The metallic bond consists of positive atoms surrounded by a cloud of electrons. These free electrons are easily excited resulting in the material being very conductive of heat and electricity. Main properties of a metallic bond include ductility, malleability, high melting point, insoluble in water, good conductor of heat and electricity.
Ionic Bond
For an ionically bonded material, an atom of one element gives an electron to an atom of another element. As a result, one element consists of positive ions and the other, negative ions. Since unlike charged bodies attract, there is a force of attraction between the atoms forming an ionic bond. Sodium chloride is an example.
Covalent Bonding
In a covalent bond, atoms bond together by sharing electrons. The bonds between the atoms in an ethylene molecule are covalent bonds.
BCC Crystal Structure
The body-centred cubic (BCC) structure is not a close-packed arrangement, which allows easier activation and movement of atoms across each other. BCC metals tend to be more brittle. The metals based on this BCC structure include iron, tungsten, chromium, vanadium, sodium, potassium and molybdenum.
FCC Crystal Structure
The face-centred cubic (FCC) structure is close-packed and FCC metals are ductile.
Point Defect
A point defect is where one atom is missing in the crystal lattice structure.
Line Defect
A line defect is where a full line of atoms is missing.
Dendritic Growth
When small seed-like cells begin to form tree-like cells on cooling, the process is referred to as dendritic growth.
Sacrificial Protection
Sacrificial protection occurs when a reactive metal like zinc comes in contact with a metal to be protected, e.g. steel. The zinc becomes the anode and the steel, the cathode. Corrosion occurs at the anode and this is sacrificed, thus protecting the cathode. This means that the anode has to be replaced from time to time.
Ferrite
Iron that contains less than 0.02% carbon dissolved in solution is almost pure. It has a body-centred cubic structure.
Pearlite
A mixture of alternate layers of ferrite and cementite. It is mostly ferrite and gives off sheen similar to mother of pearl. Formed at 0.83% carbon.
Eutectic Point
A liquid to solid change occurs at this point. It happens at 1140˚C for the iron carbon alloy with 4.3% carbon. Liquid steel changes to solid austenite and cementite.
Eutectoid Point
A reaction that occurs in the solid state when solid austenite changes to solid pearlite. It happens at 723˚C for the iron carbon alloy with 0.83% carbon.
Allotropy
Allotropy is the ability of a material to exist in more than one crystalline structure. Steel is a good example as it exists as BCC when cold and FCC austenite when heated above its upper critical temperature. Carbon is another example as it exists as graphite or as diamond under extremely high pressure.
Allotropic
Any metal that can exist in more than one form is called an allotropic metal. Steel is in the body-centred cubic form at room temperature but changes to face-centred cubic above its critical temperature.
Partial Solubility Alloy
An alloy of two metals will dissolve in each other to a limited degree. The lead-tin alloy is an example.
Solid Solution Alloy
When two metals are completely soluble in each other in both the liquid and solid states. When viewed under a microscope, a solid solution appears like a pure metal, e.g. copper-nickel and iron-chromium.
Eutectic Alloy
A mixture of metals that is completely soluble in the liquid state but insoluble in the solid state, e.g. the cadmium and bismuth combination.
Solvus
The transition from one solid form to another solid form of an alloy is called the solvus line. It is shown on the lead-tin diagram.
Solidus
A solidus line represents the end of solidification in an alloy.
Re-Crystallisation
During cold working, distorted nuclei are formed and then replaced by new crystals during re-crystallisation. This is achieved by heating and cooling the component during annealing and allowing large grains to form.
Optical Pyrometer
This method compares the intensity of light from the filament of a lamp. Current flow from the lamp can be adjusted, using a variable resistor, to match the light from the furnace. When the filament seems to ‘disappear’, a temperature reading can be taken.
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The Structure of Materials
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