Coldworking Flashcards
What type of deformation provides strengthening of a material?
What happens to tensile strength and yield strength?
What happens to Ductility?
-plastic deformation
- They both increase
-Ductility decreases
What does the application of stress greater than the yield strength provide?
It increases dislocations, and dislocation density increases.
What should we expect for the strain hardening exponents of metals?
It should be low doe HCP metals, and higher for BCC and FCC metals
What does plastic deformation do to thermoplastic polymers?
It strengthens them
When stress applied is greater than yield strength, what force between polymer chains is impaired?
Van der waals force
What does strain hardening do to polymer chains?
The chains straighten and become aligned in the direction of the applied stress.
What happens to dislocations in ceramics?
Dislocations are usually not mobile
What characteristic do ceramics have, and how does it make them act?
Ceramics contain voids, and it makes brittle
What is cold working?
It is the deformation of metal below recrystallization temperature.
What are advantages of cold working?
- Allows metals to be strengthened & at the same time the desired shape to be generated
- Offers excellent dimensional tolerances and surface finish
-It is a cheap method of manufacturing small parts in bulk - Some deformation processing techniques can only occur if the material is strain hardened or cold worked.
What are some disadvantages of coldworking
- Some materials can be cold worked
only up to a small degree such as HCP
magnesium - Ductility, electrical conductivity &
corrosion resistance are reduced - Lack of control over residual stresses
can impair the material properties - Cold working for strengthening can’t
be used for parts exposed to high
temperatures during operation
What is annealing?
Heating to a specific elevated temperature & cooling the material very slowly.
What are the three stages in annealing?
Recovery, Recrystallization, and Grain growth
What occurs during recovery?
Low temperature process
- Heat causes the dislocations to move
- Dislocations move and form the
boundaries of a polygonised sub grain
structure
- Residual stress is removed due to
rearrangement of dislocations
- This process is sometimes called stress
relief anneal
- Dislocation density is unchanged
What does recovery do to electrical conductivity, and corrosion?
It restores high electrical conductivity, and improves corrosion resistance.
What is recrystallization? What happens to dislocation density?
- Process of nucleation of new small grains at the cell boundaries of the polygonised structure by heat treatment
- Dislocation density is reduced.
What is recrystallization temperature?
Temperature at which new small grains with low dislocation density appear
What is grain growth?
It is the phenomenon of increased grain size at high enough temperature.
What do favored grains do to smaller grains?
They consume them
What happens to grain boundary area?
It decreases
Is it a desirable or undesirable phenomenon?
It is undesirable
Are recovery and recrystallization necessary for grain growth?
They are not necessary
What is hot working? Cold working? Warm working?
- Deformation process conducted above 0.6 times the melting temperature of a metal in Kelvin
- Deformation below 0.3 times
- Deformation between 0.3-0.6 times melting temp
What does repeated cold work and annealing cycle allow us to do?
Increase the total amount of deformation
What happens to the effect of strain hardening at high temperature?
The effect is lost.
What does welding do the the joining regions?
It causes the regions near the joints to be heated above the recrystallization and grain growth temperature.
What are characteristic features of ductile fractured surfaces?
- Necking for thick samples
- Flat fractured surface where micro voids nucleate & coalesced
-Shear lip at 45° angle to the applied stress giving a cup & cone appearance of the fractured surfaces.
What is a ductile fracture?
Microvoid formation at grain boundaries & interface between grain/impurity particles
What are some characteristics about brittle fracture?
- Crack initiation at small flaws
- Stress concentrates at crack tip
-In some cases, crack propagates along specific crystallographic plane by cleavage (transgranular path) - Cracks may also propagate along the grain boundaries (intergranular path)
What are microstructure characteristics of brittle fractures?
- Fractured surface is flat & perpendicular to the applied stress
- Crystalline appearance since each fractured grain is flat & they have different orientation
- V-shaped chevron markings form pointing back to the crack initiation or lines radiate in a fanlike pattern from the crack
origin
What happens to cracks in plastic deformation?
Cracks become blunted in metals and polymers
What does the strength of ceramic or glass depend on?
It depends on the probability of finding a flaw that exceeds a certain critical size
For ceramics and glasses, what can you tell me about the distribution of strength?
The distribution of strength is wide and follows the Weibull distribution.
