Metals and Alloys Flashcards
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.
Why does the number of dislocations increase the strength?
Because it is more likely that the dislocation will intercept with another dislocation when it moves.
Does the movement of dislocations involve plastic deformation or elastic deformation?
Plastic deformation
Four strength-hardening mechanisms
- Cold Working/ Work Hardening
- Recrystallisation
- Solid Solution Hardening
- Precipitation Hardening
What is cold work?
Cold work is the plastic deformation of a material at a temperature below the recrystallization temperature.
Give examples of work hardening
Rolling, forging
Why does work hardening increase the strength?
It increases the number of dislocations in the material, increases the dislocation density, this means that it is more likely when a dislocation moves that it will be intercepted by other dislocations.
Which metals and alloys can be strengthened by work hardening.
All metals and alloys, including steel containing carbon
Why does recrystallization increase the strength?
Because it reduces the size of the grains. Increases the density of grain boundaries and dislocations cannot pass through grain boundaries.
Why does reducing the size of the grains, why is the strength increased?
Because of the empirical Hall-Petch relationship. There is a lower likelihood that grains will be oriented in the same direction. This means that the distance that grains can slip will be decreased and this increases the strength of the material.
What are the three stages of recrystallization for a work hardened metal?
- Recovery
- Recrystallisation
- Grain Growth
Describe recovery
During recovery, some of the stored internal energy is relieved through dislocation motion due to its enhanced atomic diffusion at elevated temperatures. This leads to a reduction in the number of dislocations.
Describe recrystillation.
After recovery is complete then the grains are still in a high energy state. Recrystallisation is the formation of a new set of strain free, uniaxial grains with low dislocation densities. The driving force to produce new grain structure is the internal energy difference between strained and unstrained material. The new grains form on small nuclei and grow until they consume the parent material.
Describe grain growth
After recrystallization, the strain-free grains will continue to grow if the material is left at elevated temperatures. As the grains increase in size, the total boundary area decreases as does the total energy. Large grains grow at the expense of the smaller grains.
What is precipitation hardening?
Precipitation hardening results in the formation of secondary phases. Small particles of a second phase are dispersed through the primary lattice. Dislocations are inhibited when they come in contact with these small particles.
Give an example of precipitation hardening
The distribution of gamma prime in a gamma matrix of the nickel superalloy used in airplane turbine blades.
How does solid solution hardening increase the strength of the material?
There are interactions between the impurity atoms and dislocations. This increases the strength. Solid solution hardening is the deliberate addition of impurities or alloying.
What are the four types of solid solution hardening
- vacancy - vacancies play a key role in creep and diffusion
- Substitutional impurity atoms - a different type of atom is substituted into the place of the primary atoms in the lattice. The atoms are usually of a different size which distort the lattice.
- Self-interstitial - an interstitial atom occupies a place outside the normal lattice position. It is the same type of atoms to the others. There is a change in the coordination of the atoms around the defect.
- Interstitial atoms - squeeze into the spaces between the host atoms. These rarely have the same size and distort the lattice.
Give an example of substitutional impurity
Carbon in iron lattice
Give an example of an interstitial impurity
Chromium in an iron lattice
What is the relationship between the number of impurities and the strength of the material?
An increase the number of impurities, increases the yield strength and the ultimate tensile strength of the metal.
What is annealing?
This is the softening of a metal by allowing it to recrystallize. This is a heat treatment in which the mechanical properties and the microstructure are altered.
What properties change with a finer grain size?
The strength according to the Hall Petch empirical formula. The fracture toughness of the material.
What system displays complete solid-solid solubility?
Copper-Nickel
What system displays partial solid-solid solubility?
Lead-Tin
What system displays zero solid-solid solubility?
Iron-Carbon
What is the eutectic point?
The eutectic point is the point at which a liquid transforms isothermally and adiabatically into two solid phases.
What is the eutectoid?
A eutectoid reaction in which a solid phase transforms reversibly and isothermally into two solid phases.
What phases are present in the copper-nickel system?
Alpha, alpha+liquid, liquid
What is the alpha phase in the lead-tin phase diagram?
Tin dissolved in lead
What is the beta phase in the lead-tin phase diagram?
Lead dissolved in tin
Which part of the graph is bigger and why?
Tin dissolved in lead takes up a larger portion of the graph because tin atoms are smaller than lead atoms. It is easier for tin to be dissolved in lead than the other way round.
Why is the copper-nickel system simple?
Because the copper and nickel atoms are about the same size.
What is the solidus line?
The solidus line is the line below which there is only solid phase present
What is the liquidus line?
The liquidus line is the line above which only a liquid phase is present.
What is at the intersection of the solidus and liquidus lines?
At the intersection of these two lines is the melting point of the pure component.
