Metals and Alloys
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The unit cell of a face centred cubic structure
4 atoms at the corners of each face and 1 atom at the centre of each face. Connect the atoms of the faces using a dashed line.
Give examples of metals with a fcc structure
Aluminium, Copper, Gold, Nickel, Iron above 925oC
The unit cell of a body centred cubic structure
4 atoms at the corners of each face and 1 atom at the centre of each cube. Connect the diagonals of each face to the atom.
Give examples of metals with a bcc structure
Iron below 925oC, chromium, vanadium
What is a phase?
It is a region of a material with a specific chemical composition and atomic arrangement
What is a grain?
Grains are crystalline. They are areas of phase with different orientations in their crystal structure.
What is a grain boundary?
It is the interface between two areas of phase - each of the phases having different crystal structures. They are single phase of materials.
What is a dislocation?
A line running through the crystal along which atoms are misaligned.
What are the two types of dislocations?
• Edge dislocation - Screw dislocation
What is an edge dislocation?
It is an incomplete plane of atoms
What is a screw dislocation?
A dislocation in the lattice structure of a crystal in which the atoms are arranged in a helical pattern that is normal to the direction of stress.
Difference between edge and screw dislocations
Screw dislocation properties are like the edge dislocation properties but the lattice is parallel to the dislocation rather than normal to it
What are the importance of dislocations?
They cause the permanent deformation of crystalline materials and this arises from the motion of the dislocations.
The motion of dislocations.
Permanent deformation arises from the motion of the dislocation.
Do moving dislocations increase or decrease the strength?
Moving dislocations decrease the strength of the material.
Do moving dislocations increase or decrease the ductility of the metal?
It increases the ductility of the material.
When do dislocations move?
Dislocations move when the force per unit length exceeds the lattice resistance.
What is the dislocation density?
The dislocation density is the length of the dislocation per unit volume of a crystalline material.
Does the dislocation density increase or decrease with plastic deformation?
The dislocation density increases with plastic deformation
The dislocation density of annealed metals
10^6 to 10^8 cm/cm^3
The dislocation density of cold-worked metals
10^11 to 10^12 cm^-2
What is the exception to the dislocation density?
Silica crystals - these have a dislocation density of 10 to 10^4 cm^-2
Why is a pure metal soft and weak?
A pure metal is soft and weak because the dislocations move easily through the lattice.
How is the strength and hardness of a metal increased?
The strength of a metal is increased by increasing the number of dislocations in the material.