Chapter 7 Flashcards
What is cold working?
The plastic deformation of a metal at a temperature below that at which it recrystallizes.
What is critical resolved shear stress?
The shear stress, resolved within a slip plane and direction, required to initiate slip.
What is dislocation density?
The total dislocation length per unit volume of material; alternatively, the number of dislocations that intersect a unit area of a random surface section.
What is grain growth?
The increase in average grain size of a polycrystalline material; for most materials, an elevated-temperature heat treatment is necessary.
What is lattice strain?
Slight displacements of atoms relative to their normal lattice positions, normally imposed by crystalline defects such as dislocations, and interstitial and impurity atoms.
What is recovery?
The relief of some of the internal strain energy of a previously cold-worked metal, usually by heat treatment.
What is recrystallization?
The formation of a new set of strain-free grains within a previously cold-worked material; normally, an annealing heat treatment is necessary.
What is recrystallization temperature?
For a particular alloy, the minimum temperature at which complete recrystallization occurs within approximately 1 h.
What is resolved shear stress?
An applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane. Formula: τR = (F/A)*cosλcosΦ
What does slip mean?
Plastic deformation as the result of dislocation motion; also, the shear displacement of two adjacent planes of atoms.
What is a slip system?
The combination of a crystallographic (slip) plane and, within that plane, a crystallographic (slip) direction along which slip (i.e., dislocation motion) occurs. Both have high density.
What is solid-solution strengthening?
Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility.
What is strain hardening?
The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.
What are the three types of dislocations?
- Edge
- Screw
- Mixed
What direction do edge dislocations move in?
Parallel to the applied shear stress
What direction do screw dislocations move in?
Perpendicular to the applied shear stress
What are some dislocation characteristics of metals?
- Dislocation motion occurs relatively easily
- Have non-directional metallic bonding
- Closed-packed planes and directions for slip
What are some dislocation characteristics of ceramics?
- Dislocation motion is relatively difficult
- Covalent bonding is directional
- Ionically bonded have few slip systems
What are the characteristics of an FCC slip system?
- {plane} <direction></direction>
- Dislocation motion on {1 1 1} planes
- Dislocation motion on {1 1 0} directions
- Total of 12 independent slip systems
How is the required tensile stress to cause yielding calculated?
σy = τCRSS/(cosλcosΦ)
What are the characteristics of plastic deformation in polycrystalline materials?
- Grains change shape due to slip
- Grains are rolled, they go from equiaxed and randomly oriented (isotropic) to elongated in the rolling direction (becomes slightly anisotropic)
What are three mechanisms to strengthen/harden metals or to decrease dislocation mobility?
- Grain size reduction
- Solid solution strengthening
- Strain hardening (cold working)
What does reducing grain size do?
- Increases grain boundary area
- More barriers to dislocation motion
- Increases yield strength, tensile strength, and hardness
What is solid solution strengthening?
Adding small interstitial (impurity) atoms introduces tensile stress which changes to compression and partially counteracts. The opposite occurs when large interstitial atoms are added. Higher shear stress is required to cause dislocation motion
What is strain hardening (cold working)?
Plastically deforming metals at room temperature makes them harder and stronger. Deformations often cause reductions in cross-sectional area
What happens during cold working?
- Yield and tensile strength increase
- Ductility decreases
How is dislocation density calculated?
Total dislocation length/unit volume
How is dislocation density affected by deformation (cold work)?
It increases, causing the distance between dislocations to decrease, which hinders dislocation motion through repulsive forces
What happens during annealing?
Reverses effects of cold working
1) Recovery (reduction of dislocation density)
2) Recrystallization (new, small grains form)
3) Grain growth (consume and replace
Decreases tensile strength and increases ductility
What is recrystallization temperature TR?
- The temperature at which recrystallization reaches completion in 1 hour.
- TR decreases with increasing cold working
- TR decreases with increasing purity
What is the difference between cold working vs hot working?
- Hot working is deformation above TR
- Cold working deformation below TR
How does grain size influence properties?
- Small grains are relatively strong and tough at low temperatures.
- Large grains have good creep resistance at high temperatures.
