Internal Structures of Materials – Engineering Notes

Internal Structures of Materials – Engineering Notes

Bonding

Ionic Bonding: An ionic bond is the force of attraction between oppositely charged ions in a compound.

Ion: An ion is an atom that has gained or lost an electron.

Cation (+Ve): A cation is 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 anion is an atom that has gained an electron. Non metals tend to gain electrons or are electron acceptors.

Metallic Bond: A rigid lattice of positive ions (cations) suspended in a sea of valence electrons. Metallic bonds produce high melting points, ductility and malleability in material.

Covalent Bonding: Covalent bonding is the sharing of electrons.

Structures

BCC (Base centred cubic)

  • Atoms are positioned at the corner of cube.
  • Are brittle and less ductile than FCC structures.
  • E.g. Tungsten, Sodium and Chromium.

FCC (Face centred cubic)

  • Atoms are more tightly packed (more dense) in FCC structure.
  • FCC structures are Malleable and ductile.
  • E.g. Gold, silver, Aluminium, Nickel and copper.
Base centred cubic structure vs. Face Centred cubic structure.

Base centred cubic structure vs. Face Centred cubic structure.

Crystalline Structures: Bonded in a repeated structure e.g. BCC & FCC (Metals).

Amorphous Structures: Have no bonding pattern, arrangement is random. E.g. glass.

Allotropy: Allotropy is the ability of a material to exist in more than one crystalline form. Steel has a BCC form when cold, but exists as FCC austenite when heated above the upper critical point.

Dendritic Growth

  • Solidification starts at one point forming a nucleus.
  • It grows in a similar manner to a seed, forming a seed crystal.
  • As it grows, branches called dendrites grow in a tree like structure.
  • The dendrites spread out and meet other dendrites and a crystal is formed.
  • Grain boundaries are produced by impurities bonded together.
Dendritic Growth.

Dendritic Growth.

Defects

Vacancy Site Defect

  • Forms when an atom is missing from a structure.
  • All the other atoms in the lattice are forced into the empty space and it becomes deformed in shape

Interstitial Defect: A foreign atom of a smaller element has moved into the space between atoms of the
lattice of the base metal.

Substitute Defect: A substitute defect is where a foreign atom, usually larger in size replaces an original
atom in the structure. This causes the lattice to become deformed.

 

Defects

Defects

Dislocation

  • A dislocation occurs when there is a line fault (incomplete layer of atoms) in the metal when a shearing force is applied.
  • This causes the fault to be pushed along until it reaches the edge of the crystal (grain boundary).
  • When this happens it causes the grain to be deformed or change shape.
  • A weakness is created which could lead to failure in the metal.
  • Dislocations can be prevented by alloying and cold working.
Dislocation

Dislocation

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