imperfection Flashcards
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Reduce grain size
The size of grains, average grain diameter, influences mechanical properties.
Adjacent grains normally have different atomic alignment and packing densities are not as high at the grain boundary interface.
- Grain boundaries are barriers to slip. The slip plane has to change direction across the grain boundary
- Barrier “strength” increases with increasing angle of misorientation.
- If dislocations accumulate at grain boundaries stress can build up and induce new dislocations in adjacent grains.
Fine-grained material (smaller grains) is harder and stronger than coarsely grained counterparts as the higher total grain boundary area impedes dislocation
Grain size can be regulated by solidification rate from the liquid phase and through heat treatment – annealing. Toughness as well as strength improved by grain size reduction.
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Formation of solid solutions
Lattice strain exists around dislocations:
The addition of large or small atoms into vacancies within the lattice structure can alter the strain fields around dislocations.
- Impurity atoms distort the lattice & generate lattice strains.
- These strains can act as barriers to dislocation motion.
- Small impurities tend to concentrate at dislocations (regions of compressive strains).
- Impurity partially cancels dislocation compressive strains by imposing tensile strains on the crystal lattice.
- This results in reduced dislocation mobility and increases strength.
- Large impurities tend to concentrate at regions of tension caused by dislocations.
- Large impurities impose compressive strains on the crystal lattice.
- In addition to reduced dislocation mobility, due to increased lattice stability, higher stress is needed to first initiate plastic deformation.
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Cold working (strain hardening)
- Ductile metals become harder and stronger as they are plastically deformed.
- Typically at room temperature (for most metals), but certainly well below the melting temperature (i.e. CW has a very low energy input).
- Dislocation structure in Ti after cold working.
- Dislocations entangle with one another during cold work.
- Areas of compression and tension around the dislocation (within the lattice) repel one another.
- Dislocation motion becomes more difficult resulting in a harder and stronger metal
As CW procedures are typically carried out at room temp, energy from the mechanical processes can be stored as stress within the crystal lattice.
• Electrical conductivity and corrosion properties can also be adversely modified by cold work.
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Precipitation strengthening
Express the Hall-Petch equation used to calculate yield strength and use it to resolve the following issue
Express the equation used to calculate the degree of plastic deformation is expressed as a percentage of the cold work
Express the equation used to calculate the dislocation density (pd)
Identify and discuss the annealing method:
Recovery
Identify the three methods of annealing
There are three annealing stages:
- Recovery
- Recrystallization
- Grain Growth
Identify and discuss the annealing method:
Recovery
Identify and discuss the annealing method:
Grain Growth
• At longer times, average grain size increases.
- Small grains shrink (and ultimately disappear)
- Large grains continue to grow
An empirical relationship exists between grain diameter (d), temperature (“K”) and time (t)
Identify and discuss the annealing method:
Recrystallization
• New grains are formed that:
- have low dislocation densities
- are small in size
- consume and replace parent cold-worked grains
Define Recrystallization Temperature
- Hot working → deformation above TR
- Cold working → deformation below TR (typically room temperature)
A cylindrical rod of brass originally 10 mm (0.39 in) in diameter is to be cold worked and the circular cross section maintained during deformation (i.e. drawn). A cold-worked tensile strength in excess of 380 MPa (55,000 psi) and a ductility of at least 15 %EL are required. Furthermore, the final diameter must be 7.5 mm (0.30 in).
Describe the process required to meet the above specification.
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Reduce grain size
The size of grains, average grain diameter, influences mechanical properties.
Adjacent grains normally have different atomic alignment and packing densities are not as high at the grain boundary interface.
- Grain boundaries are barriers to slip. The slip plane has to change direction across the grain boundary
- Barrier “strength” increases with increasing angle of misorientation.
- If dislocations accumulate at grain boundaries stress can build up and induce new dislocations in adjacent grains.
Fine-grained material (smaller grains) is harder and stronger than coarsely grained counterparts as the higher total grain boundary area impedes dislocation
Grain size can be regulated by solidification rate from the liquid phase and through heat treatment – annealing. Toughness as well as strength improved by grain size reduction.
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Formation of solid solutions
Lattice strain exists around dislocations:
The addition of large or small atoms into vacancies within the lattice structure can alter the strain fields around dislocations.
- Impurity atoms distort the lattice & generate lattice strains.
- These strains can act as barriers to dislocation motion.
- Small impurities tend to concentrate at dislocations (regions of compressive strains).
- Impurity partially cancels dislocation compressive strains by imposing tensile strains on the crystal lattice.
- This results in reduced dislocation mobility and increases strength.
- Large impurities tend to concentrate at regions of tension caused by dislocations.
- Large impurities impose compressive strains on the crystal lattice.
- In addition to reduced dislocation mobility, due to increased lattice stability, higher stress is needed to first initiate plastic deformation.
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Cold working (strain hardening)
- Ductile metals become harder and stronger as they are plastically deformed.
- Typically at room temperature (for most metals), but certainly well below the melting temperature (i.e. CW has a very low energy input).
- Dislocation structure in Ti after cold working.
- Dislocations entangle with one another during cold work.
- Areas of compression and tension around the dislocation (within the lattice) repel one another.
- Dislocation motion becomes more difficult resulting in a harder and stronger metal
As CW procedures are typically carried out at room temp, energy from the mechanical processes can be stored as stress within the crystal lattice.
• Electrical conductivity and corrosion properties can also be adversely modified by cold work.
Discuss the following method of preventing dislocation movement and therefore strengthen metals:
Precipitation strengthening
Express the Hall-Petch equation used to calculate yield strength and use it to resolve the following issue
Express the equation used to calculate the degree of plastic deformation is expressed as a percentage of the cold work
Express the equation used to calculate the dislocation density (pd)
Identify and discuss the annealing method:
Recovery
